As the title notes, I've got a very slight 60 Hz hum in both channels of my mainline. The hum seems to have a slight pulsing characteristic, with the pulses having a period of about 2 seconds. This hum is entirely independent of input selection and attenuator setting. I think the hum is slightly more pronounced in the high impedance position (which I normally don't use).
I'm using Focal Utopias under the low impedance setting. I've also tested using a pair of Sennheiser HD 650s on the high impedance setting and can hear the same hum. I think it's quieter on the 650s than on the Utopias.
As I said up front, this hum is very slight, to the extent I first thought it was background environmental noise (like a refrigerator). It's not particularly bothersome and is completely imperceptible with any signal input. I just figured I'd note it here to see if there's a known cause for this kind of hum.
