QuoteDoes "uni assignments" mean university assignments?. If so, I sympathize. Sometimes I want to quit school so I can spend more time playing guitar and building stuff. I'm not going to, though.

Haha, yeah it does... Its pretty tempting. If it wasn't for the fact that I've already been there 5 years and only have one more to go, I'd think about it...

I love the stereo idea, especially the ease with which stereo effects could be incorporated; pretty much the only reason I didn't didn't go that way in the first place is that at the moment I only have one speaker!
JM, I'm starting to realize that paralleling these guys is, as you say, probably way more trouble than its worth. But since I'm halfway there, I think I'll try a few more troubleshooting options before I totally can the idea. As it stands, the boards are identical in terms of input and feedback loop layouts, they are both mounted to the same thick heatsink, the LM's have the same batch no., and the resistors are matched at least to 1%. All of this makes me suspect (optimistically...) that there's some problem with the design itself; I've posted this on diyAudio and have been given a few suggestions.
I probably won't have a chance to try anything for a few days because I'm bogged down with uni assignements (I feel pretty guilty even spending the time to write this post because I have one due in... 8 hours. Sh*t.) but when I do, I'll let you know how it goes.
Thanks   

haha ok, so i'll probably end up looking at this from that perspective and calling it one of the hundred pieces of junk i need to get through before i end up with something i'm proud of.
i have to admit the main thing thats keeping me going with this is stubbornness :lmao:; having read about other people building these successfully i'm frustrated by not being able to do it myself, also i'm extremely curious as to what the problem actually is.
I think I'll redesign the PCB and make sure there are no grounding issues, with both chips on one PCB.
If that fails I'll give it up and take one out, that way i've still got a single lm3886 gainclone. (even if it is only 50W into 8Ohms  )

Hey,
here's the new schem; this is how the amp is looking right now after some tinkering.

Phil; yeah, I saw that guy's site and pretty much based my circuit off that with a couple of small changes, eg. the inclusion of the volume pot. The pinouts are correct, I just had to mess around with the symbol in EagleCAD to get the wires to match up with the pins.

Teemu; after it initially didn't work, I hand matched all resistors in the signal path (input, feedback network) to about 0.5%. I think the app. note only really specifies the use of DC servos in the 200W bridged/parallel circuit, it seems to suggest you can get away without them, as the successful builds on DIYaudio seem to have.

JM Fahey; I was a little skeptical about it when I first thought about attempting to build it, but after I saw the guy's site that Phil posted, as well as a bunch of mostly successful builds of this type on DIYaudio I decided the project must at least be plausible? Not that I'm disagreeing with you; you clearly know more about this that I do, but why exactly would it be impossible on two boards? I understand that connecting them by wires introduces another potential source of interference, but i tried to keep the wire connecting the inputs as short as possible (about 3-4cm, the boards are mounted one above the other), and i can't imagine that connecting the outputs by a wire could be a problem interference-wise.

The power supply is an unregulated 24-0-24 t/f with 9,400uF at each rail, which is putting out about +/-38V.
I have tried with 4Ohm and 8Ohm loads as well as unloaded and the same problem occurrs.

Most testing has been done unloaded, as I can't imagine that this problem can be good for speakers.
At this stage, both boards work perfectly with the outputs unconnected, but whenever i connect them, they oscillate rail to rail at high frequency and as a result heat up very quickly.
I have connected the positive and ground terminals of my scope probe to each output (positive end of probe to output of one board, ground lead of probe to output of other board) with the boards unconnected, in an attempt to get an idea of how different the ouput signals were, and there was no visible difference *except* when the volume pot (25k, log) was between about 2 and 4 0'clock, when there was a high frequency signal of about 150-200mV difference between the two outputs. When the volume was turned up more that this, this signal dissappeared and the difference once again became negligibly close to 0mV.
I tried to tested this because I was under the impression that the biggest problem with the parallel setup is that differences in the outputs cause the two amps to drive each other, although I don't know if that would actually cause the oscillation I'm seeing at the output when the boards are connected.

I'm pretty confused. I'm open to the possibility that the fact that the two amps are on two different boards is causing problems, as JM Fahey suggests, but I can't think of any other possibilities as individually the boards work fine and there appears to be so little difference between the outputs (except for that weirdness I just described).

Anyway, thanks for the replies. Anyone have any other ideas with this new info or should I just scrap the whole thing?

Yeah thats basically what you do, or you could use something like a sallen-key active filter.
This TI doc has plenty of info on active filters: http://focus.ti.com/lit/ml/sloa088/sloa088.pdf
Google active filters and you should find plenty of info.

  I think the schems I posted are unclear...I'll redraw and post it soon.
I'm not trying to bridge them; bridging would give me twice the voltage swing and twice the current draw from each amp as they see half the load impedance each. I'm paralleling them so they see twice the load impedance giving half the current draw from each amp.
The circuit as a whole is pretty much taken from the LM3886 application notes, with some alterations of my own which I've probably screwed up causing my problems.

 

just got a 'scope on it and, with a square wave input, as i increase the volume pot there is high frequency ringing on the square wave which gets worse as i increase volume, then at about 2/3 volume it breaks out into high freq. oscillation. this is happening with both boards in exactly the same way when they are not connected to one another.
does this sound like a ground loop or a problem with my feedback loop? i can't figure it out.

hey guys,
i've just finished a paralleled LM3886 based power amp and i'm having a weird problem.
The two LM3886 amplifiers are on separate boards and have their inputs connected by a wire between the two boards.
One board has the zobel network and damped series inductor, the output of the other board again is connected to this one by a wire.
When i turn it on, even if the volume pot is set to 0, a weird low pitched noise, which isn't particularly loud, comes out of the speaker and the mains fuse blows after a second or two.
If I disconnect the wire connecting the outputs of the two boards however, both of them work fine. this problem only occurrs when the outputs are actually connected.
I've read that if the two circuits outputs aren't well matched, they might try to 'drive each other'? i have used 1% resistors in all places that have an effect on the output signal.
I've attatched the schems of both boards. Where it says 'to board 1' etc are the points at which the two boards are connected.

Anyone have any idea what the problem might be/ how i can fix it?

edit: forgot to mention, the series inductor is 12 turns of enamelled copper wire around the 10 Ohm resistor on the output of the first board. and the reason I have used 5 parallel resistors at the output of each is that the LM3886 documentation recommends .1 Ohm resistors but I couldn't get 1% accuracy .1 Ohm resistors with high enough power rating, so I used 5 x 1 Ohm resistors to get close. 

I've put the discrete idea on hold for a while and decided to go with the gainclone.
I've built a single LM3886 gainclone in an MDF box which is sounding fine but I have a few questions;
I'm using a 100VA transformer with a 4Ohm speaker and its getting fairly hot. With only one LM3886, it should be pulling maximum about 68W right? Its not painful to touch, but definetly very warm. How hot should a transformer usually run?

Also, the chip is attatched to a fairly large heat sink (black anodized aluminium, 125x75mm with about 45mm fins) but that also is getting fairly hot when playing at moderate volumes.

There's very little DC offset at the output (~10mV) and nothing seems wrong, and I know the LM3886 has inbuilt thermal shutdown which has never kicked in so I guess it can't be running *too* hot, I'm just wondering if this sounds normal?

sorry phatt, you're exactly right. i have two 3886's that i'll parallel. that should put me up to the 80 watt region right?

hey dogbox,
I just found out about those little valves, I'm pretty interested in getting some to put into a preamp.
How much power do you plan on getting out of the IC amp? LM386's can only really dissipate about 1 watt, which isn't much. And the THD quoted on the datasheet is 0.2% at 125mW, which is fairly high compared to other IC amps. I think you should check out the LM3886, it can produce much more power and has better distortion figures and is pretty much as simple to use as the LM386. That way if you decide you want more power you don't need to build a whole new amp. The only problem you might have is the power supply, the 3886 needs a much higher voltage than the 386; the best way to do it is to run it off AC which is a bit more complicated than using batteries. The 386 isn't a bad option if you only want a tiny practice amp though.

Hi phatt,
yeah I saw those kits at Jaycar; I didn't want to buy one to begin with but I've pretty much ended up building an LM3886 gainclone type amp which is basically the same thing, just with a little more power. Thanks for the tips on the preamp!
Hi Fahey,
when I get around to building that thing I'll have to do that. I had it running at really low power on a breadboard and even then T11 and 12 were getting way too hot.

Thanks Fahey.
I just got some LM3886's in the mail so I might put the discrete project on hold for a while, the schematic's attached anyway, let me know what you think. I'll attach the LTspice file if anyone's interested.
What do you think is the best way to power the preamp circuitry?

Got a speaker, Celestion Seventy 80 bought off ebay.

Hey guys, this is my first post although I've been a member of the forum for a while.
I've recently decided to design and build a discrete SS amp. So far I've built the power supply circuit and designed the power amp stage but have not yet fully tested it; it looks ok in simulation though. Initially, I intended only to build a small (~5W) practice amp, but once I started, I realized how easily I could scale the design up to be much more powerful.   
The design seems able to put out about 140W RMS into a 4 Ohm load, limited to about 100W by the rating of the power transformer I'm using.

The problem is, at the moment, I have no cab/speakers able to handle that much power. All I have available for testing is a junked 20W hi-fi speaker.
Also, I haven't fully decided on the plan for the preamp. I think I'm going to go OpAmp based with a pretty stock standard Fender or Marshall tonestack, but I want to build in a reasonable distortion circuit and possibly an octave effect as well. I'd also like to be able to switch preamps/effects without having to pull apart the whole box once I've built it. What I'm thinking of doing to get around this is initially built the poweramp in one box with a master pot to limit the output to something that won't destroy whichever speakers I happen to plug it into, and build the preamp in a separate box which I'll just plug into the poweramp, so if i want a new sound I can build another and plug that in instead.

All of this leads me to ask a few questions:

With regards to giving the poweramp a variable power output to enable me to switch speakers as i go without blowing up the smaller ones, I'm thinking of putting a pot in the feedback loop to enable me to vary the gain. Does that sound good, or would it be better to set the gain in the feedback loop to a reasonable value with fixed resistors and maybe just put an attenuator pot on the input of the poweramp?

Does anyone know a good place to get guitar/instrument speakers in Australia? I don't want to buy a cab, I want to get the drivers and build the box myself. I don't necessarily need the highest quality, at this stage I'm just looking for pretty much any guitar amp speaker that will handle about 50W.

Does anyone have any suggestions as to a decent distortion/overdrive circuit for blues rock? (I play a lot of White Stripes and Black keys type stuff)

I'm planning on running my preamp circuitry off +/- 9V or 12V for the OpAmps (so far I've been using TL072's); is it better to get this supply voltage from a separate power transformer and rectifier circuit or will voltage dividers (possibly with regulators) from the main power rails suffice? I do happen to have a small centre tapped +/-12v transformer.

Does the whole plan sound reasonable?

Anyway, sorry about the epic post, thanks for reading and any comments, suggestions or random thoughts anyone has are totally welcome!