For the best turntable performance, two ground wires are often recommended in a turntable, external preamp setup.
One ground wire would connect from the turntable to the preamp, the other from the preamp to the speakers or amplifier.
This is only necessary if you experience what's referred to as "ground hum" which is described as a buzz or hum coming from the speakers while using the turntable.
If you do not experience this hum or buzz, a second ground wire is not necessary and will not improve audio quality further.
If you believe you may need a second ground wire due to ground hum, you can perform this simple test:
- cut a length of thin speaker wire and strip the ends.
- touch one end to the ground terminal on the preamp, touch the other to the ground terminal of the amplifier or speakers.
- If a ground terminal is not available on the amplifier or speakers, the wire can be touched to a metal screw on the back panel of the speaker, or case of the amplifier.
If the hum stops, you will need another ground wire (the speaker wire used in the test can be used as a permanent ground wire if needed).