DIY Amp Tweaking

[choatie88]

I have a pro class A/B amp that's outputting odd noise for small signals, like very quiet orchestral passagse. The noise is low in volume but noticeable. Increasing the input level causes the noise to also increase in volume, but eventually with a high enough input the noise go away. Decreasing the input level back keeps the noise away for a few seconds, but then it comes back. The noise isn't quite uniform; it sounds like it has more HF energy.

I suspect that the bias of the transistors might be wrong, causing them to slip out of the linear region, does this seem plausible?

Opening the amplifier, I see 2SA1302/2SC3281 transistors in the output stage; each channel has a pot which I assume is used to adjust the bias.

Does anyone have tips or advice as to how I can figure out what's wrong and fix it? I can get to an oscilloscope, but it's a lot easier for me to find a multimeter.

[audiofan]

The first thing I would check is dc level at the input of the power amp or dc level around volume control, is there is a dc signal at one of those spot it may unbalance or change the operation point of the amp an cause some noise.

[Tube_Dude]

Hi choatie88

A noise that depends of the position of the volume pot....look like oscilation to me.(Oscilation dependent of the driving resistence).

Inspect with a scop...

Regards

[choatie88]

I'm not measuring any DC at the input or output.

Where should I look for oscillation?

[cunningham]

Originally posted by Tube_Dude
Hi choatie88

A noise that depends of the position of the volume pot....look like oscilation to me.(Oscilation dependent of the driving resistence).

Inspect with a scop...

Regards

Yes sounds like RF osciliation, or just plain old "dirty potentiometer syndrome". If scope shows RF noise after cleaning, check the feedback loop, and miller feedback caps, or any caps in the signal path for that matter................bad solder joint maybe????