Why do my transformers hum sometimes?

As someone else already stated, the likely culprit is DC offset. You can blame your power company. While an isolation transformer will stop the gear from humming, the same DC offset may cause the isolation transformer itself to hum.

I used to have this problem off and on over the years with my Bryston and Conrad Johnson amp. My Jeff Rowland Design Group Continuum 500 has an incredible effect solution. The 500's Power Factor Correction takes ANY AC and converts it to 385V DC (yes, you read that right). The amp is dead-quiet in all conditions. There no transformer hum, 60-cycle or otherwise and no hiss. (You can put your ear to the tweeter at normal listen level with the source off and you hear nothing).

It take the regeneration concept, but doesn't convert back to AC, staying in DC for quiet operation.

Dave

dc on ac mains.

Thanks for the responses guys. It sounds like I'm just going to have to live with it since any real solution would break my bank.

A brief addition. The Equitech website may be of help as it offers quite an overview of power problems. If not you might give them a call. I found an Equitech 2Q solved my hum problems. Their Balanced transformers are quite different than regular isolation transformers. Costwise you could try one of their "blowout" units, if they are available.