Why do my transformers hum sometimes?

"Transformers can hum when line voltage is +/-10% of the transformer's voltage rating." -- Tvad

Might explain why it happens on Saturdays. More people home using appliances, drawing electricity.

thanks, guys. I wondered why my isolation transformers seemed to be humming more loudly since I moved from a dedicated-line house to an apartment with inferior power. There's no help for it though; they're needed more than ever where the power is bad. I've been trying to think of ways to deaden the sound. I understand that Shunyata's have a box-within-a-box design, with soundproofing between them, to address this, but a diy attempt at this would seem to create heat issues.

same problem as you and you learn to live with it...once I installed a regenerator to keep the voltage locked in at 120V this seemed to help greatly.
Larry

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