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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
davemitchell
253 posts
 
Most hum is caused by a ground loop created when you have two or more components plugged into your electrical service with third prong grounds. You can try to cheat out the grounds on everything other than one component (usually the preamp), which gives your system a single ground path to your electrical service. Cheater plugs are available at any hardware store.

As stated in a previous post, cable TV connected to anything in the system is also a typical cause and can be solved with an isolation transformer.
jitterbug
15 posts
 
Hi Chasmo
I had a somewhat similar problem. This sometimes happens when the AC power line is not 100% grounded tho that wasnt my issue. When the AC is not 100% grounded, your speakers will probably pick up the 50/60hz AC frequency which you are hearing as a humm. My problem got solved by using a balanced interconnect between my pre and power (everything else is single ended) - dont recall if the Forte 5 has a balanced option. I also inserted a transformer based power conditioner which helped make the background quieter.
Best regards
JB
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.
avguygeorge
2,300 posts
 
I sold a dead quiet amp to a member. He had this problem. I just knew it wasn't the amp. About 2 months later he found the problem--a fluorescent fixture.
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.
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