DIY turntable earthing question...

[Dom66]

Hi Guys, I'm wondering if anyone more electronically minded can help me with this.

I'm starting a homemade turntable based on some bits I have lying around, namely an SME tonearm and the innards from an old Technics DC motor belt drive unit. The electronics are going to be mounted separately in a control box which I'm working on now.

The thing is, the Technics unit has a two wire mains input as the chassis is plastic, but there is a black ground wire coming out of the main PCB that connects to a metal plate inside that has carries some of the mechanism. (The black wire with the little PCB on the end in the picture). I'm using a metal case, so using a three wire mains input with an earth to the case.

DSCN0184.jpg

The question is, should I be attaching the black ground from the electronics to the mains earth or not ?

Any advice appreciated.

[Barry]

Will the "metal plate that carries some of the mechanism " be used in your new turntable? Was the Techics platter metal, and was it in contact with the "metal plate"?

I think that although the original Technics used a plastic case (and hence only needed a two-core mains cable), the platter was 'earthed' via the black wire which I suspect was eventually connected to the screening of the audio signal cables back to the amplifier.

As you are going to have to use a 3-core mains cable, connecting the 'black wire' might be unnecessary. The metalwork of the SME arm might also be connected to the metal case (depending on whether or not the four rubber grommets in the fixing holes on the base are removed), and as the arm also has a grounding lead, this too could create an earth loop.

Since there are several unknowns, I would arrange matters so that the black wire can be disconnected at a later date if an earth loop is created.

Sorry about the 'rambling' reply, but there is not a definite answer.

[Dom66]

Thanks for your reply Barry. I hadn't considered the platter itself. The Electronic s box and motor pod will all stand in isolation, with just the platter and tonearm on an MDF plinth insulated from each other. It sounds like I should provide a grounding point for each conductive item and the cable screens and experiment with which ones to connect together once its up and running....

[Dom66]

Update, I got this up and running, all earths connected including homemade screened cables connected with the screens connected at the box end. Result, absolutely silent, no hum at all. Well chuffed.

[moodybuilder]

Do you have any pics

[Dom66]

I've only made the control box so far, then I popped the motor back in to the Technics donor unit for now and connected it up temporarily so I have something usable while I make up the new plinth and mount the arm. I'll take pics when I get on to it though.
tt.jpg