Try grounding preamp to center screw in AC outlet, that sometimes works. You can also try lifting the ground by using a cheater plug. If you try the latter a further step having nothing to do with hum is to ground down or file off the bigger prong so it can be inserted either way. One way usually sounds better than the other.
Low level Hum in Phono Signal Path
There is a distinct low level hum noise evident through my speakers when the phono mode is on. Can't be heard from the seating area but can be heard when your ear is in front of the speaker, either side. This noise is also dependent on the volume; the higher the volume the louder the noise. What is causing this noise and can it be remedied. (Equipment involved: BAT PK 5 phono preamp, Transrotor turntable with Benz Micro Ruby 3H cartridge)