My father gave me his Sony home theatre system that he just replaced because there is a low level hum that is almost always present. When you turn the volume up, the music overpowers the hum, but with the volume low, you can still hear it. I'm sure it's some kind of interference, but I can't seem to find out what. Things I've already tried: Moving the unit (radio/power interference from outside?) Different outlets (dirty power?) Rerouting interior power cables (power cables next to signal cables causing hum?) Switching signal sources, moving speaker wires around to different connectors, switching input methods (surround sound, etc) and general troubleshooting. Looked at the boards for obvious problems, bugs, bulging caps, loose connectors, etc. So far, no dice. Some more info: The hum seems to be lessened on certain inputs (phono and FM radio are lower than CD player for instance) The amp has two speaker "rails" (dont know what else to call them) A and B. B seems to have a little bit less hum. It seems to have a little less hum on the surround modes, and moreover, a little less hum on specific types (dolby mode has less hum than simulated mode for instance) but I'm sure that just has something to do with the different sound configurations lessening it and not an actual reduction in the interference. It has several power plugs in the back for the other componants (cd player, phono, FM radio, Tape deck) but unplugging those and using wall outlets does nothing to lessen the hum. I'm fairly confident it's not an outside source of interference. Anyone got any ideas? I'm usually good with the basic stuff like above, but this one has me stumped.