I have a Dual CS 505-2 belt-driven turntable, an Ortofon 305 cartridge (soon to purchase an OM 30), RCA AH500 preamplifier and the following problem: intense hum in both channels. So intense that I can barely hear the music. The hum can be heard as soon as the amplifier is turned on, even when the motor is not used and the record is not spinning. I tried swapping the wires connected to the cartridge with no change. My turntable has a grounding wire that is not connected to anything, although I tried connecting it to the back of my computer with no change also. One interesting thing is... when I attached the grounding wire to one of the RCA connections, the hum disappeared completely, although music was playing loud enough only in the right channel; I could barely hear it in the left channel.

Any input is welcome!

Does anyone else have similar equpiment and experience a similar problem?
Any suggestions on how to ground the turntable and/or eliminate the hum?

Thank you very much in advance.