LM3886/Vox SS Amp Thread(w/ Samples!)

[Erik Lion]

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

[RDV]

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

[R.G.]

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.

[RDV]

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. 

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

Ricky

[RDV]

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

[Erik Lion]

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

[RDV]

I cranked back up and was blasting away and the chip shutdown on me(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

[RDV]

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

[R.G.]

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.

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.

[RDV]

I bought one of these 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

[R.G.]

I actually started to snag a couple of those myself. That'll certainly be better. Go for it!!

[RDV]

I think this might be an ideal transformer for an LM3886, any thoughts? Perhaps the 250va one instead?

RDV

[R.G.]

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.

[R.G.]

Go look at part number 120-225. 48Vct, 3A, $23.

[Erik Lion]

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

[R.G.]

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

[RDV]

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

[Threefish]

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

[RDV]

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

[stm]

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.