problem with paralleled LM3886s, help?

[mnturner]

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   

[rowdy_riemer]

I probably won't have a chance to try anything for a few days because I'm bogged down with uni assignements

Does "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.

[mnturner]

Does "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...

[phatt]

In a nutshell, the problem is this:
modern SS amps are practically perfect voltage sources, with "almost" zero internal impedance.
They achieve that by having massive internal gain and equally massive feedback, which for DC is almost always 100%.
That's why the output DC voltage , which could easily slam against a power rail, gets reduced to less than 50 milliVolts.
They do that by supplying to the load as much current as needed (tens of amperes if necessary) to bring output voltage to where the input differential amp says it should.
Now imagine two amplifiers with joined outputs: you have two "brains", absolutely ignorant of each other, driving the "muscle" (output transistors) to get a certain output voltage, on which they do not agree. I do not care about the load, they will try to force the other equally powerful opponent's output to submit to their will, killing each other in the process. Or at least oscillating madly, overheating, the works.
Neanderthal solution nº 1): put a 4 ohm 50W resistor in series with each output, the node feeds the speaker. It works, sort of, although you gain no power and waste a lot.
A somewhat-less-Neanderthal solution nº2) : you match all resistors to 1% or better, match chips from a large batch or at least use those from the same box, same manufacturing code, draw a special board where all traces have the same resistance and inductance, mount them to the same large, thick heatsink to minimize thermal differences, lower the 4 ohm resistors to 0.1 ohm, relax and think "what a great Engineer I am " . Well, not bad for a Neolithic village Engineer in around 4.500 B.C.
Yes, it may work, but, why worry?
Increasing chip count carries the complexity up where it pays to use a boosted-chip or even discrete design, where you can design for as much current as you want because, remember, parallelling chips you only get more current capability, nothing else.
As an example, in the 70's I sold a lot of 400W RMS into 1 ohm bass amplifiers, using 8 x selected RCA or Motorola 2N3055H as output devices. They drove 8x12" LEEA speakers (the local ALTEC licensed factory), consider I used the equivalent of 8 Altec 12" woofers in big boxes,  they *easily* beat Ampeg SVT's in Stadiums.
3) The real beauty of using 2 chips lies in using them bridged, because they supply *a lot* of power into a single 8 ohm speaker.
4) Of course, Teemu's and Armstrom's idea of using them "in stereo", each one driving its own speaker, is excellent. Even if you are not interested in stereo per se, you can send the signal from a single preamp to both of them.

Thankyou JMF, You have explained that very well. :tu:
The bridged output seems to be quite attractive,,, 4 times the power I think.
Hum,, I do have some old stereo STK modules under the bench somewhere.
Oh so many thoughts but never enough time.
Phil.