Sorry to hear about the hum... I know exactly what you're talking about. I got a cheapo Radio Shack pre-amp, and it hums, too, until I got the ground wire hooked up correctly between the turntable, pre-amp, and the computer card, or amplifier.
Another thing you could try if the power supply is built into the pre-amp you're using, AND if the power cord's plug is not polarized (one blade wider than the other) try pulling the plug out and turning it 180 deg. then plugging it in again. This sometimes helps, since the hum comes from a mis-match somewhere in the power / neutral / ground wiring.
Have fun with it. Hope you get it worked out.
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