strange opa541 problem

[matjans]

hey guys,

when listening to an opa541 based chipamp we encountered a strange problem. It seems as though in the mid-high frequencies (guitar, loud voices) the chip is distorting the signal waaaayyy too much. You can hear a sort of "crackle" in the guitars & stuff.

we've tried gains of ~22 and ~15.
supply voltage = +/- 21VDC
nu current limiting used (the opa doesn't have an active current limiting system, you have to include a resistor to do this).
offset is pretty low, ~2 / ~4 mv.
we've checked all wiring and it seems to be OK.

any ideas?

[matjans]

as per the datasheet i included a 0.1 ohm 5w resistor for current limiting. didn't help.

nasty!

[Netlist]

Can you measure the distortion with a scope? Sine wave, square wave?
Is the distortion volume dependent? Or load dependent perhaps?

[matjans]

i can't measure it (no scope at hand) but i used trueRTA to feed it a single frequency and (strangely enough) i couldn't hear any distortion... tried everything from 200hz up to about 18khz. pink noise doesn't give me any audible distortion either.

distortion is neither volume nor load dependent. tried 4ohm and 6ohm speakers at different volumes. Distortion is still there.

i tried the cd player with some other amps but it's fine.

[Netlist]

I don't know trueRTA but I see they have a built-in scope.
In your case I would only trust a good scope picture.
Did you play with different supply voltages. Isn't +/-21V a bit low?

[matjans]

o well, leave it. the guy ripped out the 541's and built one with 549's. works like a charm