I've purchased pre-amps which have their own powercord and give the signal off the LP turntable that boost it needs before going to the 'line-in' input of the computer sound card. In both cases the pre-amps input ground noise that I could not eliminate. Success was gained by utilizing a regular amplifier and taking the signal from the earphone jack and feeding that into the line-in of the sound card. This also allowed me to use the volume control and balance to optimize the incoming signal.