What does the phono ground wire actually ground?

[kirk57]

I've built a DIY unipivot tonearm from parts I had laying around, and $2 worth of birch veneer rolled into a tube. Surprisingly it works pretty well, but since the shaft is wood, I don't know how (or if) I should arrange a ground wire.

I am getting audible buzz and hum between tracks and would like to get rid of this. The only connections I have to the amp
are the four from the cartridge, no ground wire, since the tube is wood I don't know how to do this.

Have any of you built arms using non-metal arms (i.e. carbon, wood, etc.) and if so do you have any ground problems and how were they solved?

Thanks

Kirk

[Steve Eddy]

It's really not a "grounding" issue but rather a shielding issue. The ground lead is usually attached to the metal parts of the tonearm in order to keep it at the same potential as the phono stage ground and prevent the metal parts, particularly the arm tube, from allowing noise to be capacitively coupled into the tonearm leads.

Speaking of which, how are your tonearm leads being run from the cartridge? Are they in close proximity to each other (i.e. twisted or braided) or do you just have the four wires running randomly through the wood arm tube? If the latter, twisting or braiding the tonearm leads can help reduce noise pickup since the wood is not acting as any sort of electrostatic shielding for the leads.

If that doesn't help, you might consider running the leads through some braided copper tubing that's connected to the phono preamp's ground.

Also, while I'm no analogue expert, don't some cartridges have a fifth connection on them for attaching to the ground lead?

se

[kirk57]

Steve-

Thanks for the reply, hadn't thought about the sheilding aspect.

To answer your question, yes I braided the tonearm wire in pairs
(L hot and L ground, R hot and R ground).

Kirk

[sq225917]

twisting is fine for interconnects, but for tonearm wire you need more, you need a shield. ideally inside you tonearm tube.

[SY]

Better yet, run the interconnects between the cartridge and the preamp balanced, if your preamp can accommodate that. With my very low output MC, I can put hum sources quite close by the leads with no serious hums or buzzes.

[kirk57]

I did a little more investigation by hooking a wire to the ground of the pre-amp and connecting to various points on the turntable and arm. I found that most of the noise actually comes from the turntable itself; by connecting to a lug on the metal plate I get rid of the majority of the noise. I still hear a bit of buzz, and I suppose that would need to be fixed by sheilding.
Maybe I could slip some foil inside the armtube and connect that to ground.

Still I wonder if others who have built tonearms out of carbon fiber or wood have encountered this problem...

BTW, I don't think my pre-amp accomodates balanced inputs; is that the XLR connector??

Thanks

Kirk

[SY]

XLR could work; I use a DIN plug, courtesy of Morgan Jones. What sort of preamp are you using?

[kirk57]

Mostly I use a Superphon Revelation Dual Mono and vintage Pilot tube units. The connections look the same on all of them...