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Need Help Combating Power Amp hum

I have a vintage Forte Model 5 power amp purchased used on Audiogon. It is a nice sounding amp, but is plagued with a constant, low level HUMMM which is just audible from my listening chair when the room is very quiet. I understand hum modulates/distorts the music which is undesirable.
My speakers are an MTM design with 90dB sensitivity (the other owner used Magnapans and didn't notice the hum).

The amp uses a toroidal power transformer and all the signal wires are routed carefully away from the power supply components. The hum occurs even when the inputs are shorted/disconnected, ruling out my other components. Power cord polarity and grounding were also checked.

Any ideas/techchniques to eliminate this hum?
chasmo
powers
46 posts
 
I had a similar problem with several Amps. I could hear the toroidals from across the room. I now use a PS-Audio HUM buster, everthing is now quiet. Most likely I had some rogue DC on my AC line.
davemitchell
253 posts
 
Sorry Chasmo, when I read your original post and you mentioned speakers, I thought you were hearing hum through them. Sounds like transformer hum.
tbg
5,263 posts
 
Opps, me too. If you are hearing it entirely from the amp body, I would open it up and see if the toroid is loose. If not, loosen it and see if there is rubber beneath it. If not, add three small strips and retighten.
chasmo OP
12 posts
 
Thanks,
Normally I have to be within 3ft of the speakers to hear the hum, but during late-night CRITICAL listening (no traffic or neighborhood sounds and I turn off the fridge in the next room!) I can JUST detect it from my listening seat 8 ft away.
chasmo OP
12 posts
 
IT'S THROUGH THE SPEAKERS.
(this line didn't come through on my last post).
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