Improving the Non-Inverting chipamp

[GeorgeBoles]

Thank-you for your comments, Xelb.

Do you think that the layout of the board is the cause of the oscillation? Is it component selection? Could it be the 100k pot rather than using a 10 k? Could it be the crappy speakers with uncertain impedance?

(I do not have a symmetrical board because I was trying to get the big capacitors and the feedback resistors as close to the correct LM3886 pins as physically possible. Carlos made a big deal out of having the supply capacitors and their snubbers really close, and others have made similar comments about the feedback resistors. )

I presume you have a jumper between the Pins 1 and 5, because there appears to be no supply V+ supply to pin 1. I do like the overall layout style of your board. I will have a little think to see if I can make it all fit 4 similar boards along a 30 cm heat sink.

If you think it is the board as the main problem, I guess I will have to change it.

Your input is appreciated, and I will cogitate and formulate new plans,

Regards,
George.

[GeorgeBoles]

When I look closely at the LM3886 package, there appers to be little copper ends appearing through the plastic at various points. Are these structural, or do they have some internal connection?

If they have some internal connection, then perhaps it is the way I am mounting the package onto the heatsink.

I doubt it, but always worth asking.

Regards,
George.

[carlosfm]

George, are you isolating the LM3886 chips from the heatsink?
Do you have any pics?

[GeorgeBoles]

Hello Carlos.

I appreciate hearing from you.

No, the chip is NOT isolated from the heat-sink, but I have used the LM3886TF version, which I thought was supposed to be insulated electrically. I use white paste from an old kit to thermally join the chip to the sink. Also, at present the heatsink is just sitting on a piece of chip board and nothing else is connected to it ... (at least I think nothing else is connected to it!)

Pics coming in a bit, if I can get my son's digital camera!

Regards,
George.

[carlosfm]

Originally posted by GeorgeBoles
Hello Carlos.

I appreciate hearing from you.

No, the chip is NOT isolated from the heat-sink, but I have used the LM3886TF version, which I thought was supposed to be insulated electrically.

That's fine, then. It is isolated.

[GeorgeBoles]

Thank-you for your patience everyone.

Carlos, as requested, I have put some pictures of my effort on my homepage. (Rather than clogging up this board with pictures of failures, I have put them on my website, which I have just modified for the purpose. That is why it has taken so long. I have rarely done this before either!)

Pictures of my oscillating LM3886 Amp

I hope this sheds some light.

Regards,
George.

[juma]

Hi George,
your pics don't show clearly the presence of 300 pF cap between pins 9 and 10 of the LM3886 chip. Check that, and if you don't have it, add one. If you have it, try another, bigger one (but don't go over 1 nF). That should stop the oscillations. Also, try to make those wires shorter. You don't want antennae (especialy in input signal part of the circuit - potentiometer and input wires should be short as possible and twisted together).

Cheers

[GeorgeBoles]

Dear Juma,

The 300pF capacitor is the little iridescent blue shiny blob seen in the bottom right of the "bottom view". In fact, it is the reason I built a second copy of this amp, just to make sure I hadn't stuffed up the soldering on my first attempt.

Your thoughts about antennae and loops are valid. I was hoping not to have to be TOO careful, given this is just a mock-up. Also the "antennae" must have changed positions in the weeks between my two similar failed efforts. But if that is what it takes, off I go to fiddle some more ....

Thank-you for your input.

Regards,
George.

[XELB]

(I do not have a symmetrical board because I was trying to get the big capacitors and the feedback resistors as close to the correct LM3886 pins as physically possible. Carlos made a big deal out of having the supply capacitors and their snubbers really close, and others have made similar comments about the feedback resistors. )

Yes, that's right.
But as all in life, you have to make compromises...
I also share that opinion but I have a different approach.
I prefer small capacitance capacitors because they are faster and better in high frequencies.
And I imagine the copper layer as a hydraulic system, you should never concentrate the pressure(read it energy) in just one place... you will stress it

I also have 380uF near the chip.
( I use 6x 380uF and 2x 1000uF )

[carlosfm]

Originally posted by XELB
I prefer small capacitance capacitors because they are faster and better in high frequencies.

But they don't have enough reserve and tend to slow the lower frequencies.
The snubber deal with your worries.
When you parallel too many small caps you will end with high (trace) inductance, which is not good for... the high frequencies.
This has been discussed so many times...

It is possible to put 2x2,200uF caps near the chip, along with bypasses and snubbers. I don't see where's the problem.