A-75 buzz

[Dave Varner]

I am a rookie.

But somehow I managed to build an A-75 about 2 years ago and since then I have lived with a buzzing sound, not too annoying with my main speakers but really obnoxious with some single driver boxes I am now toying with.

Question: The buzz is not the traditional hum, but more like what I would suspect is related to either the diodes or voltage doubler circuit in the regulated power supply. Where should I look for the source and/or how can I eliminate (or reduce) the buzz?

Thanks,

 

[Nelson Pass]

Look on the rails for the front end using a scope,
and if they look like they don't have any ripple,
say only a few millivolts AC, then the amplifier
might be oscillating, in which case you look for high
frequency noise at the output, again using an
oscilloscope.

 

[PassFan]

Dave:
Did you add the bypass electrolytics to the driver card board? There was some slight oscillations that showed up on a few of these that were eliminated by placing an electrolytic from the + , - power inputs to ground on the driver cards. I think 150 mf, polarized. Keep - pointing to - and + pointing to + . This will mean where they come together at the ground you'll have a - to a + .

 

[Dave Varner]

Thanks for your help. I will look into it as soon as I can stand to stop enjoying it for a while...

DV