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DIY Stompboxes => Building your own stompbox => Topic started by: RDV on September 17, 2005, 03:52:55 PM

Title: LM3886/Vox SS Amp Thread(w/ Samples!)
Post by: RDV on September 17, 2005, 03:52:55 PM
I'm going to build the SS Vox preamp suggested to me in Ampage. Here it is. (http://img.photobucket.com/albums/v165/rickydon/voxtbp.gif)

I'm going to also try the limiter circuit suggested by R.G. Here it is. (http://img.photobucket.com/albums/v165/rickydon/voxtmixlmt.gif) I took the liberty of removing the other two input channels of the limiter.

These are going into my LM3886 practice amp I've built. My question is about supply voltage. I've got a seperate 12v supply that I use to run a cooling fan and whichever preamp I happen to be running. I just wander if it will work for those Vox circuits if I use 2N3904 & 2N3906 trannies. Will I have to create another bias circuit or will it work just plugging the 12v into the supply points as shown?

Thanks in advance.

RDV
Title: For R.G. or any SS Vox Expert
Post by: R.G. on September 17, 2005, 04:30:15 PM
The first preamp stages run on +20, the mixer and limiter run on +16-17.

One of the things that eats my time up when I don't have anything else to do... ?!?!? is I'm writing a service manual for Thomas Vox solid state amps. I have a lot of data, enough to drown in, and getting it into book form is slow going. I have no illusion that I'll make any money from this, but I like the amps and hate to see them junked.
Title: For R.G. or any SS Vox Expert
Post by: Doug_H on September 17, 2005, 05:36:07 PM
That's a cool looking preamp, Ricky. I wonder how it would sound as an outboard pedal?!? Hmmm...

Doug
Title: For R.G. or any SS Vox Expert
Post by: RDV on September 17, 2005, 05:51:44 PM
Quote from: Doug_H
That's a cool looking preamp, Ricky. I wonder how it would sound as an outboard pedal?!? Hmmm...

Doug
Dunno. Trying to figure out if it will run on stompbox voltage.

RDV
Title: For R.G. or any SS Vox Expert
Post by: RDV on September 17, 2005, 09:37:26 PM
Well it doesn't work at stompbox voltage!

RDV
Title: For R.G. or any SS Vox Expert
Post by: Doug_H on September 18, 2005, 08:41:35 AM
To get it to run at 9v you would have to re-bias it. Another idea might be to use a charge pump IC if it doesn't draw too much current.

I just realized something... That thing looks like a bootstrapped Darlington pair. Look at the npn boost and compare. I guess I should read the GEO article, I'm sure it already mentions that. Anyway, it makes me wonder what it would sound like to put all that fancy-schmancy tone circuitry on the output of an npn boost. :?:

Doug
Title: For R.G. or any SS Vox Expert
Post by: R.G. on September 18, 2005, 09:48:43 AM
It is indeed a bootstrapped darlington, for high input impedance. The tone controls are a reasonably simple mod to the stock amplifier tone stack, but scaled to lower impedance and with the inductor thrown in for a midrange boost - a fixed frequency wah setting.
Title: For R.G. or any SS Vox Expert
Post by: RDV on September 18, 2005, 05:33:18 PM
Boy! I am rusty. This thing works well at stompbox voltage. I made the 100 ohm to ground another 100k. DOH!!

I've only built the preamp part on perf so far, but I must say that just this part with a volume pot tagged on the end would make a cool treble booster. It reaaallly is bright. I guess I'll move on to the EQ section and see what happens.

Stay Tuned.

RDV
Title: For R.G. or any SS Vox Expert
Post by: Eric H on September 18, 2005, 09:31:12 PM
Quote from: R.G.
The first preamp stages run on +20, the mixer and limiter run on +16-17.

The ampage schematic shows the limiter at 18v single-supply  --is it incorrect?

I remember your old mystery-circuit postings, but find it more interesting now.

-Eric
Title: For R.G. or any SS Vox Expert
Post by: RDV on September 18, 2005, 09:31:50 PM
I finished the EQ section and really like it. This would be a good stomp box. I still need to get a 3-way rotary switch for the mid-boost. I may make a second one for a stompbox version. It has a very Voxy sound.

RDV
Title: For R.G. or any SS Vox Expert
Post by: RDV on September 18, 2005, 09:34:06 PM
Quote from: Eric H
Quote from: R.G.
The first preamp stages run on +20, the mixer and limiter run on +16-17.

The ampage schematic shows the limiter at 18v single-supply  --is it incorrect?

I remember your old mystery-circuit postings, but find it more interesting now.

-Eric


Since these things work on a single supply, I would think that almost any voltage will work with a higher voltage working better.

RDV
Title: For R.G. or any SS Vox Expert
Post by: Eric H on September 18, 2005, 09:43:51 PM
Quote from: RDV
Quote from: Eric H
Quote from: R.G.
The first preamp stages run on +20, the mixer and limiter run on +16-17.

The ampage schematic shows the limiter at 18v single-supply  --is it incorrect?

I remember your old mystery-circuit postings, but find it more interesting now.

-Eric


Since these things work on a single supply, I would think that almost any voltage will work with a higher voltage working better.

RDV


If you look carefully at RG's post, Ricky, He says it's plus-16v/ minus17v

--at least that's how I read it. IOW, a dual-supply

-Eric
Title: For R.G. or any SS Vox Expert
Post by: RDV on September 18, 2005, 10:14:07 PM
Quote from: Eric H


If you look carefully at RG's post, Ricky, He says it's plus-16v/ minus17v

--at least that's how I read it. IOW, a dual-supply

-Eric


He's refering to range of voltage rather than a dual supply. There is a dual supply in those Vox SS amps but I believe that the dual part is used in the power amp sections only.

Chime in if I'm wrong R.G.

RDV
Title: For R.G. or any SS Vox Expert
Post by: R.G. on September 19, 2005, 12:15:21 AM
You're right Rick. It was 16 to 17 volts, etc.

Sorry - I wasn't clear. I do try to write bipolar voltages as "+/-" and I hope I'm consistent.
Title: For R.G. or any SS Vox Expert
Post by: RDV on September 19, 2005, 07:29:06 AM
I'm having a little trouble finding a 3-way switch. I guess I'll get the Rat Shack 6-way and put 6 caps on the mid boost! I really like the .22uF one, it's nice and thick!

RDV
Title: For R.G. or any SS Vox Expert
Post by: Steben on September 19, 2005, 07:39:06 AM
euhm... won't those 820 and 100 ohm resistors suck enormous amounts of current for a stompbox?
Will it be easy to rebias it towards 8k2 and 1k for example?
Title: For R.G. or any SS Vox Expert
Post by: Doug_H on September 19, 2005, 08:38:07 AM
Quote from: Steben
euhm... won't those 820 and 100 ohm resistors suck enormous amounts of current for a stompbox?
Will it be easy to rebias it towards 8k2 and 1k for example?


I was wondering about that too.  You would need to adjust the 100k/33k series string that sets the reference voltage for the base if you changed the collector-emitter biasing.  But darlington pairs are used to provide really high current gain, so the "high current biasing" may be needed to accomodate that.

Something else I noticed were the back-to-back polarized electros (essentially non-polarized). If the individual cap values are "2" and "1", then you could easily sub those with 1u and .47u film caps, respectively.

Doug
Title: For R.G. or any SS Vox Expert
Post by: Doug_H on September 19, 2005, 08:39:23 AM
Quote from: RDV
I'm having a little trouble finding a 3-way switch. I guess I'll get the Rat Shack 6-way and put 6 caps on the mid boost! I really like the .22uF one, it's nice and thick!

RDV


Sounds cool! Should be interesting to hear how the limiter works out. :D

Doug
Title: For R.G. or any SS Vox Expert
Post by: RDV on September 19, 2005, 08:40:57 AM
Quote from: Steben
euhm... won't those 820 and 100 ohm resistors suck enormous amounts of current for a stompbox?
Will it be easy to rebias it towards 8k2 and 1k for example?
The version I'm building now is for an amp, I haven't started a stompbox version yet. I'm testing it with a very strong 9v adapter(about 850ma).

But you're right.

RDV
Title: For R.G. or any SS Vox Expert
Post by: changes on September 19, 2005, 05:04:32 PM
guys have I missed something?
Is this a vox top boost preamp schem?
How are you gonna use it?
And why is the limiter there?
If I get it right it should go in this order: vox preamp+limiter+power amp
What type of trannies and diodes should I use?
Title: For R.G. or any SS Vox Expert
Post by: R.G. on September 19, 2005, 06:47:08 PM
It's not a Vox Top Boost preamp. It's the preamp from the much-maligned Thomas Vox Super Beatle (and Royal Guardsman, and Buckingham and Viscount) amplifier.

I got laughed at a lot when I would admit that I *like* the sound of the Thomas Vox solid state stuff, at least the big-head ones. The preamp is run so that it does not distort on its own (except for the built-in distortion booster, which is a silicon fuzz face) and does include a limiter before the power amp.

Nasty solid state distortion comes from high gain circuits with high feedback flat-lining sharply or activating their internal protection circuits if you believe the hifi tweakos. The TV amps avoided this by (1) using fairly low gain with low-ish feedback on their power amps (2) putting a limiter in front of the power amp that clips before the power amp does. When these amps clip, you hear the limiter clipping, not the power amp running out of gas. And that lets the limiter keep things better controlled.

I guess I have to be a nut. I own:
1 Super Beatle
1 Royal Guardsman
2 Viscounts
1 Buckingham
1 tube Berkeley
1 solid state Berkeley 2
1 Berkeley 3
1 Cambridge Reverb (Thomas, not Korg)
2 tube Pacemakers
1 Nova
1 Churchill PA head
1 box of miscellaneous parts
A chassis and transformers for an AC30 clone someday.

I'm still looking for a Scorpion. Never did think much of the bass amps, though.

Give the "Thomas Vox Amps" pages at GEO a read.
Title: For R.G. or any SS Vox Expert
Post by: RDV on September 19, 2005, 06:58:35 PM
Quote from: changes
guys have I missed something? Possibly

Is this a vox top boost preamp schem? No. It's a Vox SS Brilliant channel Preamp circuit from Ampage.

How are you gonna use it? See the first post in this thread.

And why is the limiter there? Because R.G. suggested using it.

If I get it right it should go in this order: vox preamp+limiter+power amp. Yep.

What type of trannies and diodes should I use? I used just some 2N3904 in the preamp. I haven't tried the limiter yet, but may use the same and some Si diodes.


RDV
Title: For R.G. or any SS Vox Expert
Post by: RDV on September 19, 2005, 10:39:25 PM
Sorry if I seemed a bit snotty there Changes, I had a headache when I came home from work and was a bit short with everyone.

I was just looking for a nice preamp for my little LM3886 based practice amp. It might be around a 20 watter at best with the tranny I used.

Everything I've tried(previously) has been disappointing. I just wanted something clean and clear that will work decent with effects at a low volume for playing at my little work desk.

I think I've found it with this little Vox preamp. I really like it with the mid-boost circuit engaged. It's very natural sounding and almost has that steel guitar sort of country sound but without much twang to it. It almost sounds compressed in a good way with quite a bit of sustain to be so squeaky clean. I'll try to record a little sample or two in the next few days.

I didn't use the limiter cause with the 12volt supply I'm using, it doesn't produce enough gain to overload anyways, plus I'm a very lazy sod at heart.

Thanks a ton to Mr. R.G. Keen for his kind assistance. :D

RDV
Title: Re: For R.G. or any SS Vox Expert
Post by: RDV on September 27, 2005, 09:00:38 PM
Here's some samples recorded by a SM58 mike into a Mackie 1202 Mixer set flat into my soundcard. The distorted one is pretty good I think. This was my first try and will get better I hope.

Mid-Boost(.22cap) with Transistor Obsidian on Low-Gain w/Les Paul (http://mysite.verizon.net/res0o2zg/sitebuildercontent/sitebuilderfiles/mb_ob.mp3)

Clean w/no Mid Boost (http://mysite.verizon.net/res0o2zg/sitebuildercontent/sitebuilderfiles/cl_md.mp3)

Switching between the 3 caps in the mid boost. (http://mysite.verizon.net/res0o2zg/sitebuildercontent/sitebuilderfiles/mb_cl_bk_sw.mp3)

RDV
Title: Re: For R.G. or any SS Vox Expert
Post by: Eric H on September 27, 2005, 10:26:18 PM
Here's some samples recorded by a SM58 mike into a Mackie 1202 Mixer set flat into my soundcard. The distorted one is pretty good I think.

Sound great from here.

You probably already mentioned it, but what size speaker is that?

-Eric
Title: Re: For R.G. or any SS Vox Expert
Post by: RDV on September 28, 2005, 07:17:24 AM
Sound great from here.
You probably already mentioned it, but what size speaker is that?

I didn't say, but I'm running the LM3886 amp into an old Peavey Stereo Chorus 210 cabinet that was found beside the road by my bass player in a rich neighborhood. It was missing the amp part, but had a couple of Eminence 10"s in there and sounds great.

RDV
Title: Re: For R.G. or any SS Vox Expert(Now with Samples!)
Post by: Doug_H on September 28, 2005, 08:05:39 AM
Sounds good. I like the clean sound. The cap switching sounds good too.

Doug
Title: Re: For R.G. or any SS Vox Expert(Now with Samples!)
Post by: vanhansen on September 28, 2005, 09:40:08 AM
Very nice, Ricky.  That first sample with the Obsidian sounds great.  Has smooth, overdriven Bassman tone to it.
Title: Re: For R.G. or any SS Vox Expert(Now with Samples!)
Post by: RDV on September 28, 2005, 10:17:59 AM
I'm waiting on some caps to come, then I'll be modifying the preamp to take the positive voltage from the power supply from the amp which will be about 25 volts. I figure the clean headroom will be much greater than with the 12 volts it's getting now. I'll probably record it again afterward. I may need the limiter then. :icon_wink: :icon_biggrin:

RDV
Title: Re: For R.G. or any SS Vox Expert(Now with Samples!)
Post by: Eric H on September 28, 2005, 10:31:48 AM
I'm waiting on some caps to come, then I'll be modifying the preamp to take the positive voltage from the power supply from the amp which will be about 25 volts. I figure the clean headroom will be much greater than with the 12 volts it's getting now. I'll probably record it again afterward. I may need the limiter then. :icon_wink: :icon_biggrin:

RDV
25v sounds good.
 Build that limiter  --inquiring minds want to know  ;D

-Eric
Title: Re: For R.G. or any SS Vox Expert(Now with Samples!)
Post by: RDV on September 28, 2005, 06:29:48 PM
Here's a better sounding clean with no mid-boost sample.

Voxy Clean (http://mysite.verizon.net/res0o2zg/sitebuildercontent/sitebuilderfiles/clean_arpeg.mp3)

RDV
Title: Re: For R.G. or any SS Vox Expert(Now with Samples!)
Post by: RDV on September 28, 2005, 06:53:41 PM
One more with that old-time grunge feel with the Obsidian and Mid-Boost.

Spoonvox (http://mysite.verizon.net/res0o2zg/sitebuildercontent/sitebuilderfiles/spoonvox2.mp3)

RDV
Title: Re: For R.G. or any SS Vox Expert(Now with Samples!)
Post by: Doug_H on September 28, 2005, 07:13:33 PM
Here's a better sounding clean with no mid-boost sample.



Love it! It really sounds great. ;)

Doug
Title: Re: For R.G. or any SS Vox Expert(Now with Samples!)
Post by: RDV on September 28, 2005, 09:07:03 PM
Thanks Doug!

Here's a brave one with a backing track! My son says I'm rushing the beat, everybody's a stinkin' critic!!
Anyways, it's clean with the mid-boost on and I start out on the back PU and then switch to the front of my Epi LP.

Vox Blues in B (http://mysite.verizon.net/res0o2zg/sitebuildercontent/sitebuilderfiles/bminor.mp3)

RDV
Title: Re: For R.G. or any SS Vox Expert(Now with Samples!)
Post by: spudulike on September 29, 2005, 05:18:15 AM
Ricky - what did you use for the 500mH inductor in the tone section ?

I presume an audio mini-transformer winding would do - but I dont have an inductance meter.  This is different enough to be interesting and the soundclips sound great (nice playing btw).
Title: Re: For R.G. or any SS Vox Expert(Now with Samples!)
Post by: RDV on September 29, 2005, 07:38:33 AM
Ricky - what did you use for the 500mH inductor in the tone section ?
I presume an audio mini-transformer winding would do - but I dont have an inductance meter.  This is different enough to be interesting and the soundclips sound great (nice playing btw).
Thanks for the compliment!
I used a Crybaby inductor I had in my junk drawer, but I think one of those little Radio Shack mini audio transformers might do the trick as well. The effect might be less pronounced, but that might not be a bad thing.

RDV
Title: Re: For R.G. or any SS Vox Expert(Now with Samples!)
Post by: Doug_H on September 29, 2005, 08:18:35 AM
Thanks Doug!

Here's a brave one with a backing track! My son says I'm rushing the beat, everybody's a stinkin' critic!!
Anyways, it's clean with the mid-boost on and I start out on the back PU and then switch to the front of my Epi LP.

Vox Blues in B (http://mysite.verizon.net/res0o2zg/sitebuildercontent/sitebuilderfiles/bminor.mp3)

RDV

That sounds great. Nice warm clean. ;) I love your playing too, very mature, making every note count.

Sounds like you have a good sounding amp! I'm sure this crowd would love to hear you try the limiter too, since they love anything that distorts. ;D ;D

Doug
Title: Re: For R.G. or any SS Vox Expert(Now with Samples!)
Post by: RDV on September 30, 2005, 03:46:40 PM
Yet another sample as I do get carried away. Obsidian with Les Paul with no delay. In yer face.

Metallivox (http://mysite.verizon.net/res0o2zg/sitebuildercontent/sitebuilderfiles/metallivox.mp3)

RDV
Title: Re: For R.G. or any SS Vox Expert(Now with Samples!)
Post by: PenPen on September 30, 2005, 04:10:26 PM
Heh, metal on a Vox. Thats a new one. Sounds like a very verstile amp. I saved those two schems, I think I'm going to have to put that on my TODO list now.

I have an idea cooking up, using the bare MarshaValve circuit in another post as an input buffer, into this, then the limiter, then into another power stage, maybe the Ruby or something with the 386 for practice. Kicking around the idea of using the Bassman 135 power tube arrangement for a bigger head. But you just convinced me of how good that Vox preamp is, I'm definately using that as a preamp!
Title: Re: For R.G. or any SS Vox Expert(Now with Samples!)
Post by: RDV on September 30, 2005, 04:19:49 PM
Heh, metal on a Vox. Thats a new one. Sounds like a very verstile amp.

Just remember that all the distortion came from the pedal.

RDV
Title: Re: For R.G. or any SS Vox Expert(Now with Samples!)
Post by: PenPen on September 30, 2005, 05:07:22 PM
Just remember that all the distortion came from the pedal.

RDV

Of course. I've just found that some amps sound great clean, but as soon as you stomp on a box, it sounds bad. Conversely, I've had many amps that sounded fantastic distorted with my Rat or DS-1, but sounded horrible the moment I switched them off. I've only heard a couple of amps I like the sound both ways, and I think that Vox there is one of them. I'm going to try it to see, it could end up not being what I'm after, but I'll never know unless I try. What I'm really after is an extremely clean base, but gets gritty and darker when pushed with a boost, and then can sound great in full gain Rat/Big Muff distortion. I'm hoping the combo I mentioned can be tweaked to give me something close to that.
Title: Re: For R.G. or any SS Vox Expert(Now with Samples!)
Post by: R.G. on September 30, 2005, 09:47:48 PM
This is kind of fun. I've told people how good the Thomas Vox solid state stuff sounds for a couple of decades off and on, and got reactions varying from incredulity to laughter.

I still think they sound good.
Title: Re: For R.G. or any SS Vox Expert(Now with Samples!)
Post by: p1_ind on September 30, 2005, 10:09:09 PM
I agree with R.G.
I repaired a solid state Thomas Vox Cambridge Reverb last year for my brother and loved the sound of the amp.

They are not fun to work on (wiring nightmare), but great sounding.

By the way I don't remember if I thanked you R.G. for the schematic and help fixing it.  But thanks!  My brother was very happy to hear it working properly.
 
Adam
Title: Re: For R.G. or any SS Vox Expert(Now with Samples!)
Post by: RDV on October 02, 2005, 02:05:47 PM
UgLyVoX (http://mysite.verizon.net/res0o2zg/sitebuildercontent/sitebuilderfiles/uglyvox.mp3)
(Uglyface + Vox/LM3886) =  :P :P :P :P :P :P :P :P :P :P :icon_wink:

RDV
Title: Re: For R.G. or any SS Vox Expert(Now with Samples!)
Post by: RDV on October 06, 2005, 07:37:18 AM
Well I got my 10uF 50v caps in yesterday and replaced the little 16v stompbox ones in the preamp and ran the positive power supply rail(27.5v) to the preamp hoping for increased performance, less ground loop noise etc.

It is a bit louder, but still has a hum. I suppose my tranny (http://www.mouser.com/index.cfm?handler=displayproduct&lstdispproductid=416095) is too close to the rest of the amp.

I don't think the tone is quite the same either.

More testing is needed though.

I think perhaps an emitter folllower before the power amp would help things also, though I'm not quite sure which one to use.

RDV
Title: Re: For R.G. or any SS Vox Expert(Now with Samples!)
Post by: PenPen on October 06, 2005, 10:23:13 AM
Well I got my 10uF 50v caps in yesterday and replaced the little 16v stompbox ones in the preamp and ran the positive power supply rail(27.5v) to the preamp hoping for increased performance, less ground loop noise etc.

It is a bit louder, but still has a hum. I suppose my tranny (http://www.mouser.com/index.cfm?handler=displayproduct&lstdispproductid=416095) is too close to the rest of the amp.

I don't think the tone is quite the same either.

More testing is needed though.

I think perhaps an emitter folllower before the power amp would help things also, though I'm not quite sure which one to use.

RDV

I always wondered about the noise caused by a transformer. In my preliminary sketches for an amp I planned to maybe shield the coils of the transformer with some kind of steel/alu hood run to ground to try to block the noise of it from getting into the circuit. I noticed on my friend's Bassman 135 head Fender mounted the transformers upside down so that the coils where outside the chassis, with just the leads coming through the inside. Now I wonder if they did this to prevent noise. You could try just putting some kind of shroud around the transformer coils, not touching the transformer itself of course, making sure the shroud also touches the chassis, to see if it clears up any of the noise. Thats what I'd try, anyway.

Note, I am a total newb at amps, so maybe R.G or someone else with more knowledge will chime in too.
Title: Re: For R.G. or any SS Vox Expert(Now with Samples!)
Post by: RDV on October 06, 2005, 10:45:44 AM
I mounted the whole shebang on a piece of crappy brown Radio Shack perfboard so the tranny is right there just a few inches from the chip-amp. Tonight I'm going to separate it and just run wires to the rest of the circuit.

RDV
Title: Re: For R.G. or any SS Vox Expert(Now with Samples!)
Post by: Eric H on October 06, 2005, 11:35:58 AM
I mounted the whole shebang on a piece of crappy brown Radio Shack perfboard so the tranny is right there just a few inches from the chip-amp. Tonight I'm going to separate it and just run wires to the rest of the circuit.

RDV
If you leave enough slack in the wires, try rotating it while it's on, as well. You can "aim" the magnetic field for least hum.

-Eric
Title: Re: For R.G. or any SS Vox Expert(Now with Samples!)
Post by: RDV on October 06, 2005, 09:15:02 PM
Well it's not the proximity of the tranny, nor is it filtering. I moved the tranny and it still hummed. I added a couple more 4700uF for a total of about 10,000uF of filtering on each rail.

I guess it's gonna hum a bit.

It's not a real loud hum, just a bit irritating when I read how the guys make their Gainclone amps virtually hum-free.

RDV
Title: Re: For R.G. or any SS Vox Expert(Now with Samples!)
Post by: Eric H on October 06, 2005, 09:29:41 PM

It's not a real loud hum, just a bit irritating when I read how the guys make their Gainclone amps virtually hum-free.

RDV

 Star grounding.

-Eric
Title: Re: For R.G. or any SS Vox Expert(Now with Samples!)
Post by: R.G. on October 06, 2005, 11:06:00 PM
Eric is very close to the truth.

To do a ripple and hum-free class AB amp is very demanding on the wiring. The( +) and (-) power supply leads both conduct full-peak-current half-wave-rectified signal current on alternate signal polarities. The ground for the speaker must NOT be returned anywhere but to the power supply ground (perferably that's the star ground). The rectifier current pulses must not share any length of wire with either signal ground or speaker return.

The only way to make current go where you want it to is to leave it no option - make only one wire for it to flow through.
Title: Re: For R.G. or any SS Vox Expert(Now with Samples!)
Post by: RDV on October 07, 2005, 03:15:31 AM
I've gotten my 1st significant reduction in the hum  by establishing a central "Star" ground. I really did a bang-up job of screwing this thing up thinking I could wire it like a pedal. Uh Uh. Gotta wire like an amp. My next amp project will go much differently. I've still got to come up with a booster/buffer after the volume control.

Jeez

I've still got further to go but I'm worn out.

RDV
Title: Re: For R.G. or any SS Vox Expert(Now with Samples!)
Post by: Elektrojänis on October 07, 2005, 01:58:54 PM
After star grounding, how about making a separate RC-filter for powering the preamp part... Or even an LC-filter.

Power amps need a lot of current, so they need to be powered straight from the rectifier/filter caps.

Preamps on the other hand usually don't need that much current but can use some extra filtering. Even if this one eats more current than a typical spomp box, it can probably live with 1-10 ohms (the R-part of the filter) in series with the power supply (and after that a cap to gruond ofcourse, the C-part of the filter).

That's what I'd try anyway...
Title: Re: For R.G. or any SS Vox Expert(Now with Samples!)
Post by: Eric H on October 07, 2005, 02:06:20 PM
After star grounding, how about making a separate RC-filter for powering the preamp part... Or even an LC-filter.

Power amps need a lot of current, so they need to be powered straight from the rectifier/filter caps.

Preamps on the other hand usually don't need that much current but can use some extra filtering. Even if this one eats more current than a typical spomp box, it can probably live with 1-10 ohms (the R-part of the filter) in series with the power supply (and after that a cap to gruond ofcourse, the C-part of the filter).

That's what I'd try anyway...
That's an excellent suggestion.

-Eric
Title: Re: For R.G. or any SS Vox Expert(Now with Samples!)
Post by: RDV on October 07, 2005, 03:09:33 PM
About all I hear now is my little cooling fan, so I'm pretty happy with it now.

I'm going to do another one soon with a much better transformer and heat sink, and all the lessons I've learned here will be applied.

Thanks all.

RDV
Title: Re: For R.G. or any SS Vox Expert(Now with Samples!)
Post by: octafish on October 27, 2005, 01:47:29 AM
Hey Ricky I hope you check this. I just want to clarify this one. The cap selector rotary switch is the the amount of mid boost selector and the switch between the inductor and the 10K resistor is a mid boost on/off switch. Is this correct?
Title: Re: For R.G. or any SS Vox Expert(Now with Samples!)
Post by: RDV on October 27, 2005, 07:15:44 AM
The cap selector rotary switch is the the amount of mid boost selector and the switch between the inductor and the 10K resistor is a mid boost on/off switch. Is this correct?
No, and yes. The cap selector just switches between different caps for different mid frequencies to be boosted. If you go back to the samples and play the one where I'm twisting the cap selector you can hear it changing the peak of the mid boost up and down. The switch between the inductor and the 10k is the on/off switch for the MRB.

HTH

RDV
Title: Re: For R.G. or any SS Vox Expert(Now with Samples!)
Post by: Ge_Whiz on October 27, 2005, 08:47:59 AM
Nice one, Ricky - I'll buy the album. "RDV - The Thomas Vox Sessions".  :icon_biggrin:

Those non-polarized electrolytic capacitors in the preamp - does the labelling indicate a pair of capacitors, each 2uF or 1uF, or a non-polarised arrangement of actual capacitance 2uF or 1uF, please?

Phil
Title: Re: For R.G. or any SS Vox Expert(Now with Samples!)
Post by: RDV on October 27, 2005, 09:09:50 AM
Nice one, Ricky - I'll buy the album. "RDV - The Thomas Vox Sessions".  :icon_biggrin:

Those non-polarized electrolytic capacitors in the preamp - does the labelling indicate a pair of capacitors, each 2uF or 1uF, or a non-polarised arrangement of actual capacitance 2uF or 1uF, please?

Phil

Thanks Phil, I'll send you a free copy when it gets pressed!

I used 1uF Phillips MKT film caps, though I would imagine you could use anything in there that would equal the right value. I just prefer films if I can fit them. I believe they're indicating a non-polar electrolytic on the schem though.

Cheers

RDV
Title: Re: For R.G. or any SS Vox Expert(Now with Samples!)
Post by: R.G. on October 27, 2005, 09:44:23 AM
Quote
Those non-polarized electrolytic capacitors in the preamp - does the labelling indicate a pair  of capacitors, each  2uF or 1uF, or a non-polarised arrangement of actual capacitance 2uF or 1uF, please?
They varied from model to model. If you have room, put in 2.2uF polyester/mylar or polypropylene. If you must, use 2.2uF NP/bipolar electrolytic. The caps do need to be nonpolar in some fashion, because there are signal conditions when they are polarized both ways.
Title: Re: For R.G. or any SS Vox Expert(Now with Samples!)
Post by: Ge_Whiz on October 27, 2005, 11:55:49 AM
Got it. Should have read the GEO pages more carefully first - thanks.
Title: Re: For R.G. or any SS Vox Expert(Now with Samples!)
Post by: Ge_Whiz on October 27, 2005, 12:38:46 PM
Slightly off-topic, forgive me - but Ricky, is your Obsidian 'stock' with BS250s? Great sound.

Title: Re: For R.G. or any SS Vox Expert(Now with Samples!)
Post by: RDV on October 27, 2005, 02:35:36 PM
Slightly off-topic, forgive me - but Ricky, is your Obsidian 'stock' with BS250s? Great sound.
No problem. That's the old Transistor version with 2N5089s.

RDV
Title: Re: For R.G. or any SS Vox Expert(Now with Samples!)
Post by: octafish on October 27, 2005, 07:31:23 PM
Thanks Ricky. Cleared that up. So maybe you could use a 4 position double pole rotary switch to switch between off and the different cap values. I have a six position switch that has two "sides" so one side would be the on wired to three poles and then a break. On the other side you would have your three different value caps and your choice of filter caps. You could probably double connect the 100n (.1uF) or whatever your choice is so that it is also connected in the off position.
Hmmm that makes sense to me, or am I missing something?

Just for interests sake I'll be coupling it with this little class A design from RED circuits.
http://www.redcircuits.com/Page80.htm (http://www.redcircuits.com/Page80.htm)
Looks cool huh?




Title: Re: For R.G. or any SS Vox Expert(Now with Samples!)
Post by: RDV on October 27, 2005, 07:40:56 PM
I'm using a 6PDT which I only have three caps wired to. I have 3 positions of 'off'. It makes a decent 'Jazz' sound in the off position. Maybe I'll do a sample of that.

RDV
Title: Re: For R.G. or any SS Vox Expert(Now with Samples!)
Post by: Erik Lion on November 09, 2005, 06:11:10 AM
Going back to the limiter circuit: http://img.photobucket.com/albums/v165/rickydon/voxtmixlmt.gif
Question for R.G. - and others.
Do I understand it correctly that this is the circuit used in some/all Thomas Vox SS amps to prevent the poweramp from clipping?

I'm considering to try/use it with a bass amp project I'm working on. The preamp is a DIY (schem: http://www.albertkreuzer.com/preamp.htm ) - modded with a passive Baxendall tonestack instead of the active tone controls - into an old HH 130 watt power amp. (I'm aiming for a '60 bass sound).

I've never seen a complete Thomas Vox schematics, but I take it the limiter circuit would go in series with the preamp - just ahead of the power amp, right?
I'm running my preamp on 25V and it would been easy to make a 18V tap - but are there any other problems to consider, or changed to do for making it work?

Erik
Title: Re: For R.G. or any SS Vox Expert(Now with Samples!)
Post by: Erik Lion on November 09, 2005, 05:26:41 PM
One more question. What would be a good choice for the transistors and the diodes in the limiter circuit?

Erik
Title: Re: For R.G. or any SS Vox Expert(Now with Samples!)
Post by: Erik Lion on November 11, 2005, 01:02:17 PM
Anyone - please?
Title: Re: For R.G. or any SS Vox Expert(Now with Samples!)
Post by: RDV on November 11, 2005, 04:06:53 PM
When in doubt, use 2N5087,  2N5088 & 1N4148(1N914).

RDV
Title: Re: For R.G. or any SS Vox Expert(Now with Samples!)
Post by: petemoore on November 11, 2005, 06:25:18 PM
  well..I just say 'excellent'
  Thread,
  Samples,
  Work,
  Preamp
  Sounds/looks great Ricki !!! ...RG...Eric...everybody who posted here.
  Thanks for the learning lessons we can all draw from /!!!
 
Title: Re: For R.G. or any SS Vox Expert(Now with Samples!)
Post by: Erik Lion on November 12, 2005, 06:13:09 AM
Thanks RDV. Have you decided not to do the limiter?

Erik
Title: Re: For R.G. or any SS Vox Expert(Now with Samples!)
Post by: RDV on November 12, 2005, 11:38:52 AM
Have you decided not to do the limiter?

Yeah, cause the preamp as built doesn't have enough gain to make the poweramp clip.

RDV
Title: Re: For R.G. or any SS Vox Expert(Now with Samples!)
Post by: B Tremblay on November 12, 2005, 05:06:12 PM
Ricky, do you like the feel of the Bass and Treble controls?  I breadboarded the preamp (sans limiter) and they seemed fairly subtle.  The Boost switch is a dramatic effect, but I was able to perceive only the slightest difference between the tone controls set at their extremes. However, I may have made an error somewhere.
Title: Re: For R.G. or any SS Vox Expert(Now with Samples!)
Post by: RDV on November 13, 2005, 04:11:33 AM
Brian:

I find them to be somewhat like all Vox-Style tone controls; only one spot where they sound good(depending on the guitar of course).

RDV
Title: Re: For R.G. or any SS Vox Expert(Now with Samples!)
Post by: B Tremblay on November 13, 2005, 08:08:42 AM
Cool, thanks.  I have a couple other questions as well (not necessarily directed at Ricky).

What's the overall gain of the preamp?

It looks like the tonestack has been rescaled (maybe by a factor of 100) from the values I've seen in most classic amp tonestacks.  Is this an effort to pad down the output?  How is impedance affected?
Title: Re: For R.G. or any SS Vox Expert(Now with Samples!)
Post by: RDV on November 13, 2005, 01:14:19 PM
I tested the raw preamp circuit(without tone circuit, just an outcap) and it was reaalllyy loud and quite trebley. The tonestack drags the output way down, though it's a lot louder with the MRB kicked in. If you guys  could do your ROG thing with this circuit and stompbox it that would be way cool.

HTH

RDV,
Title: Re: For R.G. or any SS Vox Expert(Now with Samples!)
Post by: petemoore on November 13, 2005, 01:18:44 PM
  I just recently repaired, smoked, re-fixed, a Dyna Power Amp.
  It has no pre-amp... :icon_wink:.
Title: Re: For R.G. or any SS Vox Expert(Now with Samples!)
Post by: petemoore on November 13, 2005, 06:16:03 PM
  RDV, Could you elaborate on:
  'It is a bit louder'
  I've builta  circuit using the schematic [first line] from the title post, it boosts 'mildly', am I expecting too much gain from two transistors or does my circuit not 'jibe'...]error?[, I'm trying to figure that out.
  ya know the 'ol <'but it is right' / but is it right>?
Title: Re: For R.G. or any SS Vox Expert(Now with Samples!)
Post by: RDV on November 13, 2005, 08:43:25 PM
  RDV, Could you elaborate on:
  'It is a bit louder'
  I've builta  circuit using the schematic [first line] from the title post, it boosts 'mildly', am I expecting too much gain from two transistors or does my circuit not 'jibe'...]error?[, I'm trying to figure that out.
  ya know the 'ol <'but it is right' / but is it right>?

It's not very loud with the tone controls as they are with those small values and all and would probably benefit from a gain recovery stage ala BMP.

RDV
Title: Re: For R.G. or any SS Vox Expert(Now with Samples!)
Post by: petemoore on November 13, 2005, 09:58:53 PM
  Nice Build  :icon_wink:.
  Great application for my 'spare' inductor, puts out a really nice 'Pure SS Crank' tone on it, I put some Reverb on it and started to really enjoy the 'Zone' it's in.
  Sounds like it would make for very direct sounding presence in a stage mix.
Title: Re: For R.G. or any SS Vox Expert(Now with Samples!)
Post by: RDV on November 17, 2005, 07:11:34 PM
I've decided to buy my amps at the store. I added the first stage from the limiter and also tried a BMP vol recovery stage. Both caused an avalanche of hum, as well as the desired volume. I just don't know what I'm doing. To all those who do, god bless you.

RDV
Title: Re: For R.G. or any SS Vox Expert(Now with Samples!)
Post by: RDV on November 24, 2005, 10:56:05 AM
Quote from: Fatboy
I just don't know what I'm doing...

Boy, truer words were never spoken; however I decided to not let the thing beat me. I put it to the side about a week or so and just tried to suss it all out in my little pea-brain. Being self-taught at everthing you do can be a painful process(psychic pain that is).

So... on turkeyday eve I decided to apply a few theories.

First... I disconnected the preamp and decided to find the rest of the ground loops in the poweramp section. To do this I looked the PCB on GGG which was a huge help(thanks Dean & JD). The last ground loop was between the 100µF cap on the standby & the PS filters on the negative side. Once that was corrected I fired up the poweramp section and plugged my new $10.00 graphic EQ in and Shazaam!!

No more poweramp hum(@ least from ground loops).

Second... I remembered that the preamp sounded sweetest running at 12v off the PS for the cooling fan. I wired that back along with extra filtering and Shazaam!!

It sounded good again(though not very loud).

Third... I decided to attack the 300lb. gorilla of the volume recovery circuit. I used the one at the front of the limiter circuit with a 10µF coupler cap and Shazaam!!

It was loud(but popped and crackled when I touched the strings).

Fourth... The crackle & pop thing amounts to the bad wiring in my old house. The outlet I'm using is a 3-prong modern type outlet, but the ground is not a real ground but wired to the neutral(white) wire which is a disturbing situation concerning my personal safety. I decided to attack this one the "Hartley Peavey way" which is adding a high voltage .01µF ceramic cap between neutral(white) and the chassis ground and Shazaam!!

No more crackle & pop(well, maybe just a little).

Anyways, this thing now sounds good and is loud and noise is at an acceptable level.

The next amp will include PCBs and proper transformers and a chassis.

AMEN.

Happy Thanksgiving!!

RDV



Title: Re: For R.G. or any SS Vox Expert(Now with Samples!)
Post by: Eric H on November 24, 2005, 11:06:25 AM
Very nice sleuthing, Sherlock.

Sometimes you just have to put it down for awhile.

-Eric
Title: Re: For R.G. or any SS Vox Expert(Now with Samples!)
Post by: B Tremblay on November 24, 2005, 11:15:45 AM
Well done!

I'll be pretty disappointed if you don't decide to name it Shazaam!  Or, how about Shazaamp?
Title: Re: For R.G. or any SS Vox Expert(Now with Samples!)
Post by: R.G. on November 24, 2005, 11:22:58 AM
Quote
Have you decided not to do the limiter?
Yeah, cause the preamp as built doesn't have enough gain to make the poweramp clip.
From what I see you now have additional gain. I think you'll like the results if you include that limiter. It turns the clipping behavior of the amp into soft limiting from the hard limiting you'll get from the power amp itself.
Title: Re: For R.G. or any SS Vox Expert(Now with Samples!)
Post by: RDV on November 24, 2005, 11:43:34 AM
Here's a modified schem of the preamp as it stands.
(http://img.photobucket.com/albums/v165/rickydon/voxtbpRDV.gif)

RDV
Title: Re: For R.G. or any SS Vox Expert(Now with Samples!)
Post by: RDV on November 24, 2005, 12:25:02 PM
Quote
Have you decided not to do the limiter?
Yeah, cause the preamp as built doesn't have enough gain to make the poweramp clip.
From what I see you now have additional gain. I think you'll like the results if you include that limiter. It turns the clipping behavior of the amp into soft limiting from the hard limiting you'll get from the power amp itself.
There are some things I don't understand about the limiter section. I'm using a 12v supply for the preamp and I don't know if that's going to be sufficient at all the voltage points.

I also don't know what the limiter pot does. Does it act as a headroom level? Or maybe like a Master volume? It scares me a bit; fear of the unknown.

We've also got to remember that I still pretty limited by the 56va transformer I used. It's getting loud, but I don't think I've got the juice to really make the chipamp clip. It's also about a foot and a half from my head where it's set up, so I'm only gonna turn it up so loud LOL.

I'm enjoying this a lot though, and I sure appreciate your help R.G. I don't know if too many guys are really interested in building their own SS amps, but I think it's sorta neat.

RDV

RDV
Title: Re: For R.G. or any SS Vox Expert(Now with Samples!)
Post by: RDV on November 24, 2005, 12:41:16 PM
Oh, also thanks to you other fellows who've been following this protracted thread and offering hints and encouragement. It's all much appreciated.

Ricky
Title: Re: For R.G. or any SS Vox Expert(Now with Samples!)
Post by: stm on November 24, 2005, 01:24:25 PM
Ricky, it sounded sad to me when you decided to give up on this. I can understand the frustation you experienced at that time, 'cause I've seen that too.

Now I'm glad to see how well you kicked the ass of every underlying problem. Certainly some time to cool down things helps a lot. I think issues keep running in background, then, some time later a new idea or inspiration appears. I'm sure this is typical for all humans.

The day of my wedding I was in the shower prior to dressing up and go to the priest. Then suddenly I saw the subroutine and lines of code where the error was--I've been struggling for over a month with a piece of hardware and DSP code but it kept hunging up.  I got married, went to Dominican Republic for honey moon, took an extra week of vacation to setup the new home, and then three weeks later I returned to the office and fixed the damn bug in five minutes. Human brain is an extraordinary machine!

A final word of advice: your words of appreciation are in complete contradiction with your avatar image!!!  :D
Title: Re: For R.G. or any SS Vox Expert(Now with Samples!)
Post by: R.G. on November 24, 2005, 01:39:17 PM
Quote
There are some things I don't understand about the limiter section. I'm using a 12v supply for the preamp and I don't know if that's going to be sufficient at all the voltage points.
That's a reasonable point. I think it can be re-converted down to 12V. I'll take a look.

Quote
I also don't know what the limiter pot does. Does it act as a headroom level? Or maybe like a Master volume? It scares me a bit; fear of the unknown.
How the heck do you ever burn up parts with that kind of attitude?  :icon_biggrin:

The limiter is a variable clipper. The limiter pot sets the voltage at which the limiting occurs. I refer you to John Greene's post here for the action of the limiting pot. It is not a master volume, or a headroom level. It's like having a pair of clipping diodes ahead of your power amp that can be set to clip at any of a range of clipping voltages, not just +/- 0.6V.

Quote
We've also got to remember that I still pretty limited by the 56va transformer I used. It's getting loud, but I don't think I've got the juice to really make the chipamp clip
Making the chip amp clip is not the point. The idea is to have the limiter limit the signal **before** the chip amp clips. Having the limiter stop the signal at the input from getting large enough for the chip amp to clip means that the chip amp itself is always in its linear region. It never clips. The limiter prevents it from ever having to.

And as to whether you have enough signal to make the chip amp clip: The chip amp is an opamp with a monster output stage. It's only compensated to be stable for gains of 10 or greater, but it's an opamp nevertheless. What would happen if we drive it from +/-30V supplies, and set it up for a gain of 100? The input needed to make the output swing +30V is only 0.3V. You could make it clip driving it with some humbucker pickups directly. If we set if for a gain of 10 (the lowest we can), then a voltage of 3V peak drives it to clipping on the output.

If we have +/-30V supplies and a gain of 10, and also a limiter in front of it set to clip at +/-2.7V, then the limiter prevents the input signal that the amp sees from ever reaching 3V peaks, and so the chipamp never clips - the limiter keeps the input from going that high (or low).

If you put two clipping diode sets in series in front of the chip amp, then the input to the chip amp is limited to +/-1.4V peak by the diodes. The chip amp can then have any gain lower than the power supply divided by 1.4V and not clip.

Does that make sense?
Title: Re: For R.G. or any SS Vox Expert(Now with Samples!)
Post by: RDV on November 24, 2005, 03:32:27 PM
A final word of advice: your words of appreciation are in complete contradiction with your avatar image!!!  :D
That's Johnny Cash flipping the bird to "the Man". It's not intended to reflect my attitude toward friends.  :icon_wink: :icon_wink:

RDV
Title: Re: For R.G. or any SS Vox Expert(Now with Samples!)
Post by: RDV on November 24, 2005, 03:36:54 PM
Quote from: R.G.
Does that make sense?
Of course. All your posts make sense. I'll probably make another daughter-board(the 4th) and try the limiter for the heck of it. This whole contraption is on that brown RS perf. I'll report back with the results.

RDV



Title: Re: For R.G. or any SS Vox Expert(Now with Samples!)
Post by: octafish on November 24, 2005, 04:56:07 PM
Lokking forward to your results. I'm less than thrilled with the tone stack though, while I like the MRB, the treble and bass control seem to do too little to me. I may replace it with a baxandall/james stack. If I had the nous I'd add the MRB to the james but I'm pretty limited.
Title: Re: For R.G. or any SS Vox Expert(Now with Samples!)
Post by: RDV on November 24, 2005, 06:27:13 PM
I added the limiter to the mix and while subtle I think it's working fine. As I suspected to get it working I had to turn the amp up WAY louder than I ever will just sitting here at my desk which is also in our TV room. I couldn't have it up very long like that but I think as I turned up the limiter level(lowering voltage I think) it seemed to lessen the volume a bit. It still basically sounds the same though which is good cause I like a loud clean amp.

As far as the tone controls, I like them though all the action happens in the 1st half turn. I like the voicing they have. Too wide ranging tone controls have always been hard for me to dial in.

RDV
Title: Re: For R.G. or any SS Vox Expert(Now with Samples!)
Post by: R.G. on November 24, 2005, 07:15:06 PM
I did some reverse engineering on your power. With a 36Vct transformer, you get 36*1.414= 50.9V peak to peak. That's 25V peak, less a couple of volts for sag and amplifier losses, so you could put maybe 22V peak across an 8 ohm load. Thats 15.6Vrms, and into 8 ohms that's 30W.

And it's an honest 30Wrms out of that chip. That's LOUD. The Vox AC30 is 30W rms, and it's LOUD.

If your speaker is 90db and 1W/1M (and that's common for guitar speakers; some are more like 96db), then 20db up at 10Wrms is 110db, and from there to 30W you get another 4.7db - the spl out of that thing can easily be 114db at  one meter.

That's not a small amp.
Title: Re: For R.G. or any SS Vox Expert(Now with Samples!)
Post by: RDV on November 24, 2005, 07:23:46 PM
You're not kiddin' about loud. You could gig with this thing! It's as loud or louder than my old Peavey Bandit 65. It's zonking the holy livin' crap out of the little eminance 10"s I've got it plugged into. I think an all inclusive PCB for this amp would be really cool. You could sure build a worse SS amp(& many have!).

My meter tells me it's producing 27.5v +/-

Whee!!

RDV
Title: Re: For R.G. or any SS Vox Expert(Now with Samples!)
Post by: R.G. on November 24, 2005, 07:40:27 PM
Does your transformer get hot when you run it full blast for an hour or so?
Title: Re: For R.G. or any SS Vox Expert(Now with Samples!)
Post by: RDV on November 24, 2005, 07:53:20 PM
Does your transformer get hot when you run it full blast for an hour or so?

I couldn't take this thing full blast for an hour in my house! :icon_wink:
I've not felt much heat from it at all though, even if it stays on all day.
One reason may be the 12v cooling fan that runs over the whole thing as long as it is on.
I used a small heatsink for the chip, so I had to use the fan.

RDV

Title: Re: For R.G. or any SS Vox Expert(Now with Samples!)
Post by: R.G. on November 24, 2005, 10:52:39 PM
OK, special advice.

As I tell people a lot, power amps are essentially a power supply with some other junk on them that lets a little of the power out carefully. That's not a complete description. Power amps are also heat-exchange engines as well as power supplies.

Before you make yourself a sealed up amp, go to ebay and buy one of the flat-back heatsinks that come up there every now and then. You can get on for under $10 that's heavy with lots of fins. 3" along the fins, 6" wide, and maybe 1.5" to 2" fins would be good. Make that heatsink part of your amp. Ditch the fan. You have the beginnings of a very good amp.
Title: Re: For R.G. or any SS Vox Expert(Now with Samples!)
Post by: RDV on November 25, 2005, 06:24:29 AM
I use a 50watt Marshall to gig with, but am retiring to devote a bit more time to my children(@ least for a while) after New Years, but were this amp to be anything but a bench amp I would do many things differently. It's not going to do any traveling in it's present state for certain. A total of 6 seperate pieces of brittle brown perf and two PTs make this one go right now, so I don't really know if it could ever be made exactly road-worthy. I've got another LM3886 that may be going to be made into a real amp with chassis, heatsink, PCBs, proper 300va tranny etc.

I sort of knew going in that this one would be a learning vehicle and perhaps nothing else. The fact that it's turned out so well is just icing on the cake.

I will definitely be on the lookout on eBay for heatsink material and torroidals.

RDV
Title: Re: For R.G. or any SS Vox Expert(Now with Samples!)
Post by: Erik Lion on November 26, 2005, 12:57:17 PM
R.G. - on the original Vox limiter schem the circuit calls for two supplies - 18V and 30V.
I see that RDV is using only 12V for both. What's the disadvantages of only 12V - if any?
I take it that the higher 30V into the trim pot on the originals must be there for a reason.

I've tried to build the limiter (twice actually, uuhhh), but I'm having some trouble (going back to my power supply, I think), so I too have decided to put the project away for a couple of weeks.

Erik
Title: Re: For R.G. or any SS Vox Expert(Now with Samples!)
Post by: RDV on November 26, 2005, 02:12:50 PM
I'm not sure my limiter is doing anything. I wish someone could run a simulation for me and let me know if it is doing anything with 12v running everything. HINT HINT. I suppose I could feed it the  pre-regulator voltage which is around 14 or 15 volts if a voltage differential is required. It's just sounding so good I hate to mess with it.

I added 3 more caps to the MRB since I had a 6 pole switch. I added a .47µF, .68µF, and a 1µF. It's about a versatile midrange machine now babe!

RDV
Title: Re: For R.G. or any SS Vox Expert(Now with Samples!)
Post by: R.G. on November 26, 2005, 03:24:12 PM
OK, but this is gonna cost you a beer.

The limiter as you drew it limits between +/-1V and +/-0.5V with 1N4148 diodes for the clipppers. You probably can't hear it because the LM3886 amp from GGG has a gain of 34. With +/-18V supplies, it clips at something like +/-16V - that's a guess, the datasheet really doesn't say. So the power amp as shown clips with an input of 16/34 = 0.47V peak, below the lowest level that the limiter limits.

You don't need all that gain in the power amp. You could change the 33K feedback resistor to 10K for a gain of 11 overall, still stable. You would then need an input of 16/11 = 1.45V to clip the amp, and the limiter will do the limiting. A gain of about 16 would let you get the two pretty close, so you could just piggyback another 33K on top of the one that's there now. And then you should be able to turn the limiter pot and hear the difference. High resistance is low limit voltage, low resistance is higher limiting level.
Title: Re: For R.G. or any SS Vox Expert(Now with Samples!)
Post by: RDV on November 26, 2005, 04:08:36 PM
OK, but this is gonna cost you a beer.
Heh heh, I think everyone on this forum owes you a beer. The truth of the matter is that you should never have to pay for a drink of any kind. Ever. If I still lived in Arkansas, I'd drive to Austin with a case or two of your choice, but I'm a 'fer piece'(as we would say back home) from Texas or Clintonville where I'm at now.

You know, you're a pretty nice guy for a Texas Republican.  :icon_wink: :icon_wink:

I'll parallel a 33k and report back to the thread that wouldn't leave.

Ricky
Title: Re: For R.G. or any SS Vox Expert(Now with Samples!)
Post by: RDV on November 27, 2005, 04:40:44 PM
The limiter works as advertised. Oh my ears! I had to crank the crap out of the amp to really hear the difference, but it works great. I can hardly believe how freaking loud this thing will get. It's unreal. With the limiter full on, the output reaches a ceiling(a paint peeling, ear blistering one albeit), and with it all the way down it's about like standing behind a jet engine! I'm not exagerating. I still didn't hear any amp breakup though, but with the MRB on the 'cone cry' distortion was out standing.

The only amp I can compare it to was an old Gibson G-30(?) SS amp I had when I was a kid. It was a paint peeler as well.

I had to wait for the wife to leave the house to try this as she still has some hearing left. I can only imagine what it would do with a more powerful transformer.

RDV
Title: Re: For R.G. or any SS Vox Expert(Now with Samples!)
Post by: Erik Lion on November 27, 2005, 04:55:01 PM
Talking about limiters. By accident I just came across another take at a soft limiter - simple and passive, and softer I think than the Vox.
http://www.dogstar.dantimax.dk/tubestuf/dzart-6.htm

Erik
Title: Re: For R.G. or any SS Vox Expert(Now with Samples!)
Post by: RDV on November 27, 2005, 05:00:45 PM
I cranked back up and was blasting away and the chip shutdown on me(http://www.diyaudio.com/forums/images/smilies/hot.gif)(for a couple of seconds), the fan doesn't cut it at gig volume. But then R.G. said that didn't he? ::)

It will however work for the low-volume stuff I do at my desk.

I'm ordering a proper heatsink very soon though, and perhaps a chassis.

One thing I noticed this this time though is that there is a nice mild distortion that the limiter adds that is quite "cranked AC30-ish".

Nice.

RDV
Title: Re: For R.G. or any SS Vox Expert(Now with Samples!)
Post by: RDV on November 27, 2005, 05:19:19 PM
Talking about limiters. By accident I just came across another take at a soft limiter - simple and passive, and softer I think than the Vox.
http://www.dogstar.dantimax.dk/tubestuf/dzart-6.htm

Erik

That looks pretty cool. Have you tried it?

RDV
Title: Re: For R.G. or any SS Vox Expert(Now with Samples!)
Post by: R.G. on November 27, 2005, 07:21:30 PM
Quote
Talking about limiters. By accident I just came across another take at a soft limiter - simple and passive, and softer I think than the Vox.
Yep. That's one of the "death of the thousand diodes" limiters from the waveshaper school. They work GREAT if you have enough signal voltage to make the diode clamps negligable.
Quote
One thing I noticed this this time though is that there is a nice mild distortion that the limiter adds that is quite "cranked AC30-ish".
Yeah. That's what I like about it. It turns out that Dick Denney, the engineer half of Jennings Musical Instruments, made several trips to the Thomas Organ factory when Thomas was just starting up their solid state stuff, and advised them on some things. According to a published letter of his back to the boss in England, the TO guys had come up with a limiter that was quite pleasing in its action, and they were considering whether to keep it secret or patent it. After I read that, I walked over to the file cabinet and pulled out my file of TO schemos. There's only one limiter in there; all the 'big head' TV products have it, all the smaller ones do not.

I think it's a really nice way for a solid state amp to say "OK, that's about all I've got". Much more polite than a gauggghh-buzz like some.

You have yourself a pretty decent amp going there.

Go look at http://cgi.ebay.com/Aluminum-Heatsink-Extrusion-Sold-by-the-Foot_W0QQitemZ7565469920QQcategoryZ31489QQrdZ1QQcmdZViewItem and get yourself a foot or so, begging him to cut it into 3" long slices.

Title: Re: For R.G. or any SS Vox Expert(Now with Samples!)
Post by: RDV on November 27, 2005, 07:34:05 PM
I bought one of these (http://cgi.ebay.com/8x6x2-Aluminum-Extruded-Heat-Sink-For-Power-Amp_W0QQitemZ7566634470QQcategoryZ66946QQrdZ1QQcmdZViewItem) and I thought I'd cut it in half and make 2. There's really not too many fins now that I look at it though. What do you think?

RDV
Title: Re: For R.G. or any SS Vox Expert(Now with Samples!)
Post by: R.G. on November 27, 2005, 11:11:53 PM
I actually started to snag a couple of those myself. That'll certainly be better. Go for it!!
Title: Re: For R.G. or any SS Vox Expert(Now with Samples!)
Post by: RDV on November 28, 2005, 03:20:41 PM
I think this might be an ideal transformer for an LM3886 (http://www.partsexpress.com/pe/pshowdetl.cfm?&DID=7&Partnumber=122-615&ctab=2#Tabs), any thoughts? Perhaps the 250va one (http://www.partsexpress.com/pe/showdetl.cfm?&DID=7&Partnumber=122-625) instead?

RDV
Title: Re: For R.G. or any SS Vox Expert(Now with Samples!)
Post by: R.G. on November 28, 2005, 03:40:10 PM
Actually, that's good for about two of them.

22+22 volt secondaries is about right, total of 60Vdc, +/-30. That's close to the max you'd want to use with these chips. You should get 50Wrms out of the thing with +/-30 supplies. With 50W out, efficiency of about 79%, you need to supply 50/.79 = 63W of DC from a 60V supply. That's 0.95A of DC. A full wave rectifier makes the transformer see an RMS current of about 1.6 to 1.8 times the DC current, so the transformer is seeing maybe 1.7A. The VA rating is then 44*1.7 = 75Va.

What that tells you is that a 75VA transformer will run one of these things at a full power sine wave forever without overheating.

But music is not static. It has peaks and valleys, and transformers have enough mass to "average" the power demands in terms of their internal heating. Stereo transfomers are commonly sized to supply 1/2 to 2/3 of the continuous power rating, and they don't suffer much. You could probably get by with something under 75VA if you wanted to play brinksman.

Parts express used to sell a 22+22 EI core thing at about 80VA for $15. They were the best match to the LM3886 I ever found. I don't see them there any more.

Title: Re: For R.G. or any SS Vox Expert(Now with Samples!)
Post by: R.G. on November 28, 2005, 03:45:58 PM
Go look at part number 120-225. 48Vct, 3A, $23.
Title: Re: For R.G. or any SS Vox Expert(Now with Samples!)
Post by: Erik Lion on November 28, 2005, 05:07:05 PM
Hurraaa - succes at last. Today I pulled all components from my perfboard TV limiter project - and started all over again (for the 3th or 4th time). 15V DC supply for both the transistors and the diode bias trim pot. Omitted Q1 and biased Q2 with a pair of (390k/68k) resistors. Bingo - it works! Nice looking squased waveforms on my scope.
Haven't had the time to test how it sounds though - maybe tomorrow.

Erik
Title: Re: For R.G. or any SS Vox Expert(Now with Samples!)
Post by: R.G. on November 28, 2005, 05:36:29 PM
It's a little big and heavy, but it gives you other secondaries to run your preamp, and it's major cheap:

http://cgi.ebay.com/Transformer-18v-30v-48v-CT_W0QQitemZ3850112009QQcategoryZ4665QQssPageNameZWD1VQQrdZ1QQcmdZViewItem
Title: Re: For R.G. or any SS Vox Expert(Now with Samples!)
Post by: RDV on November 28, 2005, 09:18:30 PM
It's a little big and heavy, but it gives you other secondaries to run your preamp, and it's major cheap:

http://cgi.ebay.com/Transformer-18v-30v-48v-CT_W0QQitemZ3850112009QQcategoryZ4665QQssPageNameZWD1VQQrdZ1QQcmdZViewItem
I bought one of them, but probably should've bought both at that price.

Now I've got the heatsink for both and tranny for another amp for $26.00!!

Thanks R.G.!!

P.S. People must think I'm a little shy of a load getting excited about building SS amps.

RDV

Title: Re: For R.G. or any SS Vox Expert(Now with Samples!)
Post by: Threefish on November 28, 2005, 10:51:02 PM
No, you certainly aren't RDV. Here in Australia, tube amps are a very expensive hobby. I'm SS all the way. I've been watching this thread with great interest. I'm up to my ears in recording stuff at the moment, but if i ever (or should I say "when") I build another amp, I'll be referrring back to everything you and R.G. have generously shared with us in this thread. My current amp is an LM1875 (~18w) chip amp with a tweaked hifi-pentode-kit preamp thingy ("......Whaaaat?").  It's a little...er...off the beaten track as we say over here, but does it for me.
I hope I don't start a war here, but I just have to share with you a great graphic I saw in a Music Thing post :
http://musicthing.blogspot.com/2005/10/when-engineers-attack.html (http://musicthing.blogspot.com/2005/10/when-engineers-attack.html)
Title: Re: For R.G. or any SS Vox Expert(Now with Samples!)
Post by: RDV on November 30, 2005, 08:16:15 AM
The chipamp is now getting hot at idle now, so I'm afraid I've got some oscillation going on. I'm wondering(out loud) if I need to add a Zobel to the output and what values to use. I've been reading in the DIY chipamp sites about "snubberizing", but I can't say it's making any sense to me.

This all may just mean that the heatsink(being too small and fan not able to keep up) issue is what is going on. I'm waiting for one to come in and then we'll see. Hopefully with a proper heatsink, it'll idle cool at least.

I just never had the chip getting hot at idle or low volumes before.

RDV
Title: Re: For R.G. or any SS Vox Expert(Now with Samples!)
Post by: stm on November 30, 2005, 08:58:37 AM
Yes, it seem it might be self oscillating. Do you have the load (speaker) connected?  These things don't like to be unloaded.  Probably if you put a voltmeter set for AC voltage reading at the output you will be able to tell if the IC is actually oscillating.

The snubber thing are usually a small value resistor with a cap to ground loading the output, like 10 ohms 1/2W + 100nF or so.  Check the datasheet to see if they suggest using something like this.

Also, the oscillation may be due to the layout and/or wiring around. Try rearranging wires, keeping input signals and control pots away from the output, etc.

Regards.
Title: Re: For R.G. or any SS Vox Expert(Now with Samples!)
Post by: R.G. on November 30, 2005, 09:21:42 AM
Every solid state power amp needs a Zobel network.
Every solid state power amp needs inductive catch diodes.
Every solid state power amp needs a damped series inductor.
Every solid state power amp needs power supply decoupling right at the circuit.

These are things, just like the wrapper on an effect, that are not particularly interesting or exciting - until it doesn't work.

stm's rignt - ten ohms and 0.1uF is the standard Zobel, and it seems to work for almost all speakers. And that wire arrangement is important.

However, he's incorrect about loading. Solid state amps and tube amps have opposite preferences about loading. Solid state amps work fine into arbitrarily high load resistances, including no load resistance connected at all. They tend to have trouble with too-low load resistances, including dying. Tube amps get unstable and oscillate, sometimes to death, when they are unloaded, but cope with overloads (too-low a load resistance, right down to dead shorts) just fine. They're different. However the conventional wisdom about musical instrument amps is not to unload them. It's a legacy of the tubes-only era.



Title: Re: For R.G. or any SS Vox Expert(Now with Samples!)
Post by: RDV on November 30, 2005, 10:15:20 AM
I'm running a 4ohm load, but I have been all along.

I'm going to install a Zobel tonight, as I have a 2watt 10ohm resistor handy, it can't hurt, and hopefully will help.

What I don't get is that it had been idling cool and not getting really HOT unless fully cranked.

It happened last night as I was playing at low volume. I turned it off and felt the chip and it was very hot.

I know there's the negative supply running through the heatsink and all, but it's not touching anything.

I know I'll have to make sure the big heatsink I'm getting doesn't make contact with chassis ground.

RDV
Title: Re: For R.G. or any SS Vox Expert(Now with Samples!)
Post by: R.G. on November 30, 2005, 10:30:58 AM
Quote
I know there's the negative supply running through the heatsink and all, but it's not touching anything.
I know I'll have to make sure the big heatsink I'm getting doesn't make contact with chassis ground.
Do yourself a major favor - isolate the chip from the heatsink with a thermal insulator pad and thermal grease. Leaving heatsinks electrically hot is like leaving a rattlesnake inside your cabinet for the next time you open it up. You can get heatsink insulators at Mouser and Digikey. The Cho-therm and Sil-pad insulators are silicon rubber loaded with thermally conductive gook and work almost as well as mica and grease. You can also get the "TF" or "T" options on the LM3886 (I forget which it is) with the heatsink tab covered in epoxy so that an external insulator is not needed, only thermal grease.
Title: Re: For R.G. or any SS Vox Expert(Now with Samples!)
Post by: Steben on November 30, 2005, 11:06:25 AM
By the way that limiter with tubes: why use a limiter if you have tubes?...(for guitar purposes of course)
Title: Re: For R.G. or any SS Vox Expert(Now with Samples!)
Post by: RDV on November 30, 2005, 11:18:15 AM
I see why they offer an insulated version(LM3886TF) because a standard TO-220 sized insulator is gonna be way too small.

Guh.

RDV
Title: Re: For R.G. or any SS Vox Expert(Now with Samples!)
Post by: Caferacernoc on November 30, 2005, 12:19:20 PM
Hi, don't mean to hijack the thread. Just a quick question on these discussed limiters. I'm still at the 9 volt stompbox level but I'm interested in these soft diode limiters since my goal is always soft, amp like distortion. Anybody have a simple schematic of something similar scaled for a pedal? Thanks.
Title: Re: For R.G. or any SS Vox Expert(Now with Samples!)
Post by: RDV on November 30, 2005, 12:33:23 PM
I registered for a sample roll of Sil-Pad 1500ST.  :icon_wink: :icon_wink:

RDV
Title: Re: For R.G. or any SS Vox Expert(Now with Samples!)
Post by: R.G. on November 30, 2005, 12:52:02 PM
Quote
Anybody have a simple schematic of something similar scaled for a pedal?
That's a problem, and it is a problem of scaling, as you mention.

In a 9V stompbox, the signal levels are smaller than they are at power amp inputs by about a 3:1 to 10:1 ratio. Diodes are used for switching in the softening, gain changing resistors. But diode drops don't scale down, they're fixed by the physics of the semiconductor in the diode. So you can do a good job of soft clipping with a waveshaper approach like this on bigger signals, but it's much harder at stompbox levels. There are ways to do this, but they involve all the normal machinations of our attempts at stompboxes.

I have an affinity for very soft distortion too. One of the best simple ways is with a CMOS inverter distortion, like the "Hot Tubes" run at quite low gains.
Title: Re: For R.G. or any SS Vox Expert(Now with Samples!)
Post by: RDV on November 30, 2005, 07:12:52 PM
I got the heatsink in. My god this thing is HUGE. I can make 4 out of this at the LEAST. The backplate is thick and the fins are at least 1.75". I'll take it to work tomorrow and work it over with a sawzall.  :icon_twisted:

RDV
Title: Re: For R.G. or any SS Vox Expert(Now with Samples!)
Post by: RDV on November 30, 2005, 08:53:37 PM
Added Zobel, still getting hot at low volume. I think that the preamp/gainstage/limiter is zapping the little 300ma tranny that's running them cause the fan is not putting out much air at all. Heatsink next.

RDV
Title: Re: For R.G. or any SS Vox Expert(Now with Samples!)
Post by: R.G. on November 30, 2005, 11:31:25 PM
HUGE is a virtue for heatsinks  :)
Title: Re: For R.G. or any SS Vox Expert(Now with Samples!)
Post by: RDV on December 01, 2005, 07:57:11 AM
Now that I think about it, I believe Mouser made a mistake and sent me 100ma transformers instead of the 300s I ordered(about a year ago). I thought they were just mis-labeled cause they said 100FG on them, but no. I should've sent em' back when I got em'. That means my pedalboard supply is only 100ma too(though it's enough for the stuff on it).

Grrrr...

RDV
Title: Re: For R.G. or any SS Vox Expert(Now with Samples!)
Post by: RDV on December 01, 2005, 10:40:44 AM
Boy, it's hard to cut an 8" x 6" x 2"(fins) piece of extruded aluminum w/out a bandsaw!  :icon_razz:
I used a chopsaw instead and it worked, but made ugly cuts.

But I've got my heatsinks.

RDV
Title: Re: For R.G. or any SS Vox Expert(Now with Samples!)
Post by: RDV on December 01, 2005, 03:58:49 PM
Some more thinking out loud; I wonder if I should've used ceramic PS bypass caps(.1µF) on the rails(next to the chip) instead of the films I used. Perhaps that is causing high-freq oscillations resulting in chip running hot...

I may replace them tonight as I just happen to have a bunch of little blue ceramics that were being sold as films on eBay.

RDV
Title: Re: For R.G. or any SS Vox Expert(Now with Samples!)
Post by: RDV on December 01, 2005, 08:40:12 PM
Kids, I didn't design this with a 5 lb. heatsink in mind. I had to move the output, and the mute stuff is on yet another daughterboard, but it's on and it's not smoking. I'm gonna let it idle a while and see how warm it gets. I know it's quieter without that fan anyways. I cut a 3" X 4" piece out for it, and as heavy as this stuff is should be more than enough.

RDV
Title: Re: For R.G. or any SS Vox Expert(Now with Samples!)
Post by: RDV on December 01, 2005, 10:46:05 PM
I unhooked one of the 10"s raising the load to 8 ohms and it's running much cooler and quieter, and hasn't shut down since installing the bigger heatsink. Unless something else bad happens I'll let the thread rest till I start on the next amp. I'm going to buy PCBs(either from Chipamps.com or GGG) for this one and a power supply PCB from Chipamp. I've got a couple of the insulated versions(LM3886TF) coming and will be using those.

Late

RDV
Title: Re: For R.G. or any SS Vox Expert(Now with Samples!)
Post by: R.G. on December 02, 2005, 11:05:09 AM
I think it may simply have been going into thermal shutdown. 4 ohms loads are hard on many chip amps.

Here's a thought - what happens if you feel rich and decide to put ...two... LM3886's in there, one for each speaker? This would cost you another $5.00.

If your power supply can take it, you now have a double-powered amp and neither one gets too hot. The switch to half-power is as simple as pulling the mute on one amp.

I have this recurring fantasy about building modular amps; modules of one enclosure/one speaker/one power supply driven by whatever drives the amp. Repairs are easy - just grab another one - and you can use as much or as little power as you want.
Title: Re: For R.G. or any SS Vox Expert(Now with Samples!)
Post by: RDV on December 02, 2005, 11:13:55 AM
I think it may simply have been going into thermal shutdown. 4 ohms loads are hard on many chip amps.

Here's a thought - what happens if you feel rich and decide to put ...two... LM3886's in there, one for each speaker? This would cost you another $5.00.

If your power supply can take it, you now have a double-powered amp and neither one gets too hot. The switch to half-power is as simple as pulling the mute on one amp.

I have this recurring fantasy about building modular amps; modules of one enclosure/one speaker/one power supply driven by whatever drives the amp. Repairs are easy - just grab another one - and you can use as much or as little power as you want.
That's sort of what I was planning for the next one as I've ordered a couple more chips. I may splurge on that 160va toroid to do that. Sort of a dual mono/stereo thing. I left half of the heatsink I bought intact for such a purpose.

Kinda fun in'nit?

RDV
Title: Re: For R.G. or any SS Vox Expert(Now with Samples!)
Post by: RDV on December 02, 2005, 11:18:27 AM
Quote from: R.G.
I have this recurring fantasy about building modular amps; modules of one enclosure/one speaker/one power supply driven by whatever drives the amp. Repairs are easy - just grab another one - and you can use as much or as little power as you want.
I wonder if any commercial versions of this exist?

Or at least, is anyone using LM3886s in a currently produced guitar amp?

Hmm... ???

RDV
Title: Re: For R.G. or any SS Vox Expert(Now with Samples!)
Post by: Johan on December 02, 2005, 11:20:08 AM


Or at least, is anyone using LM3886s in a currently produced guitar amp?

Hmm... ???

RDV

..Line-6 uses them extencivly....
johan
Title: Re: For R.G. or any SS Vox Expert(Now with Samples!)
Post by: RDV on December 02, 2005, 11:46:11 AM
..Line-6 uses them extencivly....

I wonder how they're getting 75 watts out of a 68 watt chipamp?  :icon_wink: :icon_wink:

RDV
Title: Re: For R.G. or any SS Vox Expert(Now with Samples!)
Post by: R.G. on December 02, 2005, 11:53:29 AM
Quote
I wonder how they're getting 75 watts out of a 68 watt chipamp?
Oh, that's simple. Advertising watts is smaller than real watts.

Quote
I wonder if any commercial versions of this exist?
Not in that variation. I did find a wall o' modules at one NAMM. They were a different variation, though, each with a preamp itself.
Title: Re: For R.G. or any SS Vox Expert(Now with Samples!)
Post by: RDV on December 03, 2005, 01:46:19 AM
Yes I know, this thread is probably starting to get old now, but I had to post this.

I decided to drag my little 112 open-back cabinet with a Peavey 8 ohm Black Widow out and see how it sounds with the amp and it sounds real, real good. Huge in fact.

Better still, I turned off the amp and felt the heat sink and it was barely warm at all. ??? ???
It had been BLAZING a few minutes earlier through one 10"at very low volume.
The little 10"s in the cab I've been using must be VERY inefficient, or even half-blown.

Jeez! All that worrying about the amp, and the problem was the damn speakers.

Guh.

RDV

Edit: I wonder if that's what happened to the amp part of the 210 cab(It was a Peavey Stereo Chorus 210)?
Title: Re: For R.G. or any SS Vox Expert(Now with Samples!)
Post by: petemoore on December 03, 2005, 02:09:43 AM
  The 2x 10's were putting an 8ohm load on the amp?
  Half blown I would think could put some hefty 'spikes' as far as ac resistance on the output,  [especially if continuity is intermittently lost during cone movement] ,
which wouldn't show with DC testing, . Something like that might warm the heat sink  :icon_twisted:. 
Title: Re: For R.G. or any SS Vox Expert(Now with Samples!)
Post by: RDV on December 03, 2005, 02:14:45 AM
  The 2x 10's were putting an 8ohm load on the amp?
  Half blown I would think could put some hefty 'spikes' as far as ac resistance on the output,  [especially if continuity is intermittently lost during cone movement] ,
which wouldn't show with DC testing, . Something like that might warm the heat sink  :icon_twisted:. 
In parallel they were 4 ohms, and seemed to check OK. But the amp would run just as hot with only one of em' at 8 ohms.

RDV
Title: Re: For R.G. or any SS Vox Expert(Now with Samples!)
Post by: petemoore on December 05, 2005, 01:34:44 PM
  I finally got back to, and finished debugging and trying this thing out.
  The Vox Preamp with inductor...cool sound...
  Does a good job adding character to the sound.
  A little bit 'wah' tonish, the inductor can be heard, as can the transistors, of which I found I'm enjoying a lower gain Ge for Q1, and a lower gain Si for Q2.
  "Clear Bright' is the standout tone I'm hearing, this is with treble up some/some more.
  It certainly reminds me of the 'Vox Beatle' .. Thomas Organ SS amp I had for a time.
  I can see this as a way to get cleanish highly characterized tone that would effortlessly cut through a stage mix. Careful with the cutting edge though, as it can be quite sharp.
  Dialing back the guitar volume...Big Change in tone, rolls off highs, but not to dullness right away, between say 6 and 10, a pretty radical shift in frequency responses and amount of mid hump resonance can be 'mixed in'...ie higher GVolume levels and you really hear the inductor inducting, lower settings sound more...normal.
  I think it's the perfect place for my 'spare' inductor.
  Kind of turns the guitar, and amp into like a whole nuther horseteam...HB's sounded more like SC's, and tube amp sounded like a 'brightly colored SS...
Title: Re: For R.G. or any SS Vox Expert(Now with Samples!)
Post by: RDV on December 05, 2005, 03:01:47 PM
Yeah Pete, ya have to watch the treble on it or the blood will pour from your ears!(http://www.tributecity.com/forums/images/smiles/tomato.gif)

I wanted to ask everyone in general their opinions on aluminum chassis versus steel. There's not a real big price difference. I'm thinking of putting together another amp together and mounting it in a chassis in my little Peavey 210 Chorus cabinet. I'll have a version of the Vox pre in there, but there'll be preamp out/poweramp in jacks also.

The amp I've built so far is in an old(like 20+ years) Dean Markley K-20 combo amp cabinet(w/8" speaker long gone), with the preamp and controls up in the little chassis, and the powersupplys/poweramp in the bottom. The K-20 had been long burned out, though I ordered the TIP-120 and Tip-125 that power the amp today and will be restoring it.

RDV
Title: Re: For R.G. or any SS Vox Expert(Now with Samples!)
Post by: didier on December 06, 2005, 11:48:01 AM
boy,

i would be so happy if i made my own working amp...

good to hear this (simple looking) amp sounds so great!
i'll save the schematics maybe for later use...

keep on rocking  :icon_cool:
Title: Re: For R.G. or any SS Vox Expert(Now with Samples!)
Post by: RDV on December 07, 2005, 06:51:24 PM
I just bought these (http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&item=7564192947) with which I'm going to build a dual mono version. I got a deal. I'll be able to incorporate these into the structure of a head of sorts(as sides with a chip per side). I'm gonna make this one be a slave in stereo or dual mono. It's gonna kick a$$!! It'll be a 'swiss army knife' of amps, being able to carry music or the noise I generate!

RDV

Title: Re: For R.G. or any SS Vox Expert(Now with Samples!)
Post by: RDV on December 18, 2005, 02:24:33 AM
Hi again:

I finally worked out the last bit of hum in my TV/LM3886 amp. I had the mains ground hooked to the CT on the transformer(rather than to the star ground). Duh.

Was reading over at ESP and saw this which told the tale. (http://sound.westhost.com/psw-f2.png)

Going to start a Dual Mono LM3886 amp very soon(waiting on PCBs). I'm gonna have a loud computer!

RDV
Title: Re: For R.G. or any SS Vox Expert(Now with Samples!)
Post by: RDV on December 21, 2005, 05:18:43 PM
I'm wondering(out loud of course) what values I might tweak in this preamp to reduce the extreme treble I'm getting with me strat. The Les Pauls sound peachy through it, but I'd sort of like to rig up a bright/mellow switch. Perhaps the .0047µF or .033µF caps before the volume pot? If anyone has suggestions I'm all ears!

Thanks

RDV
Title: Re: For R.G. or any SS Vox Expert(Now with Samples!)
Post by: RDV on December 21, 2005, 09:23:56 PM
I just had a strange experience with the LM3886. In "Gainclone" mode(straight into the poweramp) I just played a good 30 minutes of blasting music from Mp3. It is incredible as a hifi stereo amp. It blew away my old Peavey Monitor head(which is supposed to be 130 watts... yeah right!). I know what those guys on the gainclone web-ring are talking about. These amps sound really good. An application for us here on this forum would certainly be as a studio monitor amp.

I'm building as stereo one as soon as a few more parts come in. I'm thinking of making a switchable input between a guitar preamp or line ins for the computer playback.

Uhh... I gotta find some speakers. :icon_wink:

RDV
Title: Re: For R.G. or any SS Vox Expert(Now with Samples!)
Post by: RDV on December 21, 2005, 09:37:30 PM
I forgot to say that with prerecorded music(bypassing the TV Preamp/Limiter) this amp doesn't even get hot, barely warm even. With the guitar amp thing going it heats up almost instantly. I wonder if I should cap couple the input of the poweramp section?

RDV
Title: Re: For R.G. or any SS Vox Expert(Now with Samples!)
Post by: R.G. on December 21, 2005, 10:19:36 PM
Quote
with prerecorded music(bypassing the TV Preamp/Limiter) this amp doesn't even get hot, barely warm even. With the guitar amp thing going it heats up almost instantly. I wonder if I should cap couple the input of the poweramp section?
It's always a good idea to cap couple if you can to prevent rogue DC from getting in.

I'm not too surprised that it doesn't warm up on prerecorded music. There's a generally accepted rule that prerecorded music has a crest factor (peak power to average power) of about 20db. The peaks are 20db larger than the average level. So if you play it cleanly, the average level barely warms the heatsinks. Not so for guitar by itself, especially with a square wave fuzz going into it. The worst thing you can to to your amp is not to play full-bore sine waves or music through it. The worst thing you can do is to play half-maximum swing square waves. That means that each output device is alternately handling half the power supply voltage and half the full current. That's the peak power dissipation point in the output transistors. Playing louder square waves actually lets them cool off some from there.

... of course, you could have some DC leaking in too...

Title: Re: For R.G. or any SS Vox Expert(Now with Samples!)
Post by: RDV on December 24, 2005, 04:27:57 PM
Big thing. Huge heatsink for guitar amp. Smaller for stereo amp. I'm not sure at this point that these chipamps are too great for guitar. They get so hot.

Still Working.

RDV
Title: Re: For R.G. or any SS Vox Expert(Now with Samples!)
Post by: RDV on December 24, 2005, 07:17:58 PM
Changed to huge heatsink. Still gets hot and shuts down. At a stinking loss. Must've done something wrong somewhere but don't know what in the world it could be. I hope my PCB versions turn out better than this perf one, it just gets worse and worse. Don't get me wrong, it sounds good but heats up and shuts down in 5 to 15 minutes.

Arg.

RDV
Title: Re: For R.G. or any SS Vox Expert(Now with Samples!)
Post by: RDV on December 24, 2005, 08:45:40 PM
DEAD CHIPAMP WALKING. BIG FLASH, LOUD CRACK.

AMP GO BOOM.

RDV
Title: Re: For R.G. or any SS Vox Expert(Now with Samples!)
Post by: B Tremblay on December 24, 2005, 08:56:29 PM
Loud crack, flash! - it's a gas, gas, gas!

Seriously though, I'm sorry to hear that happened.  I've been following your adventures with great interest.
Title: Re: For R.G. or any SS Vox Expert(Now with Samples!)
Post by: RDV on December 24, 2005, 09:47:10 PM
Oh man, just when I thought I had the overheating thing licked the little daughter-board with the mute resistor & cap made some solid contact with one of the rails and ccccrrraaaaaccckk!!!! with the accompanying requisite blue flash(at the chip). I tried another fuse before I figured out the deal. I think the chip's DOA but I'll have to wait till the stores open back up to find out. I sure as hell ain't sticking no tinfoil in there cause I'd have to take it out of my tinfoil hat! :icon_wink: :icon_wink:

RDV
Title: Re: For R.G. or any SS Vox Expert(Now with Samples!)
Post by: RDV on December 25, 2005, 01:10:42 PM
The chip survived but still gets hot as nuts even with a huge heatsink. I don't get it because when I first built this thing it hardly got hot at all. I just don't know what went wrong. All I can figure at all is when I had lots of hum, it ran cool, but when I established a star ground it got quieter but started getting hot. Perhaps I grounded something that shouldn't have been.

RDV
Title: Re: For R.G. or any SS Vox Expert(Now with Samples!)
Post by: R.G. on December 25, 2005, 04:37:33 PM
I think you're having a wiring problem. The tab of the LM3886 is not connected to ground, it's connected to V-. If you accidentally connect the tab to ground, you might have a similar problem.

Check that out.
Title: Re: For R.G. or any SS Vox Expert(Now with Samples!)
Post by: RDV on December 25, 2005, 04:56:15 PM
I going to check the PS wiring right now. I'm not sure what you mean by the tab, do you mean the outside casing? That's at the - supply by default. I'm not the sharpest tool in the shed.

RDV
Title: Re: For R.G. or any SS Vox Expert(Now with Samples!)
Post by: RDV on December 25, 2005, 04:57:47 PM
I going to check the PS wiring right now. I'm not sure what you mean by the tab, do you mean the outside casing? That's at the - supply by default. I'm not the sharpest tool in the shed.

RDV

Oh jeez, I get it.
To much Xmas dinner. :P

RDV
Title: Re: For R.G. or any SS Vox Expert(Now with Samples!)
Post by: RDV on December 25, 2005, 05:11:55 PM
Everything's looking alright voltage-wise. I'm using one of my ebay aquired big-ass transformers with all the secondaries. I'm using the 48vct@2A to run the amp part and the 18vct@500ma to run the preamp part. I'm getting 32.5v +/- after rectification.

One thing I did notice is that I'm using an underated cap(25v) at Cm(mute) which is connected to the negative supply to ground. I'm going to replace that now and do another heat test and see what happens.

Doing this stuff has helped me through a season I usually detest. I gave my wife a Taylor Baby and she cried tears of joy. It was pretty nice as I'm usually Scrooge-city. Maybe I'm more Grinch. :icon_wink: :icon_wink:

RDV
Title: Re: For R.G. or any SS Vox Expert(Now with Samples!)
Post by: RDV on December 25, 2005, 06:01:29 PM
I added the mute circuitry to the main perf with the proper value cap. It didn't help.

After it got hot this time I checked the rail voltages and they had sagged from 32.5v to 27.3v, so I'm started to suspect my rectifier. I think I now know why the chipamp(gainclone) guys seem to really overdo it on the rectifiers. They don't use bridge rectifiers but make their own with separate MUR860 diodes. I used a 6 amp bridge rectifier which I thought would be enough, but I'm not too sure now.

I repeat. ARG.

RDV
Title: Re: For R.G. or any SS Vox Expert(Now with Samples!)
Post by: R.G. on December 26, 2005, 06:18:20 PM
Quote
After it got hot this time I checked the rail voltages and they had sagged from 32.5v to 27.3v, so I'm started to suspect my rectifier. I think I now know why the chipamp(gainclone) guys seem to really overdo it on the rectifiers. They don't use bridge rectifiers but make their own with separate MUR860 diodes. I used a 6 amp bridge rectifier which I thought would be enough, but I'm not too sure now.
Hmmm... a 6A rectifier bridge should be plenty. Six amps at 65V is about 390W, so you're well under the bridge's abilities. If both rail voltages sag to 27.3 and stay there, you're probably oscillating.
Title: Re: For R.G. or any SS Vox Expert(Now with Samples!)
Post by: RDV on December 26, 2005, 06:22:13 PM
I'm suspecting this is a layout problem, resulting in oscillation. I'm not sure but I'm going to let it set a while and finish the ones I'm building with PCB(waiting on tranny).

RDV
Title: Re: For R.G. or any SS Vox Expert(Now with Samples!)
Post by: RDV on December 27, 2005, 12:14:51 AM
I deep-sixed the original chipamp and built up a couple of PCB chipamps. I bought a dual-mono set which is enough for two complete amp and PS sets. This is the only way to go. I went back to the smaller heatsink(1/4 of a really big heavy one) w/out a problem(barely warm). Sound is very sweet. One of the best clean sounds I've heard along with the TV preamp.

Me happy.

RDV

PS: I do not recommend perfing a chipamp. Too many ways to screw it up and make it oscillate. YMMV of course.
PSPS: You can perf these, but you must keep the bypass caps and FB resistor VERY close to the chip(if not soldered directly to the legs of the chip!
Title: Re: For R.G. or any SS Vox Expert(Now with Samples!)
Post by: RDV on December 29, 2005, 09:54:00 AM
A couple of real-world observations about LM3886 chipamps.

1. The chip works great for 8 ohm loads and lousy for 4 ohm loads. There's just no other way to say it. Even if you use a lower voltage transformer(which is reccomended for 4 ohm loads) it'll still run hot and possibly ruin the chip(it happened to me).
EDIT: #1 is wrong, it was a careless perf layout and less than stellar parts choices. With the right heatsink, 4 ohms is no problem unless you use a way to high voltage transformer.

2. A good PCB layout(such as the one at GGG) is essential to prevent ground loops and fatal oscillations. I'd reccomend a PCB for the rectifier/filter section also.

3. It's a hifi amp, not a guitar amp, which is why the modeling amp companies are using them. What you put in them signal-wise is exactly what you'll get out of them. I'm having good luck using a 6-band graphic EQ pedal to make various distortions work well with it. That said: It's got to be one of the ultimate warm clean amps ever.

4. The SPIKe protection built into the chip is a bummer. When you're really loud with it and whack a chord with a HB PU it WILL activate and cut out the sound(completely) during the initial attack. There'll be no nasty clipping alright, just no sound. Not good for an electric guitar amp IMHO.
Edit: Number 4 is wrong because the limiter was causing these problems.

5. They're still pretty cool though. :icon_wink: :icon_wink:

RDV
Title: Re: For R.G. or any SS Vox Expert(Now with Samples!)
Post by: RDV on January 06, 2006, 12:58:55 AM
New Chapter in the never ending saga of trying to semi-design an SS amp.

I've never given up on the original chipamp I perfed in the beginning of this. It was heating up and shutting off no matter how much heatsink or what size load I put on it.

The last couple of things I tried seemed to do the trick. I replaced the polys(.1F) on the PS bypass with monolithic ceramic and moved them right by the chip. I changed the zobel to a 2.7 ohm and another monolithic ceramic. I had removed the 10k pot early on so in case that was the problem I installed a 22k from input to ground. The feedback resistor it seems was hanging by a thread and broke off when I touched it. EEK. Replaced that also and fired it up without the preamp and it didn't get hot and sounded okay too. Cool. I've spent about 2 weeks trying to figure this out.

I used one of the eBay multi-tap transformers R.G. pointed me to so I didn't have to run a second tranny for the preamp. The only problem with that was I was using a regulator I'd fixed up for a wall-wart which didn't work out too well with the 25v I was getting from the 18vct after rectification. I think it was somehow limiting the current a bit and only allowing about 14v max. I took another LM317T and just built the standard regulator circuit from the datasheet which allows me a range from a few volts all the way up to max(about25v). I set it to 18v like the Thomas/Vox schem said and all is good. Now the limiter is really doing it's thing and acts like a soft clipping distortion control even at lower volume if the control is cranked.

Woo Hoo, on to the stereo one!

RDV
Title: Re: For R.G. or any SS Vox Expert(Now with Samples!)
Post by: nelson on January 06, 2006, 07:24:03 AM
RDV

You have inspired me to build a solid state bass amp. I have always put it off waiting to gather the money and transformers for a tube bass amp. Reason being I have disliked how solid state bass amps sound when pushed presumably because the power amp IC is hard clipping (only tried cheap ones).

Instead, I am going to use this handy little limiter circuit, make a jfet emulation of the tube bass amp I was going to build and use an LM3875 (single supply) power amp IC.

Thanks for the inspiration.

I think this will also give me a little more experience in amp building before I tackle my champ project, the parts have been in my possession for a while, I just dont feel ready to tackle it yet.

Title: Re: For R.G. or any SS Vox Expert(Now with Samples!)
Post by: PenPen on January 06, 2006, 10:19:39 AM

I just wanted to chime in here, please keep updating this thread, RDV. I read this thread every time you update it, there is a wealth of info here!

I really want to try making a hybrid now, and your reports on what worked and what didn't really does help prepare one for the common problems in SS amp development. I'm planning on building with that new LM4651 chip as the power stage, hopefully if I run it at around 80W I can avoid some of the heat issues you are encountering.

Great work on the current build btw!
Title: Re: For R.G. or any SS Vox Expert(Now with Samples!)
Post by: Eric H on January 06, 2006, 12:02:16 PM

I just wanted to chime in here, please keep updating this thread, RDV. I read this thread every time you update it, there is a wealth of info here!


I'll second that, Ricky --great stuff.

-Eric
Title: Re: For R.G. or any SS Vox Expert(Now with Samples!)
Post by: RDV on January 07, 2006, 04:14:01 PM
Well thanks guys, here's some more.

I hooked an LED across the power rail to the limiter and discovered some things. It dimmed a great deal when I would hit a chord, and if I hit it too hard it would completely blank out the sound, which to me makes it useless, so I've unhooked it. I'm taking a guess that there's supposed to be a higher voltage going to the limiter pot than to the rest of the circuit, which I did not have. I may not fool with it any more since I like the amp fine with no limiter. If anyone has tried the circuit and gotten better results please let me know, but for now it just removes all the clean headroom from this amp(and there's a bunch!). The only breakup I'm hearing without the limiter is the one little 10" woofer I'm running it through crying for help!

RDV
Title: Re: For R.G. or any SS Vox Expert(Now with Samples!)
Post by: RDV on January 07, 2006, 08:31:48 PM
With all going well I've hooked up the other speaker again for a 4 ohm load, and it's running cool as a cucumber as they say.

Can't truly say how good it sounds because I've had a TERRIBLE head cold since the 1st. Can't hear crap, sounds like I'm underwater. Starting the year feeling like doodoo was NOT one of my resolutions!!

SWEET.

RDV
Title: Re: For R.G. or any SS Vox Expert(Now with Samples!)
Post by: Joecool85 on January 08, 2006, 12:01:32 AM
Did you build your lm3886 to match the GGG schematic or did you just do it exactly the way the chipamp.com stuff is set up?
Title: Re: For R.G. or any SS Vox Expert(Now with Samples!)
Post by: RDV on January 08, 2006, 09:58:56 AM
Did you build your lm3886 to match the GGG schematic or did you just do it exactly the way the chipamp.com stuff is set up?
I followed the GGG schem, them started experimenting using a bunch of various schems as guides. The way my perfed one stands right now is kind of a combo of GGG's and chipamp's along with a PS borrowed from Rod Elliott (http://sound.westhost.com/). A good place to look as they have a chipamp forum is DIYAudio.com (http://www.diyaudio.com/)

RDV
Title: Re: For R.G. or any SS Vox Expert(Now with Samples!)
Post by: RDV on January 08, 2006, 02:08:09 PM
Just did some serious blasting with the amp on 5 with the MRB off and my RDV pedal into a 6 band graphic with an abusive setting: 100hz @ +10db, 200hz @ +5db, 400hz @ flat, 800hz @ +15db, 1.6khz @ flat, and 3.2khz @ +15db.

My ears are smoking and bleeding, but there was no nasty SS distortion at all, these amps have a lot of headroom!!

This was with the 4 ohm load and the heatsink did get quite hot, but did not trigger the thermal shutdown. This was louder than my Marshall! I'd suggest a really big heatsink for 4 ohm loads, but what I'd really suggest is keeping it at 8 ohms and not having to worry about it. If you can wire up an 8 ohm 412 cab this thing will light it up with the right pedals in front of it.

Instead of shutting the amp down I'm letting it idle and it's cooling down nicely. I may have to buy a nice strong 12" 8 ohm speaker for this one. I've got some Peavey Black Widow 12"s, but they're too strong, you know, icepick in your ear style.

I'm building a stereo pair, so I'm not gonna have to worry much about them getting hot as I'm gonna run 8 ohm loads with all 3!

Wish my hearing luck!

RDV
Title: Re: For R.G. or any SS Vox Expert(Now with Samples!)
Post by: petemoore on January 08, 2006, 06:13:17 PM
  Well you've got me interested too now..
  But first I'd like to price and source the chip, I've tried N.Semiconductor [data sheet there] and a host of other places, but haven't found a price or availably chip without 99 of it's buddies 'tagging along' [min 100 order].
Title: Re: For R.G. or any SS Vox Expert(Now with Samples!)
Post by: PenPen on January 08, 2006, 06:27:07 PM
  Well you've got me interested too now..
  But first I'd like to price and source the chip, I've tried N.Semiconductor [data sheet there] and a host of other places, but haven't found a price or availably chip without 99 of it's buddies 'tagging along' [min 100 order].

Check Digikey, Futurlec, and Bdent.com. They have this chip without the minimum order crap.
Title: Re: For R.G. or any SS Vox Expert(Now with Samples!)
Post by: RDV on January 08, 2006, 06:33:20 PM
http://www.partsexpress.com/pe/showdetl.cfm?&DID=7&Partnumber=LM3886TF (http://www.partsexpress.com/pe/showdetl.cfm?&DID=7&Partnumber=LM3886TF)

RDV
Title: Re: For R.G. or any SS Vox Expert(Now with Samples!)
Post by: petemoore on January 08, 2006, 07:20:08 PM
  Whew, that's Steep !!!  :icon_razz:
  inexpensive clean power in seems. Very hard to resisto.
Title: Re: For R.G. or any SS Vox Expert(Now with Samples!)
Post by: RDV on January 08, 2006, 07:23:32 PM
  Whew, that's Steep !!!  :icon_razz:
  inexpensive clean power in seems. Very hard to resisto.
Oh mama me-o mama me-oh!!

Couldn't resist-o

RDV
Title: Re: For R.G. or any SS Vox Expert(Now with Samples!)
Post by: RDV on January 14, 2006, 07:25:39 PM
Just finished my LM3886 stereo version, and fired right up. No smoke. A month of planning and work finally coming together perfectly.

A thank you to Brian at chipamps.com for the PCBs. They were brilliant!

Pictures forthcoming.

...but. What do I do now?

RDV
Title: Re: For R.G. or any SS Vox Expert(Now with Samples!)
Post by: R.G. on January 14, 2006, 10:16:45 PM
The LM3886TF is $5.99 each at Digikey in ones.
Title: Re: For R.G. or any SS Vox Expert(Now with Samples!)
Post by: RDV on January 14, 2006, 10:24:05 PM
...test the damned thing, that's what!

I ran the summed output of a Sony Discman into one channel, the output of the Thomas/Vox preamp into the other side and played Won't Get Fooled Again and BaBa O'Reilly using the backing tracks I purchased from a nice gentleman from the north of England. Cranked. With the top cover on the amp.

I opened it immediately afterwards to check heatsink warmth and the results were barely. Warm, that is. The side running the backing tracks was considerably warm-er, but still nowhere near hot, and that's with a 5-piece band playing through it!

Long Live Rock!!(I need it every night)

RDV

Title: Re: For R.G. or any SS Vox Expert(Now with Samples!)
Post by: RDV on January 14, 2006, 10:55:25 PM
...did it again, but twice as loud, at least loud enough for a restaurant/lounge gig(that would tolerate the Who). The bass was vibrating my guts if you know what I mean. I've gotta stop before the po-lice come to shut down the band(tee hee). Took the cover off again and the backup track channel heatsink was hot but nowhere near shutdown hot. The guitar side was warm, but not hot. If I were to use it gigging, I'd use both channels for backing tracks and vocals only, so I think it would be no problem. At worst, I've got a serious fan if needed that runs off straight mains power.

RDV
Title: Re: For R.G. or any SS Vox Expert(Now with Samples!)
Post by: RDV on January 15, 2006, 05:43:25 PM
Okay, okay... back to the limiter. I hope you're all able to follow along through the distractions. This thread is similar to my life! :icon_wink: :icon_wink:

I decided to give the limiter another go, taking the 25vdc coming off my 18vac rectified by way of a 1000F cap. This voltage goes to the limiter pot.

This changes the ballgame considerably. Now the formerly treble-heavy amp becomes a bass-heavy amp that's about to kill the 12" I've got it hooked to. I don't know why, perhaps the .001F to ground at the input of the limiter circuit does take some treble out. I think I've got to replace the linear bass pot with an audio which should help a little.

No longer does the output squash out all the way anymore which I do believe means that's what the problem was(the voltage differential that is).

I hope I've made myself unclear. :icon_razz:

The jury is totally still out on this limiter because when you turn the thing all the way down so you can hopefully get your clean headroom back it starts an audible hum. I probably need to isolate that 25v somehow more from the regulated 18v that comes after it. I don't have a clue how to do this or if it can be done.

I'LL BE BACK.

My wife's out of town this weekend which is why I'm so full time into this.

RDV
Title: Re: For R.G. or any SS Vox Expert(Now with Samples!)
Post by: RDV on January 15, 2006, 06:46:29 PM
Sigh.

The clipping of the limiter is quite harsh.

I need a drink.

+

RDV
Title: Re: For R.G. or any SS Vox Expert(Now with Samples!)
Post by: Joecool85 on January 15, 2006, 09:30:21 PM
You should just do a different preamp/eq and be rid of the limiter.
Title: Re: For R.G. or any SS Vox Expert(Now with Samples!)
Post by: PenPen on January 15, 2006, 09:33:03 PM

No, keep the preamp. That thing sounds great. Honestly, if you aren't clipping the power stage, I say screw the limiter.
Title: Re: For R.G. or any SS Vox Expert(Now with Samples!)
Post by: Threefish on January 15, 2006, 09:34:34 PM
Darn. Don't you hate that. You've got it working and ....
....this isn't what I wanted!

Ditch the preamp/eq just because of the limiter? After all this work?? No way!!!
The setup without limiter sounds great doesn't it? The samples sound great to me. I can totally understand the challenge, but do you really need the limiter?

This is a great thread RDV. Thanks for the regular updates. I'm sure I'm one of the many following this with great interest.
Keep at it.
Title: Re: For R.G. or any SS Vox Expert(Now with Samples!)
Post by: RDV on January 15, 2006, 10:43:36 PM
Thanks guys.

I've reached another of those 'let it set a few days and think about it' periods. I'm thinking of building a cab sim in place of the limiter. I want something to tame a bit of that SS skronk.

I shall return.

RDV
Title: Re: For R.G. or any SS Vox Expert(Now with Samples!)
Post by: R.G. on January 15, 2006, 11:02:43 PM
The Achilles heel of the LM3886 is that it sounds ...really... bad if you ever trip its protective circuits by overdriving it. Practically any limiter you put in front of it to stop it from doing its protective dance will sound better than listening to it save itself. The Thomas Vox limiter isn't the final word on limiters by any means, but do use something other than the innate overload of the LM3886. Scale the input to three or four diodes or something.
Title: Re: For R.G. or any SS Vox Expert(Now with Samples!)
Post by: RDV on January 16, 2006, 12:14:29 AM
The Achilles heel of the LM3886 is that it sounds ...really... bad if you ever trip its protective circuits by overdriving it. Practically any limiter you put in front of it to stop it from doing its protective dance will sound better than listening to it save itself. The Thomas Vox limiter isn't the final word on limiters by any means, but do use something other than the innate overload of the LM3886. Scale the input to three or four diodes or something.
If I could somehow not get that rather harsh clipping that's occurring WAY before the LM3886 would even consider clipping, it wouldn't be a problem.

I haven't scrapped it yet. I may try a bit lower voltage on the preamp while still maintaining the 24.1v I've got across the limiter pot. It may or may not have an effect.

I also don't know if I should have gain matched the two transistors in the limiter. Another thought is I used an MPSA18 in the gain stage before the limiter which may have been a poor choice considering everything else is 2N3904 or 6.

This is pretty new territory for me, and I'm not a smart(Re: Educated) man. I'm curious like a cat, or is that stupid?

I'm going to listen with fresher ears tomorrow.

RDV
Title: Re: For R.G. or any SS Vox Expert(Now with Samples!)
Post by: RDV on January 16, 2006, 01:47:47 PM
Hi:

I reduced the preamp voltage to 15v and may go lower. It seems to decrease the harshness of the clipping.

Further testing is needed.

Oh, and I did have a drink.

Vodka.

Absolute.

RDV
Title: Re: For R.G. or any SS Vox Expert(Now with Samples!)
Post by: PenPen on January 16, 2006, 02:29:52 PM

Oh, and I did have a drink.

Vodka.

Absolute.

RDV

I prefer Grey Goose.

Vodka tonic always gets the brain going for me too ;)
Title: Re: For R.G. or any SS Vox Expert(Now with Samples!)
Post by: RDV on January 18, 2006, 04:07:39 PM
LM3886 @ Futurlec for cheap (http://www.futurlec.com/Linear/LM3886Tpr.shtml)

Not the insulated version though. :(

RDV
Title: Re: For R.G. or any SS Vox Expert(Now with Samples!)
Post by: Joecool85 on January 18, 2006, 06:43:16 PM
I'll pay the extra couple bucks for the TF. 

Can we have some pics of your amps?   ;D
Title: Re: For R.G. or any SS Vox Expert(Now with Samples!)
Post by: RDV on January 18, 2006, 06:55:12 PM
I'll pay the extra couple bucks for the TF. 

Can we have some pics of your amps?   ;D
I keep forgetting to bring home a camera from work. They're really nice digital Kodaks and better still, I know how to use them. :icon_redface:

RDV
Title: Re: For R.G. or any SS Vox Expert(Now with Samples!)
Post by: Joecool85 on January 18, 2006, 07:39:56 PM
It's cool.  I just wanna see.  It sounds to me like you got some nice stuff over there  8)
Title: Re: For R.G. or any SS Vox Expert(Now with Samples!)
Post by: RDV on January 19, 2006, 10:29:16 PM
Here we go. 12 hour work day, another one tomorrow, vodka for supper, crappy camera, everybody yelling at me. Wife crazy.
Enjoy Pictures.

Thomas/VoxDean Markeley & Stereo Amp & Mackie recovered from Dumpster.
(http://img.photobucket.com/albums/v165/rickydon/Amp1.jpg)


Insides of TV/Vox/Chipamp showing poweramp.
(http://img.photobucket.com/albums/v165/rickydon/Amp4.jpg)

Inside of stereo amp.
(http://img.photobucket.com/albums/v165/rickydon/Amp8.jpg)


Blurry pic.
(http://img.photobucket.com/albums/v165/rickydon/Amp10.jpg)


...and another.
(http://img.photobucket.com/albums/v165/rickydon/Amp6.jpg)

RDV
Title: Re: For R.G. or any SS Vox Expert(Now with Samples!)
Post by: RDV on January 19, 2006, 10:53:37 PM
The controls on the guitar amp aren't what they seem. They are, left to right: Input,  Bass, Treble, MRB cap selector, MRB on/off, Volume, Limiter, Line Out.

RDV
Title: Re: For R.G. or any SS Vox Expert(Now with Samples!)
Post by: Joecool85 on January 20, 2006, 07:15:47 AM
WOW the heat sinks on that stereo guy are HUGE!

Also, since it seems you used one of the kits from chipamp (which I knew about) did you just use the schematic/resistor/cap values for their kit, or adjust it to be a guitar amp?
Title: Re: For R.G. or any SS Vox Expert(Now with Samples!)
Post by: RDV on January 20, 2006, 10:48:07 AM
WOW the heat sinks on that stereo guy are HUGE!

Also, since it seems you used one of the kits from chipamp (which I knew about) did you just use the schematic/resistor/cap values for their kit, or adjust it to be a guitar amp?
I just used his values, there's not a lot of difference really. As far as I know the one at GGG is just from the app notes of the LM3886 datasheet.

I probably could've just used one of the heatsinks for that amp but I wanted to be able to push it hard if need be. Right now it's just hooked to the Mackie mixer with my computer running into it with 2 of those little metal speakers that Radio Shack used to sell with 4" woofers.

That stereo one will get really loud with good speakers!

Is that the same Dean Markely as your practice amp?

RDV
Title: Re: For R.G. or any SS Vox Expert(Now with Samples!)
Post by: Joecool85 on January 20, 2006, 11:30:02 AM
Nope, here is mine:

(http://cordova.asap.um.maine.edu/~raymondj/imagehost/dean.jpg)

Behind it is the gutted Fender, and the rest of the stuff thats next to my bed  :icon_biggrin:
Title: Re: For R.G. or any SS Vox Expert(Now with Samples!)
Post by: Joecool85 on January 20, 2006, 11:41:12 AM
If I was to buy the stereo lm3886 set from chipamp.com, along with a transformer, some pots and jacks, could I build an amp that would have one input jack for the guitar, and drive two separate cabs with 50w a piece?  If so...I may do that, its so much cheaper than doing a GGG board, chipamp.com power supply PCB, buying all necessary components etc.  Well, not a lot cheaper, but some, and instead of having a 1x50 amp, I could (in theory) have a 2x50 amp.  And I was worried about using their values etc for guitar use, but if it worked for you...I'm sure it'd work for me.  Then I'd just have to build a decent preamp/eq setup and bing bang boom, I got me 100w worth of guitar amp  :icon_twisted:

Then I can finally run what I really want to do.  (both custom built cabs, built by me) - a 4 x 8 cab and a 1 x 12 cab.  I think it'd sound sweet, and be pretty portable.
Title: Re: For R.G. or any SS Vox Expert(Now with Samples!)
Post by: RDV on January 20, 2006, 12:22:49 PM
You could certainly do that by just paralleling the inputs.

That chipamp guys got a special for about 30 bucks where you get a stereo LM3875 amp kit. It would be plenty of power, and cheap.

RDV
Title: Re: For R.G. or any SS Vox Expert(Now with Samples!)
Post by: Joecool85 on January 20, 2006, 12:27:52 PM
WOW!  56watts @ 8ohms!  And a stereo one, so 2 x 56w for $35!  Sounds good to me.  Is there any downfall to using a lm3875 setup instead of lm3886?  Also, would it take the same transformer and stuff?  You the man RDV!
Title: Re: For R.G. or any SS Vox Expert(Now with Samples!)
Post by: Joecool85 on January 20, 2006, 12:29:52 PM
Also, it mentions that the lm3875 comes with rectifier boards...but the lm3886 kit comes with power supply boards...does this matter?

**edit**
In doing a little research, it seems that this is better in some ways for me.  I only care about 8ohm operation really, and it does that @ 56w.  The lm3886 only does 50w @ 8ohms, but is capable of 68w at 4ohms (although really, really hot).
Title: Re: For R.G. or any SS Vox Expert(Now with Samples!)
Post by: RDV on January 20, 2006, 03:33:00 PM
The rectifier board on that also has a place on the bottom of the board to place big filter caps like the ones I used on my stereo version (http://www.mouser.com/index.cfm?handler=displayproduct&lstdispproductid=830309&e_categoryid=386&e_pcodeid=01402). I looked at the caps I used(see link) and Mouser's out of them but they will fit perfect on the bottom of that(LM3875) rectifier board and they were cheap.

Is that Dean Markley a keyboard amp? Mine is ancient(like me)! I bought it in 1985.

RDV
Title: Re: For R.G. or any SS Vox Expert(Now with Samples!)
Post by: Joecool85 on January 20, 2006, 04:05:39 PM
I'm confused...where do the 10000uF caps go?  it lists using 1000uF caps or something if I remember right.

And my dean markley is a K-20X.  It is a guitar amp and has a built in overdrive channel that you can only switch by the button on the front.  8" speaker, 15w amp.  It's alright for practice etc.  Not insanely loud, but enough to bug the neighbors if its up all the way.  As far as age goes, its not very old, I bought it new in 2002.
Title: Re: For R.G. or any SS Vox Expert(Now with Samples!)
Post by: RDV on January 20, 2006, 04:22:18 PM
I'm confused...where do the 10000uF caps go?  it lists using 1000uF caps or something if I remember right.

Look Here (http://www.chipamp.com/lm3875rev3.pdf) & Here (http://www.chipamp.com/images/ps-rev3.gif).

RDV
Title: Re: For R.G. or any SS Vox Expert(Now with Samples!)
Post by: Joecool85 on January 20, 2006, 04:38:58 PM
Ah ha!  I see.  Now, did you use the lm3886 kit, or the lm3875 kit? 

Does it really make a difference? 

What is the difference between a power supply PCB and a rectifier PCB?

And is it just me, or does the kit come with everything you need, except for the 10,000uF caps?

And lastly, would these work alright?
http://jameco.com/webapp/wcs/stores/servlet/ProductDisplay?langId=-1&storeId=10001&catalogId=10001&productId=157737&pa=157737PS
Title: Re: For R.G. or any SS Vox Expert(Now with Samples!)
Post by: Joecool85 on January 20, 2006, 07:38:39 PM
I might just do the stereo lm3886 kit anyway because its only $25 cheaper to do the lm3875 and I'd need to get the caps at roughly $5 a piece, so then its only $15 cheaper, plus shipping for the caps, so lets assume its around $10 cheaper...I dunno.  I don't see that either one has any advantage (lm3886 vs lm3875) except for a negligable wattage output.  I think I'd rather have everything I need right there in the kit.  Something to think about anyway.
Title: Re: For R.G. or any SS Vox Expert(Now with Samples!)
Post by: Joecool85 on January 22, 2006, 11:56:57 AM
Bump.  Any help RDV?
Title: Re: For R.G. or any SS Vox Expert(Now with Samples!)
Post by: RDV on January 22, 2006, 02:25:24 PM
I haven't been able to log in for a while.

I think you'll be satisfied either way you go.
The instructions for the LM3886 kits are more detailed for sure, and all the parts are there.
I believe that you can build the LM3875 kit as it is without the 10k F caps and be fine.
It's hard to say since I haven't built that one.
What I do know is that one was the 1st one he put out so I'm sure it works just fine without the added stuff.
In the end it's up to you.

RDV
Title: Re: For R.G. or any SS Vox Expert(Now with Samples!)
Post by: Joecool85 on January 22, 2006, 02:40:45 PM
What about dual mono?  What exactly does it do?  Like does a dual mono lm3886 put out 100w or is it still 2 x 50w?  If so, whats the point?  Why not use a stereo version?  And also, does that mean you did the lm3886 kit?
Title: Re: For R.G. or any SS Vox Expert(Now with Samples!)
Post by: RDV on January 22, 2006, 06:07:11 PM
What about dual mono?

True dual mono would mean two completely separate circuits. Two transformers, two PS boards, two poweramps.

What exactly does it do?

Offer the extra headroom of two separate circuits, plus less stress on PS.*

Like does a dual mono lm3886 put out 100w or is it still 2 x 50w?

Well, 50 + 50 does = 100

If so, whats the point?

See*.

Why not use a stereo version?

I did.

And also, does that mean you did the lm3886 kit?

Yes, the Dual Mono one, but I built it as a stereo version so I could have the other PS board for another project.

RDV
Title: Re: For R.G. or any SS Vox Expert(Now with Samples!)
Post by: RDV on January 22, 2006, 06:12:51 PM
If you'd like a mono 100 watt chipamp, you want to build the LM4780 kit (http://www.audiosector.com/lm4780.shtml).

Gee, I hope these guys send me some free stuff(at least some PCBs) for the references here.

RDV
Title: Re: For R.G. or any SS Vox Expert(Now with Samples!)
Post by: Joecool85 on January 22, 2006, 06:27:38 PM
Chipamp.com has that kit too.  Now, if I was to build that, wouldn't it give me 2 x 120w?  Is there anyway to get it bridged to get 1 x 240w?
Title: Re: For R.G. or any SS Vox Expert(Now with Samples!)
Post by: RDV on January 22, 2006, 06:37:49 PM
Chipamp.com has that kit too.

Out Of Stock.

Now, if I was to build that, wouldn't it give me 2 x 120w? 

I suppose, or two separate 120w amps.

Is there anyway to get it bridged to get 1 x 240w?

You got me. Ask here. (http://www.diyaudio.com)

RDV
Title: Re: For R.G. or any SS Vox Expert(Now with Samples!)
Post by: Joecool85 on January 22, 2006, 07:30:39 PM
Cool, you the man RDV.

The reason for all this isn't for me, but for my brother.  I'm planning on doing the lm3886 stereo kit for my guitar head, but my brother plays bass, and wants a beefy bass head.  He was kinda ok with 2 x 120, but would rather have it as 1 x 240 so that he could just play one big 4x10 or something.

Well, I put a message on diyaudio.com and I'll let you guys know what I figure out.  So far it seems really possible.  If I do construct this monstrous beast this summer, trust me, there will be plenty of pics on here for you guys to oogle at.
Title: Re: For R.G. or any SS Vox Expert(Now with Samples!)
Post by: RDV on January 24, 2006, 05:11:57 PM
Cool, you the man RDV.

The reason for all this isn't for me, but for my brother.  I'm planning on doing the lm3886 stereo kit for my guitar head, but my brother plays bass, and wants a beefy bass head.  He was kinda ok with 2 x 120, but would rather have it as 1 x 240 so that he could just play one big 4x10 or something.

Well, I put a message on diyaudio.com and I'll let you guys know what I figure out.  So far it seems really possible.  If I do construct this monstrous beast this summer, trust me, there will be plenty of pics on here for you guys to oogle at.
Check this guys projects with chipamps, UNREAL!! (http://www.mhennessy.f9.co.uk/index.htm)

RDV
Title: Re: For R.G. or any SS Vox Expert(Now with Samples!)
Post by: RDV on January 24, 2006, 08:10:45 PM
Welcome back!

I've ordered some LM4780 (http://www.national.com/pf/LM/LM4780.html#Datasheet)s and a PCB to make a parallel version which with enough transformer should make at least 100 watts into 8 ohms. Maybe 120.

I'm sick. Somebody stop me.

I'll be making a sub amp with it to run one of my Peavey Black Widow 12"s. I'm afraid I'll have to find another toroid transformer cause my stereo amp I used one in sounds so good. I listen to recorded music again. I also drink again. Oh well.

I'm thinking eBay for the tranny this time cause $40.00 is a lot to drop at a time. I mean that'll buy at least 2 half gallons of good vodka.

I'm running the guitar amp one as a sub amp right now with a smaller Eminence 12" and it sounds pretty good, but I want to make the earth move with something else besides my footsteps.

Cheers.

RDV
Title: Re: For R.G. or any SS Vox Expert(Now with Samples!)
Post by: Joecool85 on January 24, 2006, 08:21:06 PM
It will yield 80 watts at 8 ohms and 100w at 4 ohms.  I talked to brian about it  :icon_cool:  I was going to buy a dual mono kit so that its the lm4780 chip running in parallel and then I was going to bridge both chips for a total of 200w!  It looks like I have no need for this anymore though since my brother is buying a bass head instead.  If you want more info, check this thread out: http://www.diyaudio.com/forums/showthread.php?s=&threadid=71502&perpage=10&highlight=&pagenumber=1

It should be easy enough and yield tons of power  :icon_twisted:
Title: Re: For R.G. or any SS Vox Expert(Now with Samples!)
Post by: RDV on January 24, 2006, 09:02:16 PM
II talked to brian about it  :icon_cool:
It's cool he returned your email. He returned one of mine and ignored the next. I guess he gets a lot of them.

RDV
Title: Re: For R.G. or any SS Vox Expert(Now with Samples!)
Post by: Joecool85 on January 24, 2006, 09:19:11 PM
Yeah, not only did he return the email, but then he got me on aim also, just to make sure if I had more questions I could ask!  What a guy!  I tell you, with all of my hobby endeavours as of late (pedals and amps), I've met a bunch of nice people online.

Did you check out about bridging both the lm4780 via DRV 134?
Title: Re: For R.G. or any SS Vox Expert(Now with Samples!)
Post by: RDV on January 24, 2006, 09:49:43 PM
Did you check out about bridging both the lm4780 via DRV 134?
Yeah, but you gotta have a lot of transformer to make that happen the right way. Something that'll dim the lights when you turn it on, like 300va or over. Did you check that one link where the guy wired a 4780 up with just the pins. I LIKE that. I may do one like that just to see if I can.

RDV
Title: Re: For R.G. or any SS Vox Expert(Now with Samples!)
Post by: Joecool85 on January 24, 2006, 09:54:13 PM
Yeah haha.  I don't know if I'm that sure of myself to try that one.  Pretty neat though.  And 300va shouldn't dim the lights, I mean, thats pulling like 350watts or something, no biggie.  But yeah, it would be a little pricey...and huge.
Title: Re: For R.G. or any SS Vox Expert(Now with Samples!)
Post by: RDV on February 01, 2006, 08:16:25 PM
More OT than ever!

I've taken a couple of crappy little 6.5" monitors that were given to me and bought nice replacement woofers and dome tweeters from Parts Express and now have a killer little pair of monitors! For less than $40.00 USD! The x-overs were already there and looked pretty decent. I just made sure and used the same impedance drivers so the response would be the same.

My next project will be a DIY 12" sub cabinet to go with the LM4780 amp I'll soon be building. Since I retired(semi) from every week gigging I actually enjoy listening to music now. I need to research how to make a good x-over for the sub.

I haven't been building pedals lately but at least this is electronics.

Computer sounding good!

RDV
Title: Re: For R.G. or any SS Vox Expert(Now with Samples!)
Post by: Joecool85 on February 01, 2006, 08:51:57 PM
Cool, thanks for the update.  And keep us posted on the lm4780 project.
Title: Re: For R.G. or any SS Vox Expert(Now with Samples!)
Post by: RDV on February 06, 2006, 09:45:18 PM
I don't ask for much around here, but I'm asking now.

I need a simple 200Hz or so active crossover design that I can easily perf up with one dual op amp(I hope).
I've been reading a lot at the audiophile forums and at ESP and haven't really laid eyes on what I need.
I know I could go passive easily enough but I don't want to waste that much power.

I know I could(should?) post this at Diyaudio but I don't feel real comfy there.
I know there are some filtering experts here who won't BS me with audiophoolery.

Help! ...please.

RDV
Title: Re: For R.G. or any SS Vox Expert(Now with Samples!)
Post by: R.G. on February 06, 2006, 10:07:05 PM
http://sound.westhost.com/project08.htm
Title: Re: For R.G. or any SS Vox Expert(Now with Samples!)
Post by: RDV on February 07, 2006, 07:30:39 AM
http://sound.westhost.com/project08.htm
Thanks R.G.
I remember seeing that one. I just have seen so many the last few weeks it's been confusing.
I can just use the bottom half for a sub out, right? 300Hz would probably be a decent freq for this setup.
I'm using my near field monitors at full range which they seem to be fine with. They don't seem to distort so I was just going to use a one channel low pass x-over for the sub I'm building. I can use the 18vct or 30vct from one of those big transformers(The other being in the guitar amp of course) I bought. I'm going to be taking a bit of a chance hoping the 48vct @ 2A tap will run a paralleled LM4780. I'm not looking for a ton of power anyway.

Thanks again.

RDV
Title: Re: For R.G. or any SS Vox Expert(Now with Samples!)
Post by: Joecool85 on February 07, 2006, 08:23:24 AM
300hz is pretty high for most subs (to me).  Any regular 5" speaker can go that low, so I normally set my low passes to about 150hz or so.  Of course, thats for automotive subs, but I don't see as there would be much difference.
Title: Re: For R.G. or any SS Vox Expert(Now with Samples!)
Post by: RDV on February 07, 2006, 10:11:17 AM
300hz is pretty high for most subs (to me).  Any regular 5" speaker can go that low, so I normally set my low passes to about 150hz or so.  Of course, thats for automotive subs, but I don't see as there would be much difference.
You're right but my little monitors aren't too efficient in that range. They produce it, but they're specialty is from about 500 on up(I bought cheap woofers). I'm going to socket the appropriate parts and the project tells you how to raise or lower the range, so I'm sure I'll experiment.

RDV
Title: Re: For R.G. or any SS Vox Expert(Now with Samples!)
Post by: RDV on February 07, 2006, 10:39:16 AM
Based on the info in the project, will this work for 150Hz?
(http://mysite.verizon.net/res0o2zg/sitebuildercontent/sitebuilderpictures/150hz.gif)

RDV
Title: Re: For R.G. or any SS Vox Expert(Now with Samples!)
Post by: Joecool85 on February 07, 2006, 10:47:16 AM
Looks good to me, but I would wait for a second opinion seeing as I haven't built anything like that yet.  Also, if your main speakers suck below 500hz (which is really bad lol, but cheap is good), depending on the sub, it should be fine around 250hz or so.  A lot of subs are rated for as high as a couple thousand hz, they just sound like garbage up there, and plus its a waste of power if your other speakers are already handling that.  If I was you, I would try 150, 250 and 500 or so and see what works best with your setup.  Afterall, its all about the overall sound, not just one piece.  So many car audio guys don't get that.  In one car I set up, the guy had 5 1/4" stock speakers in the back, so to get a nice even sound we had to set his 10" sub to around 225-250hz or so.  If you have some nice 6x9s you can normally turn your sub down between 80 and 100hz and get great overall sound.  Afterall, the purpose of a sub is to fill in the low end.  If the low end of your speakers is 200hz, you'll want to set your sub a little above that to get a good overlap, probably 250hz or so.
Title: Re: For R.G. or any SS Vox Expert(Now with Samples!)
Post by: RDV on February 07, 2006, 10:58:25 AM
Right now I'm using a graphic EQ as my crossover(actually it's the graphic in my ancient Peavey Monitor head) with everything dumped except the 50 & 100Hz sliders and that works pretty well actually, so I'm thinking 150Hz might just be the ticket. I'll be using a pretty decent woofer (http://www.partsexpress.com/pe/pshowdetl.cfm?&DID=7&Partnumber=290-332) for the sub, but not one of those 50lb. monsters. The one I'm using right now is overkill(a 12" Peavey Black Widow in my stage monitor), and it easily overblows the rest of the system.

RDV
Title: Re: For R.G. or any SS Vox Expert(Now with Samples!)
Post by: RDV on February 08, 2006, 02:11:06 PM
Since I'm driving my sub-amp with the aux out of my Mackie mixer shouldn't I just be able to accomplish my sub filter needs by just putting a twin T low-pass filter in-line from the aux out to the amp input tuned to the appropriate freq?

What if I put this in-line between the mixer and amp:
(http://mysite.verizon.net/res0o2zg/sitebuildercontent/sitebuilderpictures/passivefilter.gif)

Wouldn't that give me a low-pass at 160Hz with a 12db cut of everything above that? I'm just lost when it comes to simple R-C circuits. It's one of the many reasons I didn't pursue an electrical engineering career. Sad but true. I never really did grasp ohm's law and it was & is very frustrating. I don't even know what the reason for putting two filters(like this) together is. If someone knows of a basic RC filtering article on-line I would be grateful for the info.

Unless someone knows the answer please don't reply as I want someone who does know to see this.

No offense meant.

TIA!

RDV
Title: Re: For R.G. or any SS Vox Expert(Now with Samples!)
Post by: R.G. on February 08, 2006, 03:04:16 PM
No, it won't. It will be a lowpass filter, and it will eventually get to a two-time-constant slope, but it won't be what you think you're getting.

The problem is that the first filter adds source impedance to what the second filter sees, and the second filter loads the first one, so the frequencies and rolloffs are not what you expect. It works if you feed each filter with a low impedance source and buffer after it.

RC filters see whatever drives them and whatever loads them as part of the filter.
Title: Re: For R.G. or any SS Vox Expert(Now with Samples!)
Post by: RDV on February 08, 2006, 03:25:03 PM
I wish you had a filter tutorial on GEOFEX.

Not that I don't appreciate everything that is there. I do.

I'm just a bit frustrated with my brain these days.

I didn't feed and care for it properly in my youth and I'm paying for it dearly now.

RDV
Title: Re: For R.G. or any SS Vox Expert(Now with Samples!)
Post by: RDV on February 08, 2006, 03:28:53 PM
I guess I'll burn some parts and experiment tonight. I've got quite a few pre-wired buffers and what-not. I might come up with something decent by accident.

Who knows?

RDV
Title: Re: For R.G. or any SS Vox Expert(Now with Samples!)
Post by: wui223 on February 08, 2006, 04:02:56 PM
i am having the same thought in my mind now.
Title: Re: For R.G. or any SS Vox Expert(Now with Samples!)
Post by: RDV on February 08, 2006, 07:16:15 PM
Tried this when I got home to see what would happen.
(http://mysite.verizon.net/res0o2zg/sitebuildercontent/sitebuilderpictures/filter.gif)

It sort of works but lowers the level a bunch.

I'm going to use this until I build my sub amp and cabinet though because along with the graphic in my old Peavey Monitor head I'm using to power my temporary sub it's filtering enough above 225Hz to give me my stereo image back to my near-field monitors. This is DIY-rigging at it's best!

I'm hearing that my old Peavey monitor cabinet is no sub cabinet. The highs and mids that were coming through before were masking the fact that it wants to blow apart!

RDV
Title: Re: For R.G. or any SS Vox Expert(Now with Samples!)
Post by: Joecool85 on February 08, 2006, 07:50:19 PM
I would use a smaller resistor.  Something on the order of 8-10 ohms.  But this is from theories bouncing around in my head...
Title: Re: For R.G. or any SS Vox Expert(Now with Samples!)
Post by: RDV on February 08, 2006, 07:57:46 PM
I would use a smaller resistor.  Something on the order of 8-10 ohms.  But this is from theories bouncing around in my head...
I was fresh out. :icon_redface:

My plan was a 10 ohm, 100F combo.

RDV
Title: Re: For R.G. or any SS Vox Expert(Now with Samples!)
Post by: Joecool85 on February 09, 2006, 08:41:30 AM
Thats about what I was thinking.

**edit**
Actually, do you have another 15ohm?  You could run it in parallel for 7.5ohms.
Title: Re: For R.G. or any SS Vox Expert(Now with Samples!)
Post by: RDV on February 09, 2006, 09:07:22 AM
Thats about what I was thinking.

**edit**
Actually, do you have another 15ohm?  You could run it in parallel for 7.5ohms.
I used this to calculate the cut-off (http://www.muzique.com/schem/filter.htm). I'm not too unhappy with the result.

RDV
Title: Re: For R.G. or any SS Vox Expert(Now with Samples!)
Post by: Joecool85 on February 09, 2006, 11:07:15 AM
Cool, good to hear it works pretty well.
Title: Re: For R.G. or any SS Vox Expert(Now with Samples!)
Post by: RDV on February 09, 2006, 03:21:32 PM
I'm very intrigued by the concept of a passive in-line crossover. Mine seems to work a bit, and here's a company that sells them (http://www.hlabs.com/products/crossover/).
I'm interested of course in how this would work(or perhaps if) without any active components.
Mine seems to work because it's just a simple 6db RC filter, but this guy is claiming a 12db filter without added components.

Any theories?

RDV
Title: Re: For R.G. or any SS Vox Expert(Now with Samples!)
Post by: Joecool85 on February 09, 2006, 09:32:38 PM
I don't know how they work, but I know they do.  Crutchfield sells similar passive crossovers.
Title: Re: For R.G. or any SS Vox Expert(Now with Samples!)
Post by: RDV on February 09, 2006, 10:41:20 PM
I don't know how they work, but I know they do.  Crutchfield sells similar passive crossovers.
My guess is they work like the one I just built. They filter well enough but they must attenuate the signal a great deal. Mine does. Even with a buffer. I guess I'm gonna run a gain stage 1st.

RDV
Title: Re: For R.G. or any SS Vox Expert(Now with Samples!)
Post by: Joecool85 on February 09, 2006, 10:54:40 PM
I dunno...I don't think they cut too much signal strength, at least not in car audio use.  For car audio you put it inline with the low signal level.
Title: Re: For R.G. or any SS Vox Expert(Now with Samples!)
Post by: RDV on February 09, 2006, 11:54:08 PM
There's some trick I don't know then.

*Sigh*

RDV
Title: Re: For R.G. or any SS Vox Expert(Now with Samples!)
Post by: Joecool85 on February 10, 2006, 07:42:46 AM
Well, I suppose it might be losing some signal strength, but since it is right before the amp you wouldn't notice because you would just turn the amp's level up more.
Title: Re: For R.G. or any SS Vox Expert(Now with Samples!)
Post by: RDV on February 13, 2006, 08:17:35 AM
Cost of Stereo LM3886 amp
This is a stereo amp I'm using to run my small monitor speakers.


2 ea. LM3886TF - $13.00
1 ea. Dual Mono LM3886 PCB from chipamp.com - $26.00*w/shipping
1 ea AC Receptacle - $.50
1 ea Computer AC Cable - $0.00
1 ea. AVEL Y236503 160VA 22V+22V TOROIDAL TRANSFORMER - $46.99 w/shipping
1 ea Chassis 2 rack space aluminum enclosure - $0.00**
2 ea 10,000F 50v Electrolytic Capacitor - $7.34
6 ea 100F 50v Electrolytic Capacitor - $.60
2 ea 47F 50v Electrolytic Capacitor - $.14
1 ea 10F 50v Electrolytic Capacitor - $.07
6 ea .1F 50v Poly Capacitor -  $1.44
8 ea 8.0 Amp 600 Volt Diodes - $10.48
1 ea 1N4004 Diode - $.05
3 ea 10k Metal Film Resistor - $.30
4 ea 22k Metal Film Resistor - $.40
4 ea 1k Metal Film Resistor - $.40
2 ea 680ohm Metal Film Resistor - $.20
2 ea 2.7ohm 2watt Metal Film Resistor - $.64
2 ea 2.2ohm 2watt Metal Film Resistor - $.64
1 ea Green LED - $0.00***


Total: $109.19




*Came with 2 PS PCBs, only used 1, saved 2nd for next project.
**Broken(and unfixable IMO) Carver CD Player from the 80s had 2 space rack mount chassis. A comparable chassis from Par-Metal.com would be around $50.00
***Freebie from the wonderful Brian Marshall. LEDs are cheap.

RDV
 
 
 
 
Title: Re: LM3886/Vox SS Amp Thread(w/ Samples!)
Post by: RDV on February 13, 2006, 08:23:30 AM
I've changed the name of this thread because it has evolved a bit and this may make it easier to find with the search.

RDV
Title: Re: LM3886/Vox SS Amp Thread(w/ Samples!)
Post by: R.G. on February 13, 2006, 09:24:13 AM
That's a good illustration of something I keep saying - a power amp is mostly power supply and other things that are NOT the power amp itself.
Here's what I think we can credit to the power amp circuit:
Quote
2 ea. LM3886TF - $13.00
1 ea. Dual Mono LM3886 PCB from chipamp.com - $26.00*w/shipping
6 ea 100F 50v Electrolytic Capacitor - $.60
2 ea 47F 50v Electrolytic Capacitor - $.14
1 ea 10F 50v Electrolytic Capacitor - $.07
6 ea .1F 50v Poly Capacitor -  $1.44
1 ea 1N4004 Diode - $.05
3 ea 10k Metal Film Resistor - $.30
4 ea 22k Metal Film Resistor - $.40
4 ea 1k Metal Film Resistor - $.40
2 ea 680ohm Metal Film Resistor - $.20
2 ea 2.7ohm 2watt Metal Film Resistor - $.64
2 ea 2.2ohm 2watt Metal Film Resistor - $.64
Which adds up to $43.88 if I added right.

And here's the non-power-amp-circuit stuff:
Quote
1 ea AC Receptacle - $.50
1 ea. AVEL Y236503 160VA 22V+22V TOROIDAL TRANSFORMER - $46.99 w/shipping
2 ea 10,000F 50v Electrolytic Capacitor - $7.34
8 ea 8.0 Amp 600 Volt Diodes - $10.48
1 ea Chassis 2 rack space aluminum enclosure - $0.00**
1 ea Computer AC Cable - $0.00
1 ea Green LED - $0.00***
Which adds to $65.31.

The power transformer itself cost more than all the power amp circuitry and purchased PCBs.

The imbalance gets even worse when we notice that all the free stuff was non-poweramp, and that the cabinet could have added more cost than the power amp circuitry; even worse when we note that the PCBs could have been made at home for under $10.

This is the kind of stuff that people forget when they decide to build a big power amp.

By the way - you did a great job of holding costs down. Getting to under $1 per watt is a tricky goal in any kind of home built power amp that looks presentable.
Title: Re: LM3886/Vox SS Amp Thread(w/ Samples!)
Post by: RDV on February 13, 2006, 09:49:57 AM
Thanks R.G.!

The true spirit of DIY should be to hold the cost down without sacrificing too much quality. Recycling should be done at as much as possible. This includes effects as well. YMMV.

My only regret at this point is not buying the 330VA transformer because I could've ran both the stereo LM3886 amp and the paralleled mono LM4780(for a subwoofer) I'm building now and put it all in the same enclosure. DOH!! I've even got enough heatsink in there to just put both LM3886 amps on one heatsink and the LM4780 board on the other! I'd just have to add two jacks! ...and a crossover ...and a separate PS for the crossover. :icon_eek: :icon_eek:

Hee hee hee. :icon_wink: :icon_wink:

RDV
Title: Re: LM3886/Vox SS Amp Thread(w/ Samples!)
Post by: Joecool85 on February 13, 2006, 05:10:06 PM
I'm using a 330va for a single lm3886.  I like overkill  :icon_cool:

Besides that, to get full power you really want 200va or so for a lm3886, and since partsexpress only has 160,250,and then 330...and the 330 is only $5 more than the 250...it seems a given to go with that.
Title: Re: LM3886/Vox SS Amp Thread(w/ Samples!)
Post by: RDV on February 13, 2006, 06:25:19 PM
I'm a little more prone to listen to R.G. Keen than Brian at chipamp.com.

RDV
Title: Re: LM3886/Vox SS Amp Thread(w/ Samples!)
Post by: R.G. on February 13, 2006, 07:27:35 PM
The subject of how much transformer to buy is a tricky one.

If you want full power at 100% duty cycle, it's easy. You add up the watts, and then compute average DC currents. From that you compute transformer heating current in RMS (they're different!) and pick your transformer VA as the product of its output voltage and the heating current RMS.

That always give a pessimistic answer, because music is not constant power. Only unnaturally compressed sound or constant signals can do full power all the time, and people won't listen to that. Music varies a lot over the time constant of a power transformer.

A transformer may take an hour to get to final temperature at a given power level. Anything that varies in that hour goes into the average. Typical music on radio stations and such has a crest factor of about 20 to 1. That is, the peak power is about twenty times the average. So mostly you get away with transformer VA ratings equal to or perhaps half of the rated RMS power of the amp. VA ratings of 100% of the power amp rating are quite conservative. VA ratings bigger than the RMS power of the amp are overkill by any yardstick I've ever seen. And those are the hifi yardsticks, where power supply sag is treated like an unmentionable social disease.

The thing is you almost can't prove than any transformer that doesn't go into meltdown isn't a good answer. Alternatively, there are many acceptable answers. It all depends on what you take as starting assumptions. Brian is probably as right as I am about transformer powers.

And yes, I'd be happy to give that underpowered 160VA transformer a home. Pay pal OK?
Title: Re: LM3886/Vox SS Amp Thread(w/ Samples!)
Post by: Joecool85 on February 13, 2006, 07:37:13 PM
I see what you are saying RG, and agree.  My big thing though, is there is no real reason to NOT go with a bigger one, except for price.  But when a 160va is $37, a 250va $42 and a 330va $47...it makes me think the 330va is my best bang for buck, and it can't hurt.  This is kinda how Brian described it to me.
Title: Re: LM3886/Vox SS Amp Thread(w/ Samples!)
Post by: RDV on February 13, 2006, 07:51:41 PM
And yes, I'd be happy to give that underpowered 160VA transformer a home. Pay pal OK?
Whoa partner, that was this morning! I later looked to see how much I payed for that "under-powered" 160VA transformer. It was about $46.99 if you include the shipping. I do wish I'd bought a bit more powerful one but I'm gonna leave it be and use one of those big bulky multi-tapped ones that you recommended for the sub-amp(hoping that 96VA will do it).

If I change my mind(again) I'll let you know. :icon_wink: :icon_wink:

I just got my sub woofer from part express today (http://www.partsexpress.com/pe/pshowdetl.cfm?&DID=7&Partnumber=290-332) and I put it in a guitar cab and I'm running it with my old PV monitor head and digging it. It's breezing my pants legs as we speak. The Darkness and Flaming Lips records are being played for sure! It's just a cheap Chinese made woofer but it keeps up with the rest of my little system.

No disrespect intended to Brian either. He thinks in audiophile terms. I don't(can't?).

RDV
Title: Re: LM3886/Vox SS Amp Thread(w/ Samples!)
Post by: Joecool85 on February 13, 2006, 08:15:36 PM
I love The Darkness, the rock.
Title: Re: LM3886/Vox SS Amp Thread(w/ Samples!)
Post by: R.G. on February 13, 2006, 11:25:46 PM
Quote
Whoa partner, that was this morning! I later looked to see how much I payed for that "under-powered" 160VA transformer...
If I change my mind(again) I'll let you know.
That's OK. Actually, I suspected you might have second thoughts...  :icon_biggrin:

Call it an IQ test...  ;)

Title: Re: LM3886/Vox SS Amp Thread(w/ Samples!)
Post by: RDV on February 14, 2006, 06:46:42 AM
Call it an IQ test...  ;)
Jeez, that's the last thing I need. I took one and didn't do too bad, then they gave my 15yr old son one and he scored about 5 points ahead of me. This wouldn't be so bad as one wants the fruit of his loins to be successful and all, but this boy pees on the toilet seat, won't bathe, and is a middle school dropout.

I couldn't be prouder.

RDV
Title: Re: LM3886/Vox SS Amp Thread(w/ Samples!)
Post by: RDV on February 15, 2006, 07:34:28 AM
Oi:

I did the obligatory "smoke test" on the parallel LM4780 amp last night and no smoke occurred.

YAY!

It seemed to sound fine and got real loud & only moderately warm with a pretty large heatsink.

Today I'm building a (ply)wooden shoebox (so to speak) to house the amp + crossover, then I'm getting going on a PS/crossover on perf.

The "shoebox" will have the 3 3/4"(H) X 6 1/8"(L) heatsink sticking out of one end(in free air) and a control plate on the other end. The controls will be: Level, a lighted on/off switch, and output terminals for the sub.

RDV

Title: Re: LM3886/Vox SS Amp Thread(w/ Samples!)
Post by: Joecool85 on February 15, 2006, 07:54:39 AM
Sounds like enough heatsink for a lm4780.  Now, is it just one lm4780 or two?
Title: Re: LM3886/Vox SS Amp Thread(w/ Samples!)
Post by: RDV on February 15, 2006, 10:18:29 AM
Sounds like enough heatsink for a lm4780.  Now, is it just one lm4780 or two?
1 for a lightweight subwoofer amp.

RDV
Title: Re: LM3886/Vox SS Amp Thread(w/ Samples!)
Post by: RDV on February 17, 2006, 02:36:17 AM
Boy.
Did I fail to build the crossover?
Yes I did.
It sounds like a fartover.

Can an NE5534 be used as a buffer?
I'm wondering.
I tried the 22pF cap between pins 5 & 8.

This sucks.
I'm about at the end of my perfboard rope.

RDV
Title: Re: LM3886/Vox SS Amp Thread(w/ Samples!)
Post by: Joecool85 on February 17, 2006, 07:59:59 AM
Here is a nice looking active one I came across:

http://www.snippets.org/filters/shivaeq.htm
Title: Re: LM3886/Vox SS Amp Thread(w/ Samples!)
Post by: RDV on February 17, 2006, 11:55:32 AM
Well I fixed the crossover this morning. I stayed up WAY past my bedtime last night trying to get it done and made a typical mistake that I make when tired. I used a 15v bipolar supply and at the end I wired up the power straight through .1F caps instead of using them to bypass to ground! DOH! Instant fart machine. I adapted the design from Rod Elliot's 2way crossover project (http://sound.westhost.com/project08.htm).

Here's the final schem.
(http://img.photobucket.com/albums/v165/rickydon/150hz.gif)

I was going to build my subwoofer cabinet this weekend also but discovered that I'd cut my sides 5/8" short. Another big DOH! I only was able to do about 50%.

I'm listen testing as I type. I think the amp is going to work out fine. It gets warm, but not hot by any means, which is good because I don't want to buy any more heatsinks(the one I'm using probably weighs at least 3 lbs. . A paralleled LM4780 is the only way to go to run a 4 ohm load with a chipamp IMHO.

Cheers.

RDV

Title: Re: LM3886/Vox SS Amp Thread(w/ Samples!)
Post by: Joecool85 on February 17, 2006, 12:06:14 PM
Hows it sound?
Title: Re: LM3886/Vox SS Amp Thread(w/ Samples!)
Post by: RDV on February 17, 2006, 12:20:16 PM
Hows it sound?
Like 150Hz on down. You know. Boom Boom Boom. There's almost 0 high freq content. It's a 18Db cut on everything above 150Hz.

Seriously, the LM4780 is just 2 LM3886s as you know. When I tested the amp with no x-over it just sounded like any other good straight poweramp. I think the 96VA transformer is going to work out here because I'm really not pushing it too hard, plus this puppy has multiple windings so I used the 30vct winding to build the 15v+/- PS for the X-Over.

It's all good for now.

RDV
Title: Re: LM3886/Vox SS Amp Thread(w/ Samples!)
Post by: Joecool85 on February 17, 2006, 03:15:16 PM
Wow, I wouldn't run smaller than a 250va for a lm4780, but thats just me.  Cool that its working out for ya though.
Title: Re: LM3886/Vox SS Amp Thread(w/ Samples!)
Post by: RDV on February 17, 2006, 03:45:42 PM
Wow, I wouldn't run smaller than a 250va for a lm4780, but thats just me.  Cool that its working out for ya though.
That's cause yer mind's been poisoned by the audiophools.

Just wandering: Have you actually built one? I'm on my 4th.

RDV
Title: Re: LM3886/Vox SS Amp Thread(w/ Samples!)
Post by: Joecool85 on February 17, 2006, 03:57:39 PM
You got me there, I haven't built one yet.  But I do like overkill  :icon_twisted:
Title: Re: LM3886/Vox SS Amp Thread(w/ Samples!)
Post by: RDV on February 17, 2006, 04:33:19 PM
You got me there, I haven't built one yet.  But I do like overkill  :icon_twisted:
It would assure you of not having any PS sag.

RDV
Title: Re: LM3886/Vox SS Amp Thread(w/ Samples!)
Post by: Joecool85 on February 17, 2006, 05:17:00 PM
On yours or mine?
Title: Re: LM3886/Vox SS Amp Thread(w/ Samples!)
Post by: RDV on February 17, 2006, 05:30:54 PM
On yours or mine?
I mean using a large VA rated transformer assures one of not losing power when cranked.

I'm listening to my subamp with a 96VA at a very reasonable volume and I don't hear anything resembling sag or distortion caused by it. I always have the option of a larger transformer.

I've had stereos had were supposedly around 400 watts that didn't have half the transformer that I'm running here on what is probably not quite 200 watts in all. I have at least 1 VA per watt of output and until that proves to not be good enough it will remain.

RDV
Title: Re: LM3886/Vox SS Amp Thread(w/ Samples!)
Post by: RDV on February 19, 2006, 04:07:29 PM
I must have been pretty successful with my amps because I have become extremely unpopular here at my house. I need a shop I can just move into. I'll just come over here to shower.

Yeah.

RDV
Title: Re: LM3886/Vox SS Amp Thread(w/ Samples!)
Post by: RDV on February 23, 2006, 07:52:29 AM
LAST INSTALLMENT
On This Thread

I finished my sub cabinet last night and it came out well. Nice tight bass without too much woofer excursion. Matches well with the rest of the system. The crossover worked out well also, there's almost no high end content at all.

I got a 200 watt computer now.
50 watts a side(stereo LM3886 amp) to rebuilt(by me) 6 ohm near field monitors with 6.5" woofers and 3/8" dome tweeters.
100 watts(paralleled mono LM4780 amp) to a 4 ohm 12" sub in a 17" X 17" X 13.5" DIY sealed cabinet.

Theses are real RMS watts, not "advertised" watts.

I've got one more LM3886 and a 56VA transformer, but I don't know what I'd use it for so it may sit awhile.

Goodbye Everybody!
Have A Happy Life.
It looks like brighter days ahead for everyone on the planet.

RDV
Title: Re: LM3886/Vox SS Amp Thread(w/ Samples!)
Post by: DougH on September 07, 2010, 09:03:42 AM
Bumped in remembrance of Ricky.

This was a very cool thread about a Vox preamp and SS amp he built. This was the "Ricky thread" I was searching for. There are a lot of great ideas in here, some things I was going to play with and never got around to.
Title: Re: LM3886/Vox SS Amp Thread(w/ Samples!)
Post by: Brymus on September 07, 2010, 03:53:47 PM
Thanks for digging this out.
RDV was a big help over at SS guitar as well.
Helping people with their "chip amp" builds.