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Buzz coming from power amp - not through speakers

Hi, first post here. I just picked up a Harmon Kardon PA5800 off ebay. It sounds great, no issues there. However, the amp itself makes an intermittent hum/buzzing noise (ie, the sound comes out of the amp unit, not through the speakers). It fades in, peaks, then fades back out with a period of about 5-7 seconds. Sometimes it's not noticeable at all, but occasionally it's loud enough to hear even with a moderate volume level through the speakers.

Anyone have ideas? DC noise on the AC line? Basic service needed, ie new filter caps? (I'm a tube guitar amp guy, not even sure this type of amp has filter caps).
logan7
logan7 OP
4 posts
 
What do you mean by tack? Considering that the noise has a very distinct pulse to it, I'd expect there's a more fundamental underlying cause, and that just dampening the vibration would be going after it directly.
oldears
386 posts
 
If it has a toroidal transformer, they have been known to buzz. You can try to reorient the transformer and change the mounting tension or possibly replace the rubber damping between the coil and the mounting washers with better materials from Herbies Audio Lab. Some of this can also be tried for the mounting of c-core transformers.
logan7 OP
4 posts
 
Yep, it's toroidal. I'll open it up and see what I can do. It was bugging the heck out of me last night, very clearly audible from across the room.
swampwalker's avatar
swampwalker
5,138 posts
 
If it's loud enough to hear over moderate volume music, I'm guessing it's not the typical transformer "buzz". The periodic nature of it concerns me. Of course, trying the typical fixes for that problem described by others can't hurt. Not sure I'd go so far as resoldering all of the joints until you figure out if there is a bigger problem.
gbart
290 posts
 
The PA5800 is not an amp I would put too much work into. When I owned one,there was a distinct drop in overall volume level anytime I switched from two-channel playback to multi-channel. I replaced it with an Onkyo AVR, which was a surprisingly better performer.
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