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

[R.G.]

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.

[RDV]

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

[RDV]

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.

RDV

[RDV]

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.

RDV

[RDV]

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

[R.G.]

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.

[RDV]

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

[RDV]

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!

[RDV]

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.

RDV

[RDV]

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(.1µF) 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

[nelson]

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.

[PenPen]

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!

[Eric H]

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

[RDV]

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

[RDV]

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

[Joecool85]

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?

[RDV]

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. A good place to look as they have a chipamp forum is DIYAudio.com

RDV

[RDV]

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

[petemoore]

  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].

[PenPen]

  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.