What is the purpose of a turntable's ground wire?

Question

EDIT: I understand that the ground wire reduces hum. I'm not looking for a qualitative answer. I'm hoping someone can break it down for me Henry Ott or Bill Whitlock style.

Many turntables, such as the ever popular Technics SL-1200 mk2 have a separate ground wire which attaches to the chassis of the mixer or preamp you have your turntable connected to. It's also a popular modification to modify your turntable such that the ground wire internally connects to the shield of the RCA cable (which is connected to the chassis of your mixer/preamp already), eliminating the need for a separate wire.

My questions is, what is the purpose of the ground wire, especially given that it seems redundant given mods such as https://www.youtube.com/watch?v=3leKhyNinNk.

For the purposes of this question, assume use of Allen & Heath Xone 32 mixer.

Below are the phono input stage, and the point in the circuit where circuit ground connects to chassis ground in the mixer.

And here are some pictures of the inside of my Gemini XL 500 II.

The small black wire connects to the tonearm shell, acting as a shield for the four signal wires. It connects in turn to the rest of the grounds in the turntable via the springs, and in turn the the ground wire post on the outside of the turntable.

Note that the turntable itself has no connection to safety ground.

A visual/schematic explanation would be fantastic.

Answer 1

This is my take on the turntable and amplifier circuit.

^{simulate this circuit – Schematic created using CircuitLab}

It looks like the ground wire is important in this schematic.

The turntable motor chassis will float at about Vsupply/2 if not grounded, and the environment will have other nearby objects with 60 Hz voltages on them. These couple to the tone arm and then cause currents to flow in the signal wire, to earth. This current gives rise to a voltage, because of the finite resistance of the signal wires. At the amplifier input, it will see a voltage which is both the pickup voltage, and some of the environmental buzz.

The ground wire earths the chassis of the turntable, mainly the tone arm, to reduce this voltage. Its resistance is lower than the signal wires, but more importantly, it earths the chassis, which is then weakly coupled to the signal wires. So if the environmental "hot" object is at ~60 V, Vsupply/2, the tone arm might also float at 60 V AC, which might induce a few uV onto the signal wires because of their resistance. With the earth wire connected, because of the fairly low (and independent) resistance of the earth path, the environment might manage to induce only a fraction of a volt on the tone arm and chassis. This has to pass through the stray capacitance onto the signal wires, so the effect is much reduced.

Connecting the earth wire to the RCA cable shield at the turntable might help a bit, but it forces this induced current to flow on the cable shields, which will transfer some voltage to the signal path.

It is my firm belief that all EMC / shielding / ground loop voodoo can be reduced to simple circuit schematics, as long as all ground paths, capacitive coupling and external sources are included.
This one is a quasi-DC problem, everything is just as you see it, a combination of L, C and R. There are no wavelength-effects, so it should yield to conventional analysis.

Answer 2

The shielded wire that runs from the rear of the turntable up through the tonearm is NOT well shielded. If you connect the cable from the turntable to the preamp, the residual hum and noise might be acceptable but most likely isn't. Especially on a turntable with a shaded-pole motor and a non-grounded AC supply cord.

Try it sometime and listen to how much hum you are hearing.

Now attach the ground wire from the turntable to the preamp ground screw provided and marvel at how quiet everything suddenly got.

That ground wire can make a huge difference in the noise floor.

Note that you do NOT want to connect any of the wires going to the cartridge to turntable chassis. The signals from a standard magnetic phono cartridge are quite low (low mV) and even less for a moving-coil cartridge. Any current that you inject into the shield wire is essentially in series with the signal and can severely degrade the hum and noise performance of the system.

I spent a lot of my younger years working as a radio broadcast (assistant) engineer and records are what the station played. Commercials were on carts but the music came from vinyl. We took great care to make sure that everything was as quiet as it could possibly be. The people with whom I worked were absolutely anal about it.

Answer 3

Typically the shielding of the audio cable from a turntable will be capacitatively coupled to the ground of the preamplifier, to pre-empt any earth loops. The earth wire provides the real chassis ground.

The 'modification' you describe is therefore based on a false premiss.