Why is amp toroid humming?

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.

I wouldn't even pop the cover off of any equipment that's still in-warranty; that could work against you. Transformer laminations are probably buzzing so it needs to be replaced. Line voltage variations shouldn't matter, even +/- 10% in a proper design. Vantage's suggestions above have merit, but I would experiment along those lines only if the unit was out of warranty. Once you go in there fooling around, the manuf. has an easy way out. Take the amp to a friend's house & power it up there for a comparison. No need to connect inputs or speakers to try that. It will probably still hum no matter where you plug in. If it doesn't, only then does your AC line appear suspect. Hey it might just be some loose mounting hardware, but get the manuf's OK - IN WRITING - before you go inside.

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.

if the fan/rheostat is, indeed, the source of your problem, then your whole system is likely affected adversely. you might try various filters or conditioners but you're unlikely to rid yourself entirely of the ac/rf intreference unless you install dedicated lines and all that go with them (lots of posts on this).

First of all: if you've ascertained that the ac isn't too high then the manufacturer should solve this problem at his expense. If you have access to a variac you can reduce the ac output in order to include or exclude that as a potential source. A too-loud hum is still too loud at half volume(it shouldn't be audible except perhaps in the very immediate vicinity). Since transformer noise is usually mechanical,the fix should be simple. The question of concern is "who pays for it?"