Amplifier Transformer Hum

If you narrow the hum down to the amplifier's transformer, try to determine which circuit the amplifier is running off of. Then check the circuit breakers in the service panel.

Chances are that your amps' circuit is on the opposing phase of some or all of your other audio circuits and therefore generating some AC noise thats affecting the transformer.

Have an electrician move all of your audio circuit breakers in the service panel to the same phase of 115 volts.

Just for grins I moved my amp's circuit over to the opposing phase where my other dedicated audio circuits are to see what sonic improvements I may find. No sonic improvements that I could tell but it cured my transformer hum and the transformer has been almost dead silent every since.
Sometimes the hum is caused by "DC Offset". The AC power quality is off a little, and the transformer has to work harder to get the right power to the amplifier or other component. It will vibrate more when it has to work harder. A power regenerator/ voltage corrector would probably fix this kind of hum. (PS Audio, Equi=tech, etc.) Sometimes Stehno's solutions will work as well, as well as a dedicated line, if the power problem is in your house and not the local utility.
Sometimes the hum is caused by a light dimmer. The dimmer works by chopping off AC cycles and this generates a lot of electrical noise. Torroidal (donut shaped) transformers are especially susceptible to this. When the light in my range hood is on the dim setting, the balanced transformer in my power conditioner buzzes.
Great Herman! We are basically describing the same thing. I never thought of dimmers as a cause.
In my case, (I have many dimmers) all lights would be all the way off but would still get the transformer hum. Dimmers on or off made no difference.

Today, the only time I get a resemblance of the transformer hum I used to get is when the microwave oven is turned on.

Again, for some time now, I've been running 3 seperate/dedicated lines to each component but still had the transformer hum until I moved the amp's circuit over to the same phase as the cd player and preamp.
Watch out for any electrical device that has a dioded high/low switch. In the low setting the chopped off wave form drives many transformers to hum. My humidifier (in low setting) drove my velodyne sub amp crazy.
What you need is the AH Offset Killer. I got one for my Rotel and now it is dead quiet. If you get one go from the amp to the offset to the wall, it has surge protection, dont use it in conjunction with a power conditioner, as it may limit dynamics.
Darryl where can one find this "AH Offset killer"?
wish I still had the e mail address, however if you search around audioasylum,you should be able to find it. It really works!