SME 3009 ground wire

[F575M]

Hi,

I replaced the 4 pin output socket of my unimproved 3009 with RCA jacks. Instead of connecting the ground wire to the ground socket I soldered it to the ground terminal of the left channel jack in order to avoid a separate ground wire from my preamp to the tonearm. Please see last photo.

Is this allowed?

Cheers,

Horacio

[Barry]

The wire on the right is connected to the metalwork of the arm (and I assume to the black wire inside the screening can, which connects to the metalwork of that part of the arm that moves). This wire should be connected to the metalwork of your preamplifier.

You have a choice as to how to earth the turntable. Either:

[1] 'Daisy chain' a wire from the metalwork of the TT to the wire you have already fitted to the arm base,

or

[2] Run a separate wire from the TT back to the metalwork of the preamplifier (so-called "star earthing").

Both versions will provide electrical safety and will not result in an earth loop. I would be very wary of using one of the signal return shields as an electrical safety earth - there's a reason why SME do not do so.

[Blackdog]

That ground wire is not for electrical safety. It is for shielding the arm against hum pick up.
I prefer to have a seperate ground wire, but Rega has and still is wiring one RCA as the screen ground. No reason you can't do it. However it may not provide as much shielding as a seperate ground.

[Barry]

That ground wire is not for electrical safety. It is for shielding the arm against hum pick up.
I prefer to have a seperate ground wire, but Rega has and still is wiring one RCA as the screen ground. No reason you can't do it. However it may not provide as much shielding as a seperate ground.

Then why do SME provide a 'daisy chain' earth wire with their RCA phono socket terminated arms? Cartridges are essentially balanced devices, so the two (twisted) leads inside the arm tube will provide immunity to common mode pickup. The metal arm tube acts as a Farady cage. Earthing it improves the effectiveness of shielding as well as providing electrical safety.

That's not to say just using the outer part of the RCA socket won't work, but a five wire system covers all options.

(Incidently Shure connect the metal body of their cartridges to one of the channel ground pins.)

[Blackdog]

SME uses that daisy chain simply because that's the way they do it. Having re-wired literally over1000 SME tonearms, it makes no difference whether there is one ground wire or their daisy chain.
I'm not disagreeing about the shielding principles, just the electrical earthing part. While that does provide that, it's not something to worry about.
The shielding of the arm tube is needed as there is very little common mode immunity. The internal wires are seldom twisted.
As for the Shure, they do that as do almost all MM cartridges, with a few exceptions.
It's better to have the 5 wire system, but using the RCA ground can and does work.

[Barry]

My comment about the extra 'earth wire' providing a mains safety earth is based on the fact that all the TTs I have come across are fitted with a two-core mains lead, and they are not double insulated. The metalwork of the TT needs to be earthed, and on many decks with an integrated arm, the safety earthing is done via the shielding of one of the signal leads.

[Blackdog]

True enough. My comment about that was, it was that the ground was never intended as a safety ground.

[F575M]

Thanks chaps, great lessons.