Sounds very much like a ground loop problem. Your cable connection is grounded where it enters the house; if this is not the same physical point as the ground for your power panel feeding the preamp, you typically have a voltage differntial betweenteh two ground points, which is manifested as a hum in the system. Easiest solution typically is to reroute the cable ground to the same physical connection as the power system ground.
Help!! Low level hum from FM cable sgnl
I'm using my cable as an the signal input to my FM tuner(MD FT-11), but there is a terrible hum now in the system. It makes no difference if I select the tuner or not(on the preamp) or if the tuner is on or not. I'm curious if a Monster Cable power conditioner (1200 I think-the one with the cable input) would filter this out or if there is another solution. Thanks