Why do turntables need a separate grounding wire?

s rassi

Active Member
The thought occurred to me when I was reattaching my jury-rigged grounding wire earlier (the Technics BD22D uses a stupid proprietary barrel plug to attach the ground wire instead of a screw). Why does there need to be a separate ground wire in the first place? Why couldn't the entire thing be grounded through the RCA leads?
 
First, the chassis and arm need to be grounded so interference picked up by them does not pollute the signal to the amp. The optimum place for grounding is the ground/chassis of the amplifying device.

Secondly, some manufacturers choose to use the shielding/ground side of one of the interconnects to accomplish ground.

A separate ground does not need to be anything special.
 
Grounding turntables is done so static charges (extra voltage or 'potential') that builds up can be discharged
away from the signal path. This build up is commonly a result of the friction of the stylus motion against the
vinyl and it travels via the headshell to the tonearm and chassis but there are other factors that may cause it to
accumulate. Since current flows from high to low potential, the 'ground', which is the lowest potential, is connected
to the turntable and it doesn't matter whether it's to the chassis or another path (such as RCA shield).
Thorens, for example, does the grounding on it's classic TD series turntables through the right channel's shield/return path.
It's important that this grounding connection always occurs just once. Multiple grounding points may result in a
'ground loop', which typically cause audible diusturbance in the form of mains frequency hum.
 
It's important that this grounding connection always occurs just once. Multiple grounding points may result in a
'ground loop', which typically cause audible diusturbance in the form of mains frequency hum.
Exactly.
 
The thought occurred to me when I was reattaching my jury-rigged grounding wire earlier (the Technics BD22D uses a stupid proprietary barrel plug to attach the ground wire instead of a screw). Why does there need to be a separate ground wire in the first place? Why couldn't the entire thing be grounded through the RCA leads?
going against all the other reasons given. i don't run a separate ground wire. all my grounds are summed inside the table, and both ground sides of the RCA cables. I have 1 potential after the point of summing.....your cables and your ground are summed at the back of your pre, mine are 1 meter ahead of that point.......{{{{shrugg}}}}}
 
Because the gain is so high on a cartridge (40db for MM, 60/+ for MC), the metal in a turntable acts like an antenna, gathering EMF and RF from all kinds of sources (even the very equipment it is connected to) so we need to have a "noise drain". I really don't like the term ground as it has electrical connotations, like with power.
 
This is a good question and I haven't seen an answer yet as to why the separate ground. All of my Technics turntables have separate grounds. I just did a repair on a Sony direct drive quartz lock automatic turntable that has no separate ground. Why do some manufacturers use a separate ground wire?
 
Because the gain is so high on a cartridge (40db for MM, 60/+ for MC), the metal in a turntable acts like an antenna, gathering EMF and RF from all kinds of sources (even the very equipment it is connected to) so we need to have a "noise drain". I really don't like the term ground as it has electrical connotations, like with power.

agreed.........however the question was of why a "separate wire", when all of the "grounding wires" could have been summed inside the table and sent down one or both RCA output cable grounds
 
In almost every line stage preamp that I have or have been in, the shields are all connected together, so whether you connect them at the source or not, they still are all connected together. I have 2 tables that carry the noise drain on one of the RCA shields, that being the AR-XA and a home-made version using AR parts that I call ARmod. I'm not saying that this is a bad practice and probably works 99.9% of the time, however, not all line preamps or phono stages are built the same, and there might be a very remote possibility of introducing noise into other line stages as most selector switch designs do not sever the shield connection. That right there may be a good enough reason why almost all turntable manufacturers have a separate noise drain wire going to the chassis.
 
...I haven't seen an answer yet as to why the separate ground.
My bet would be "history" and "tradition". The external ground wire is probably a relic of the days before polarized AC power cords and I guess it was originally regarded as a "safety ground" that was meant to eventually connect to real ground (e.g. radiator, water pipe, ground electrode). The RCA shield (one channel or both) ground scheme is not necessarily "real ground" but rather the pre-amp ground (AKA 'floating ground'). This type of grounding is not done for safety reasons but rather to discharge unwanted voltage. This typically involves grounding through some sort of capacitive coupling so I guess it's not for everyone and it's effectiveness depends on the type of motor (AC? Synchronous? Inductance? DC? Brushless? ) and the electrical "noise" it induces on the system. So.. I guess if an external ground wire worked for early National Panasonic DD's, it traditionally kept being employed with Technics turntables and if Thorens designed their classic AC synchronous motors to be quiet enough for a floating ground, they just kept going on with it. Why change something that works?
 
Because the gain is so high on a cartridge (40db for MM, 60/+ for MC), the metal in a turntable acts like an antenna, gathering EMF and RF from all kinds of sources (even the very equipment it is connected to) so we need to have a "noise drain". I really don't like the term ground as it has electrical connotations, like with power.


Exactly.

It has to do with the application of Thevenin's Theorem with the implementation of what could be called star grounding.

It (a separate ground wire) is a method to help reduce ground loops and unwanted common mode currents in the audio signal path, phono cartridge to phono stage input.

Remember that a signal does not just stop when it comes to a ground connection.

A much better and more accurate term would be return connection. For there to be a complete circuit, all current must have a path to return to the source. What is generally called the ground is actually the return path to form a complete circuit.

When the shield of one of the phono cables is used as a ground (return path) for the chassis of the turntable, there is the possibility of unwanted electrical noise, RFI/EMI that is picked up be the chassis to be imposed on the audio signal.

Because almost every implementation for the use of a turntable is likely to be different, there are any number of examples of turntables with and with out separate ground wires working okay. Application of Thevenin's Theorem will show why this is so. No smoke, mirrors or black magic involved.

As I have posted before, it is all about where the signal, whether it is the desired signal or unwanted electrical noise, comes from, where it goes, what happens to it along the way and how it gets home. This is a basic word picture of a complete circuit.





















 
Why do some manufacturers use a separate ground wire?
To keep chassis (mechanical) and signal (electrical) grounds separate. Theoretically, it's better that way, but grounding the TT chassis to the signal ground works, too. Technics chose to go with what's ideal rather than what's simpler, but it might just have been easier for them to do it this way.
 
Thank you for the very thorough answers. :)

To keep chassis (mechanical) and signal (electrical) grounds separate. Theoretically, it's better that way, but grounding the TT chassis to the signal ground works, too. Technics chose to go with what's ideal rather than what's simpler, but it might just have been easier for them to do it this way.

I expect the very low expense of having a separate ground connection also played a factor. That extra wire, two spade connectors and a screw-post probably amounted to a few extra pennies in material costs, so even if they could have gotten away without it, the savings would have been miniscule.

The next question is why did Technics decide to do a custom proprietary connection for the ground wire for the SL-BD22, replacement cords cost 30+ dollars on eBay (hence the cobbled-together solution that prompted this inquiry)
 
...why did Technics decide to do a custom proprietary connection for the ground wire for the SL-BD22
Because they could. Selling accessories could prove a lucrative business.
replacement cords cost 30+ dollars on eBay
See? That's why! eBay sellers figure out if it worked for Technics, it would work for them too and if it's quoted here, it probably will.

If it was me with that SL-BD22 I'd rip out that custom connection on general principal and hook up a standard ground wire at the expense of 20 cents and a five minutes soldering job.
 
s_r: MM, MC and MI cartridges as such just have plus and minus poles, of which the minus poles only turn into signal grounds in combination with a regular phono stage/section with unbalanced inputs. However, balanced amplification would be just as possible, in which case the minus poles have to stay pure minus poles and hence may not be combined with headshell-, tonearm- and chassis-ground (and ideally also not even generator-shell-ground in case of MMs and MIs). From that point of view separate grounding is the more universal grounding scheme. And another problematic case in combination with combined grounding would be the use of an MC step-up tranny, that doesn't sport a ground bridge between the primary and the secondary winding and would thus break the grounding path through the signal grounds.

Greetings from Munich!

Manfred / lini
 
The next question is why did Technics decide to do a custom proprietary connection for the ground wire for the SL-BD22
Good question. My SL-110 uses a binding post, so the proprietary treatment wasn't universal for Technics. Faced with the outrageous $30 charge for a replacement, I'd hack out the original socket and put in a post, myself.
 
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