Mark Rehorst wrote:
On Sat, 01 Nov 2003 16:31:45 -0400, --exray-- wrote:


Could you elaborate a bit more on what it sounds like? Is it an AC buzz
or an AC hum? Is it 60 cycles?
Is the power supply a plain-jane bridge or is it a switching type?

-Bill


It is a buzz. Connecting a scope in parallel with the speaker reveals
a very complex waveform that is in sync with the 120 Hz power supply
ripple.

A mod article in 73 magazine suggested raising the feedback
capacitance to flatten the audio response and reduce hum and noise. I
tried it and it had no audible effect on the buzz.

I also tried putting capacitance in parallel with the rectifiers.
Again, no joy.

I tried replacing the rectifiers with Schottky types. No change.

I am going to try adding a 3 terminal regulator to supply the audio
stage from a regulated source instead of the unregulated power it now
uses. If that works, maybe I'll replace the audio board with a
regulated power supply and IC power amp...

MR

Are you getting a lot of harmonics of the 120Hz?. Maybe a small value
cap like .05 across the filter caps would be worth a try.
If you redo the audio stage what about the 'modulation' of other stages?

-Bill