Thanks to all who have contributed opinions and suggestions. As Richard at Vantage Audio opined, David Belles has been helpful with suggestions and is quite willing to replace/repair the toroid if required. Of course, neither one of us want to ship the amp for repair if the problem is in the ac line. This morning, I realized that we keep a small fan running most of the time on a rheostat at the back of the house to help with ventilation in a bathroom. While not on the same cicuit, it creates line noise that is getting into my audio system ac circuit. You can clearly hear the hum level rise and fall as the fan is switched off/on. The hum level is now half what it was and if it stays at this level, is probably acceptable. Does anyone know if something like AudioPrism's Quietline ac filters are effective for this situation? Thanks, Will.
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?