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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

Showing 6 responses by chasmo

chasmo OP
12 posts
 
It hums all by itself... As already mentioned
I'm listening to just the power amp w/everything else
disconnected/unplugged.
chasmo OP
12 posts
 
The hum occurs even when the power amp is the ONLY component used (i.e. everything else was unplugged) it appears to be within the power amp itself. No balanced options in my system, as all components use single-ended RCA.
chasmo OP
12 posts
 
Interesting, but no fluorescents involved. When I listen I usually turn off all the lights - still problem with hum.
BTW audio system is on dedicated line and no hum when substituting a different power amp.
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).
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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