Why do turntables need a ground while other components dont?

Just wondering why turntables must have a ground wire.

Is it because the needle on the vinyl creates a static charge that must be grounded away?

 

Well, all the other components are grounded -- there's both a signal ground and, usually, I this modern day and age, a chassis ground carried by the third prong of the AC plug.

I think -- and someone correct me if I'm wrong -- with a turntable the arm tube and turntable bearing aren't typically connected to a grounded chassis or any part of the electrical circuit, so unlike with other components where the ground return of the interconnect and the third prong of the AC able are providing ground connections, there's nothing connecting the bearing and the arm tube to ground without a separate ground connection to the preamp chassis. So those things can be functioning like antennas for EMF noise that's then induced into tonearm wire or cartridge coils, etc, but if you attach those devices to ground, instead, they act like shields.

A phono cart doesn't have a common ground conductor in its 4-wire connection as I understand it, it's like a balanced signal with a + and a - conductor for each channel, but no ground connection, so that ground connection is like the third pin on an XLR cable in a balanced set up.

 

Some of my components only have a 2 prong plug (no ground) like the CD player.

So the electrons fine the route of least resistance via the ground? Without it they will travel through the tonearm and create static sounds?

What is creating this extra charge? Not the needle/vinyl? Anything rubbing on plastic creates a static charge, that's why plastic eyeglass lenses get dirty so much faster than glass lenses. I just thought that might be happening with the record dragging on the needle at 33 1/3 rpm.

 

I don't think it's just the friction of the needle. Unplug the ground wire and you'll hear a nasty hum even when there's no record playing.

 

My Nottingham Analogue TT doesn't use a dedicated ground wire, as its incorporated directly into the power/mains cable (3-way plug). In the end its still grounded though.

 

Rega TTs don't use a separate ground wire.

 

Think about it this way you have a twin axial wiring scheme for each channel from the phono cart with unshielded wire. One wire per channel carried the AC in goal when it's moving in one direction, the other wire carries the other phase. The tone arm encapsulates the wires and is the shield. If you don't connect the shield to ground all the hum inducing EMF radiation in the vicinity will be induced in the two cables per channel. By connecting the tone arm to ground you drain that off.

 

Thanks that makes sense.

Looks like they did away with the separate ground wire in the new ones.

 

Some arms, like the stock Regas, tie the arm ground to one of the cartridge signal wires that connects to circuit ground.

 

Tannoy speakers have a ground connector. They work without the ground cable though.

 

You could also fix the ground-wire to any metal part inside the TT, like my nephew did when I cleaned my vintage DUAL CS 701. From then on the tt was always grounded.
I guess why TT have the ground wires extra is because you have the option to add various parts, like the 4 pinned carts or tonearms from other brands.
Each electrical system has to be grounded somehow - basic principles of AC/DC ..

 

So why does my CD player not have a ground wire? Its only a 2 prong.

 

Equipment with two-pronged plugs have an internal floating ground.

 

I take it certain criteria have to be met in order to use a floating ground. I always wondered why some appliances have 3 prong while others only have 2 prong. Not even sure what a floating ground means.

 

The wires running through the tonearm aren't shielded unless the tonearm is grounded.

 

In a three slot electrical socket both the neutral slot AND the third prong "ground" slot are connected to ground (the other slot is hot and is not connected to ground). The third prong connects the equipment chassis to ground so that if for some reason the metal chassis becomes energized by the hot side of the AC line, the hot signal will flow to ground and trip a circuit breaker instead of just energizing the case and potentially giving you a nasty shock when you touch it.

 

My Cadillac sits on four tires that insulate it from ground but as long as long as the internal circuits are a complete path it runs (computerized engine, head lights, etc.) fine. Airplanes work the same way, not even rubber on the ground when in flight.

"Ground" has two meanings. One is "earth ground", i.e., connected to the planet via a rod pounded in the ground, etc. The other is simply a common point on which all or most internal circuits use as one side of the connection. Most electronic devices use the chassis as a common point all the circuits inside use and are referenced from, but that can be isolated from or connected to earth ground.

We talk about "balanced" and "unbalanced" circuits a lot. Unbalanced circuits have one side at some common point, usually the local ground, whereas balanced circuits form their own complete loop. This may be floating from or referenced to local ground which in turn may be referenced from or isolated from earth ground. If it is on a jet at FL 400 it's a fair bet it's isolated from earth ground (unless it's trailing an eight mile long wire). It can be isolated from airframe ground (your iPad in your lap) or referenced to it (all aircraft electricals are 'bonded' to airframe ground in case of lightning strike or when it lands lest it be zapped).

Telephone lines are balanced, 'tip' and 'ring', but they are not isolated from earth ground. There is one and only one connection (usually at the central office). Any time you have a ground, this is the rule, you want one ground, a really good one, but only one. If you have more than one, you have a ground loop. This invites noise and does so very well.

The turntable has a two wire AC cord to its motor and drive system and this is wholly separate from the tonearm and cartridge system. The latter has the ground wire. Usually. With compliance for UL and CE all bets are off.

 

You now your stuff, thanks. So that third prong is more of a human safety issue. Like on a corded drill etc. But on a wall socket one is hot one is neutral but I did not think it was a ground.

Meaning its not connected to the metal box like the third prong ground wire is, I thought it went back to the circuit breaker.

 

Ahh thanks so much. I was always thinking of Earth Ground types.

 

The neutral is grounded at one point, but is intended as the return for the hot side whereas intentional current should never flow through the ground wire.

 

This is the important thing for folks confused about this to remember. I'm sure you, Burt, can explain this better than I can but let me try so people can get their heads around it, and you can correct me.

The tone arm and cartridge are not connected to the motor that drives the platter which is plugged into the wall socket. So any connections made between the motor and the home's AC supply aren't also being made to the arm and cart.

Typically the phono cart has four wires -- two per channel, one of which is carrying one phase of the AC signal coming from the cartridge, the other of which is carrying the other phase. These wires are connected to circuit hot and circuit ground, respectively, of the preamp through the interconnect. That's a complete connection between cartridge and preamp for the purposes of delivering the cartridge signal to the preamp. (Remember, a phono cartridge is a passive generator, creating a voltage through magnets and coils, one moving in relation to another in response to the groove modulation. It's not powered by an outside electrical supply and is not connected to the house's power line.)

However, at this point the arm tube -- which is typically made out of a conductive material and usually is in contact with whatever shield material is built into the cartridge body via the mounting screws -- is not connected to any part of the circuit. The phono cart and the phono cart wires need to be shielded. Like a single coil guitar pickup, the cart is an excellent antenna for EMF radiation which causes hum. Also the arm tube is like an antenna for EMF. For the cartridge shielding scheme and the arm tube to function as a shield and not an source of hum, they need to be connected to ground -- to drain off that hum inducing radiation. The ground wire on turntables run from the arm tube to some ground point, typically a post on the preamp connected to circuit ground (which is typically the chassis, which is often also connected to the house supply's earth ground via the third prong if one is present), for this reason.

 

Awesome point! Amazing how such a tiny cartridge can produce voltages that mimic the original sound so faithfully.

 

Tell you what OP, I don't know the answer but I think it's a darn good question...