Bill, Though, if the problem is with the ground wiring in the tone arm wouldn't the sound of the hum change when the OP touches the arm? The thing, imo, that rules out that the hum is caused by an AC mains ground loop is the OP said when he turns off the preamp the hum stops. TT: DUAL CS 5000, DUAL 1219 Cartridge: Ortophon OM 40, Grado mono Quote: The Dual CS 5000 is a three speed, semi-auto, belt-driven turntable system with integrated low mass tonearm and pitch control. Specifications Motor: EDS5000 Drive: Microprocessor controlled quartz belt drive Platter: non-magnetic removable Tonearm: aluminium tubular tonearm, Kardan, OPS Signal to noise ratio: 80dB rumble weighted signal to noise ratio Cartridge: OMB 20E HiFi DIN 45500 Stylus: DN 20 NE Biradial Frequency range: 10Hz-28kHz DUAL 1219 Quote: Specifications Power supply: AC, 50 or 60Hz, changeable by changing motor pulley Power supply voltage: 110/117V or 220V switchable Drive: Synchronous continuous-pole motor with radial-elastic suspension Platter: non-magnetic, dynamically balanced, 3.1kg Speeds: 33, 45 and 78rpm Pitch control variation: 6% Rumble: -60db (weighted) Tonearm: extra long, torsionally rigid metal arm, 4-point gimbal suspension, skeletal head design Cartridge holder: removable, accepts all 1/2" cartridges from 1 to 12 . I assume the AC power plug on the TT is a 2 wire, non polarized type. Just for the heck of it I would unplug the TT from the wall outlet and check for the hum. If the hum is gone I would then try reversing the plug 180 in the wall outlet and check if that changes the sound of the hum. .