Phono cable routing? Too close to a transformer perhaps?
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Have you tried floating the ground on the pre-amp and or the table? This can be done simply by using one of those .99 cent grey adapters from Home-Depot that convert your three-prong plug to two prongs. There is sometimes hum generated in the ground loop so give that a try. Also, I don't know if the cartridge you have is a shielded cartridge. I used to have one of the older Grado signatures, which is a wonderful sounding cartridge but is unshielded. Whenever I turned the motor on the table on (one of the original AR tables - we're talking early 80's when I first got into this crazy hobby) the hum became audible as soon as I swung the arm out over the platter and got louder as it got closer to the motor. I think it was Merril, or some other company made a small lead plate to go between the platter and the plinth on that table to help quell the problem. It helped, but did not solve the problem entirely. Don't know if you're having similar problems, but there's two possibilities. Good luck!