Why do turntables need a separate grounding wire?

[restorer-john]

Having the separate ground wire gives you choices.

In carbon fibre or composite arms, there is an additional ground wire from the headshell collet as well as the arm bearing housing down to the common point in the TT. They can also be a source of trouble (hum/noise etc) when cartridges with an 'earth strap' to the body are used in such an arm.

I haven't had an earth hum issue for decades, but a considerable reason for that is the proliferation of double insulated source gear in my collection. It was a problem in the 70s and 80s when we got two core gear rewired for Australian standards all with chassis earths and three core leads. This played havoc when daisy chaining lots of gear (tape decks, eqs, etc).

It didn't help that the Australian importers were often lazy in the 70s, identifying active and neutral- they just cut off the little black US two core wire, wired in a three core and screwed an earth lug to a chassis or transformer bolt and changed the voltage switch. As our plugs were not reversible, some gear came with inbuilt hum out of the box.

I like my Power amps to be earthed, and looking at my current (no pun intended) system, all the other components (2xCD players, 2xDATs, Tuner, 2xPreamps, TT are all double insulated- non earthed). Essentially, it is a star-grounded setup, with the only duplication being parallel shields on high level, low impedance line sources where hum/noise is unlikely to ever be a problem.

When high gain, high input impedance FET front ends became popular, hum was tamed by going double insulated (no power point earth) which meant differential earth/ground potentials weren't an issue.

 

[gadget73]

I actually just had to make myself a ground strap tonight. Shure SC-35C that the strap had obviously been removed, and it hummed on my Type A with the plastic headshell. I made a strap from a resistor lead and it stopped humming.

 

[heyraz]

The shields of the L/R phono interconnects are connected to the ground post internally on one of my phono stages.
Once while spinning a record I heard so much static and pops I thought I had a dirty album. No hum while the platter spun with the cartridge up in the air. No static at all!
The static and popping was so bad while the stylus was on the record I was simply going to toss the album rather than attempt a cleaning. It sounded beyond salvage.

I checked the ground wire and found it had fallen off. Reconnecting it cleared up all of the static and pops.
Internally..that turntable has a wire from a wiper at the end of the spindle to ground.

What I think caused the noise and pops while the ground wire was disconnected was static building up across the surface of the spinning vinyl which then discharged into the cartridge to ground thru the best path available (the stylus).
Once I reconnected the ground, all static and pops disappeared.
Personally....I think the extra wire (ground) for that turntable provides a path for static buildup on the record surface through the spindle to ground.
Just my 2 cents.

 

[woodj]

The shields of the L/R phono interconnects are connected to the ground post internally on one of my phono stages.
Once while spinning a record I heard so much static and pops I thought I had a dirty album. No hum while the platter spun with the cartridge up in the air. No static at all!
The static and popping was so bad while the stylus was on the record I was simply going to toss the album rather than attempt a cleaning. It sounded beyond salvage.

I checked the ground wire and found it had fallen off. Reconnecting it cleared up all of the static and pops.
Internally..that turntable has a wire from a wiper at the end of the spindle to ground.

What I think caused the noise and pops while the ground wire was disconnected was static building up across the surface of the spinning vinyl which then discharged into the cartridge to ground thru the best path available (the stylus).
Once I reconnected the ground, all static and pops disappeared.
Personally....I think the extra wire (ground) for that turntable provides a path for static buildup on the record surface through the spindle to ground.
Just my 2 cents.

Very interesting.

 

[Doug G.]

I actually just had to make myself a ground strap tonight. Shure SC-35C that the strap had obviously been removed, and it hummed on my Type A with the plastic headshell. I made a strap from a resistor lead and it stopped humming.

That's why I prefer to isolate the cartridge from a conductive headshell instead of removing the strap. You never know when you might want to mount that cartridge in a situation where you need the strap.

Doiug

 

[marcmorin]

Marc, you join your commons at the turntable, the phono or line preamp joined them at the RCA inputs, so the two lines have become one in any event, being connected at both ends in your case.

Yeah, I know that. And in my minds eye I cannot possibly see how a loop could be formed when i'm simply creating that 1 potential further upstream.

 

[marcmorin]

The shields of the L/R phono interconnects are connected to the ground post internally on one of my phono stages.
Once while spinning a record I heard so much static and pops I thought I had a dirty album. No hum while the platter spun with the cartridge up in the air. No static at all!
The static and popping was so bad while the stylus was on the record I was simply going to toss the album rather than attempt a cleaning. It sounded beyond salvage.

I checked the ground wire and found it had fallen off. Reconnecting it cleared up all of the static and pops.
Internally..that turntable has a wire from a wiper at the end of the spindle to ground.

What I think caused the noise and pops while the ground wire was disconnected was static building up across the surface of the spinning vinyl which then discharged into the cartridge to ground thru the best path available (the stylus).
Once I reconnected the ground, all static and pops disappeared.
Personally....I think the extra wire (ground) for that turntable provides a path for static buildup on the record surface through the spindle to ground.
Just my 2 cents.

funny stuff static electricity is. doesn't follow the "basic rules" and 100K is a dead short to the stuff. not connecting the ground wire to the AR subchassis can make for some noisy playback

 

[rothwellaudio]

even though both have the same potential to ground when the RCAs are plugged into the pre? ie at that point both are used.

Yes. If the negative sides of the L and R coils are connected at the headshell and at the phonostage and two wires are used a loop is created. The term "ground loop" wasn't coined for nothing. A ground loop is an induction loop and magnetic fields will induce a current in the loop just like the magnetic fields within a transformer core induce current in its secondary winding.

And don't forget you're also dealing with a very high gain as far as a magnetic cartridge. This gain will gain anything, even stray hum.

The situation is actually worse than that. The RIAA curve boosts hum frequencies about 20dB more than the midband, so for a phonostage with a nominal 40dB gain the gain at hum frequencies is more like 60dB.

Having the separate ground wire gives you choices.

Exactly.

Yeah, I know that. And in my minds eye I cannot possibly see how a loop could be formed when i'm simply creating that 1 potential further upstream.

You won't create ground loop if the L and R negative sides of the cartridge are linked within the turntable and then a single negative wire goes back to the phonostage.

funny stuff static electricity is. doesn't follow the "basic rules"...

Yes, electricity does follow the basic rules. Essentially, we're dealing with the laws of physics.

Whether or not hum is created when a ground loop is created depends on other factors. The way the two (L & R) 0V references within the phonostage are dealt with can make a big difference. Simple power supply arrangements with a single 0V reference can be a lot better than attempting to create a "dual mono" phonostage with seperate L & R 0V references which are carelessly routed and linked. That's why some phonostages will produce hum when faced with a ground loop and some won't.

 

[WaynerN]

A ground loop is 2 or more lines to ground that have different potentials to each other, causing eddy current flow, which is what causes the hum. That is why there should only be one point that is ground in a system.

 

[gadget73]

That's why I prefer to isolate the cartridge from a conductive headshell instead of removing the strap. You never know when you might want to mount that cartridge in a situation where you need the strap.

I tend to agree, but that cartridge came to me used. It came off one of those wood MicroTrak arms with the chunky metal headshell. That cartridge has plastic ears anyway so I don't know why you'd need to remove the strap anyway. Shure SC-35C

 

[marcmorin]

A ground loop is 2 or more lines to ground that have different potentials to each other, causing eddy current flow, which is what causes the hum. That is why there should only be one point that is ground in a system.

correct.

 

[woodj]

funny stuff static electricity is. doesn't follow the "basic rules" and 100K is a dead short to the stuff. not connecting the ground wire to the AR subchassis can make for some noisy playback

Yes, static electricity is very unlike electricity we are familiar with and work with.
Many times what happens seems very illogical because it is very much more than energy jumping across "barriers".

 

[woodj]

A ground loop is an induction loop

Not.

 

[woodj]

A ground loop is 2 or more lines to ground that have different potentials to each other, causing eddy current flow, which is what causes the hum. That is why there should only be one point that is ground in a system.

Absolutely.

 

[lini]

wj: No, not really - 'cause eddy currents (in German: "Wirbelstöme") are a different thing. However, I don't know the correct English term for our German term "Ausgleichsströme" and have read several translations (mainly "circulating current", "compensating/compensatory current", "stray electric current" and "equalizing current"), which would all seem quite plausible, so that I don't know, which would be the actually usual term - and unfortunately Wikipedia is no help either, as their article on ground loops seems to contain neither of these terms... *sigh*

Greetings from Munich!

Manfred / lini

 

[woodj]

(in German: "Wirbelstöme")

Great word.

I don't know the correct English term for our German term "Ausgleichsströme" and have read several translations (mainly "circulating current", "compensating/compensatory current", "stray electric current" and "equalizing current"), which would all seem quite plausible, so that I don't know, which would be the actually usual term - and unfortunately Wikipedia is no help either, as their article on ground loops seems to contain neither of these terms... *sigh*

I love that description and Ausgleichsströme.
I was wondering about that eddy current description.
I do know that there is dependence on miniscule difference of potential between one conductor and the other(s), but never understand what introduced the hum.

BE WELL
John

 

[ripblade]

Not.

Actually, Roth is correct.

I do know that there is dependence on miniscule difference of potential between one conductor and the other(s), but never understand what introduced the hum.

From Wikipedia:

"Ground loop currents can be induced by stray AC magnetic fields which are always present around AC electrical wiring. The ground loop constitutes a conductive wire loop which may have a large area of several square meters. It acts like a short circuited single-turn "transformer winding"; any AC magnetic flux passing through the loop, from nearby transformers, electric motors, or just adjacent power wiring, will induce currents in the loop by induction. Since its resistance is very low, often less than 1 ohm, the induced currents can be large."

 

[heyraz]

The OP's question took a turn and became a discussion about hum.
The assumption has been that this hum is caused by "ground loop", a term which is loosely applied but not fully understood.

It's a difference in potential causing a current to flow. You only want this difference to flow in one direction (out) so you provide only one path. Like tilting the gutter on your house...you want the water (moving water is a current) to flow in one direction only.

Electricity is a lot like water when you think about it.

 

[woodj]

The ground loop constitutes a conductive wire loop which may have a large area of several square meters.

So does the loop(s) oscillate?

 

[ripblade]

So does the loop(s) oscillate?

The current induced into it does because the EMI causing it does.