Why are Turntables not Grounded?

[Yoa01]

Hello all,

So I know that turntables without a built-in preamp need to be grounded via the grounding post, but I looked inside my turntable today (Gemini XL-500 II) and noticed that the AC plug ground wasn't even connected, and never was! And, the grounding post wasn't attached to the audio grounds either. It's like each ground is supposed to be a part of a different circuit!

Why is this? And, would it be bad to wire together all the grounds?

 

[davenn]

Its not uncommon for signal ground and "earth" ground to be separate

Dave

 

[JonBoy470]

Not grounding the turntable is a very effective way to prevent a ground loop between the turn table and whatever amp you have it connected to.

Though it would be unlikely in a home setting, as the phonograph and amp are likely one on top of the other and plugged into the same outlet, the possibility exists that the ground of the turntable and the amp to be at different voltage potentials, at which point current would flow between them, which would be audible as a hum out of the speaker.

Another common place for this to happen is in a car, when an iPod is plugged into a car stereo aux input and cigarette lighter charger simultaneously.

http://en.wikipedia.org/wiki/Ground_loop_(electricity)

 

[Yoa01]

Ah, that makes sense. But what about using the audio ground? Wouldn't that be easier and cheaper instead of having to use a grounding post? In many cases, I have found that the two grounds end up connecting anyway with no issues.

 

[KrisBlueNZ]

Turntables should be grounded. Normally the preamp will have a terminal post on the back which should be connected to the metalwork of the turntable using a separate wire.

The signal wires from the pickup to the preamp are screened and the pickup is grounded through the screens.

The turntable is not connected to the pickup circuit because leakage current would run through the screen and cause hum and/or noise to become superimposed on the signal. The ground for the turntable metalwork should be through a separate wire that is physically separated from the signal wires.

When the grounding setup is correct, your multimeter will tell you that all grounded items are connected together with no significant resistance. But that doesn't tell you the full story. Currents must be kept separate.

 

[Yoa01]

Ah, I see. Thank you, Kris.