subtle fuzz from solid state

[antiunderscores]

I'm looking for some advice on getting rid of some excess noise.

The amp:
I'm having a bit of fun with an old solid state 60 watt general purpose type amp. It has a single ic in the preamp followed by a handful of discrete transistors in what I would think to be the "phase inversion" section of a tube amp followed by two tanky transistors which make up the power amp. If memory serves me correctly the ic runs at about 27 volts.

The problem:
When playing notes there is this subtle fuzz in the background. It is a noise similar to a snare buzz. It gets louder and quieter as you adjust the master volume accordingly, so its most likely in the preamp section. But as you play louder//softer the noise stays at a constant volume, until the note is very quite, then the noise is non-existent. The noise gets higher if the note your playing is higher and visa versa. The sound comes through with different different guitars//bases.


Im just looking for a push in the right direction, could it be something as simple as a noisy capacitor or resistor, or should I introduce more power supply filtering, or is the noise inherent in the layout or ic itself?

cheers.

[R.G.]

I would first look for parasitic oscillation by putting an o'scope on the output. Sometimes an amp has RF fuzzies on the signal over a certain signal level.

Next I'd check the Class AB bias of the power amp stage. That could produce the same symptoms too.

There's a myriad of other possibilities, but I'd start there.

[pdf64]

Try it through a different speaker. A damaged or rubbing voice coil etc can add fuzz to a clean tone.

[R.G.]

Try it through a different speaker. A damaged or rubbing voice coil etc can add fuzz to a clean tone.

Oh, yeah. There's that. I spent a day once chasing "crossover" distortion that turned out to be a rubbing speaker.

[Enzo]

That was my first thought, the classic description of a light voice coil rub in the speaker. COnnect the amp to a different speaker and see.