2. amps-preamps
  3. 1177011842

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 2 responses by davemitchell

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.
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.
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