Hum from Pre3 Cathode/Follower and WDKT88

[G Willis]

I have upset something moving my equipment. Since the move I have been having a problem with hum from my WD-KT88XL and Pre3 CF. The hum comes and goes.

Powering up today I am hearing a steady hum on both channels which sounds like a combination of G2 and G3 on a piano. G2 is 97Hz so I think given my imperfect pitch this is mains hum.

With my pre-amp powered down a louder, lower 50Hz hum predominates. Switching the pre back on, the 50Hz the hum diminishes and the higher hum is heard again.

The volume or input selector of the pre-amplifier has no effect on the hum.

Unplugging the interconnects causes the hum to almost vanish – there is some transformer noise which is audible over a quiet 50Hz mains hum. I think this is about the same level of quiescent noise produced by the equipment before the move.

Powering down the power amplifier causes the noise to momentarily become louder.

With my DAC plugged directly into the power-amp using the same interconnect cables there is no hum above the normal quiescent level. (It has a built in pre-amplifier)

Any ideas of how to go about looking for this? The obvious culprit is the CF but I cannot see anything wrong with it. I understand that earth loops are caused by two connected devices having different ground potentials but I am not sure how this could arise.