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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
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).
tbg
5,263 posts
 
Take to a tech unless Forte has a better idea.
chasmo OP
12 posts
 
Found the problem!
A tech friend tightened up the chassis ground screw and
the hum is gone.

Thank, Guys.
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