I don't know why they hum, but I've had 4 amps that drove me various states of crazy. I sold two, one is low level enough that I can live with it, but the fourth, a Sonic Frontiers tube amp, can be heard at 15-20 ft. in a quiet room. So, after talking to the SF Tech. Rep.-- he said the amp should not hum, and to send it in. Good Luck. Craig.
Why is amp toroid humming?
My 6 week old Belles 150A Hot Rod amp has started to emit a humming noise. The volume is such that it can be heard from 4-5 feet away in a dead quiet room. David Belles, the amp's designer, is being helpful and suggests that line voltage over 124v. could excite the toroid, as could line interference from rheostats, etc. However, the amp ran silent for the first six weeks and nothing has changed in the electrical system to my knowledge. Time of day/night makes no difference in presence or volume of hum. Disconnecting speaker cables or inputs makes no difference. Nor does changing power supply to a different house circuit. Anyone had experience with a problem like this?