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chris-man 28th April 2004 04:13 PM

200w amp trouble (LM3886T) - one chip only wants to oscillate

I've built the 200W amp as shown in AN-1192, with each LM3886T chip on a separate pcb; so the output from JFET (U5) feeds four separate circuit boards.

When 2 or 3 of the circuit boards are wired together, there is no problem.
When I wire up the fourth circuit board, it only wants to oscillate.
I've double checked solder joints and looked for bridges and all seems fine.

Anyone have any suggestions?


peranders 28th April 2004 04:54 PM

I think your main problem is separate pcb's and using these high speed (rather anyway) devices.

Is it always one particular board which gives you trouble?

chris-man 28th April 2004 06:20 PM


yes, it's the same pcb / chip that doesn't seem to want to work properly.

I can't really see why using four pcbs should create a problem, especially with just one of the boards.

skyraider 1st May 2004 07:25 AM


obviously if only one particular board keeps giving trouble, you should throw it away=)

i am looking into another way to do my sub amp now, i will have two separate amps to power two smaller subs. this should be easier and cheaper. what do you think?

chris-man 2nd May 2004 12:51 PM

Not a bad idea - bloody amp's ****ing me off a treat.
5-6 weeks work and it's still not quite there!!!

That said, I'd still stick with a single 200W amp rather than two separate amps for two subs; surely it will mean two volume controls?

Made a further discovery today - when I wire up all four boards, the DC offset is 42V, the + / - rail voltage. Yet it works fine when only two or three of the boards are wired together.
Anyone, any ideas???

chris-man 2nd May 2004 05:12 PM

heat sink wasn't isolated properly!

anyway, now two chips oscillate and two work fine. The two non-inverted circuits are causing the problem.

i'm out of ideas and am considering using a hammer to "fix" this amp

skyraider 3rd May 2004 02:44 AM

i will be happy to borrow you my hammer-)

you mean its the non-isolated chips thats causing the problems?

chris-man 3rd May 2004 09:23 AM

Thinking about it, I probably won't destroy it yet.

All the chips are the non-isolated type and earlier they were connecting to the heat sink. I've fixed that though.

Now two of the chips just oscillate. If you look on the application note, it's the two non-inverting chips U1 and U2.

peranders 4th May 2004 12:25 PM

I have seen the pictures at

and you have also my answer there.

Judging from the photos you may have huge problems.

Check Thomas Madsen's bridge amp. It should look something líke that.

Magura 4th May 2004 12:35 PM

Another little detail I would check if I was to make it work, would be the solder joints. They do not look reliable. If possible, have somebody help you solder or show you how. There is a good article about soldering on written by Karen if my memory serves me right. You could gain a lot from reading that article.


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