Need help to get rid of noise

[deletecm]

Hi,


I have an amp that somehow makes noise that looks like a square wave around 1kHZ, and it gets to the output amplified. It's only present when the amp is turned on, so it's coming from the amp itself. The output (both channels) looks like this when no input signal is present:

I'm also attaching the schematic. The amp doesn't even have digital circuits apart from a digital clock IC, and that doesn't seem to generate 1kHZ at any pin. So where is that noise coming from?

 

[MarcelvdG]

Did you by any chance test it with a multiway loudspeaker with broken-down squawker or tweeter as a load?

 

[deletecm]

There was no load at all, I only measured the output with an oscilloscope. Don't know if you can open that picture but it looks like a noisy 200mV peak to peak square wave.

 

[MarcelvdG]

That's a pity, because then I haven't a clue what might be the cause.

 

[Mark Tillotson]

Sounds like break-through from the 7-segment LED display circuitry. This is possibly getting to the amp's input section somehow (dried out decoupling capacitors perhaps).

 

[nigelwright7557]

The last time I had that type of problem turned out to be my desk top CFL lamp.
It was radiating near the amp and interfering with it.

 

[scobham]

Try to identify the problem area by scoping the various modules in the amp to see where the issue is generated. Usually this kind of problem occurs in a negative feedback loop or through coupling through the psu. I would suspect a faulty capacitor somewhere but without a bit more diagnostic info that's about all I can say.