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Topic: Turntable grounding question... (Read 6464 times) previous topic - next topic
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Turntable grounding question...

I currently have my turntable grounded via a separate ground wire (connected to the chassis and tonearm), which attaches to a grounding post on the phono stage.

I will soon be replacing the signal interconnects with some Canare mini Star Quad microphone cable (see link below), which has 4 internal wires and an outer braided shield.

I was wondering if I could get rid of the separate turntable ground wire, and just use the outer shield of the new signal cable instead?  If so, would I want to ground one end of the outer shield to the turntable chassis, and the other to the grounding post on the phono stage?

TIA...

P.S... I'll probably be using two separate Canare cables, and connecting it similar to the configuration shown here, (scroll down toward the bottom for illustration) with two of the four wires tied together in each cable.  This seems to be what Canare is recommending.

Turntable grounding question...

Reply #1 –
Fewtch, by no means am I a real turntable authority, but my impression is that with metal platters(Linn, etc)-don't worry about it, with nonmetals(rega, etc) if you're experiencing a static problem, you might try a non-felt pad(unfortunately more expensive and many claim detract from sound quality). With normal surroundings and equipment from the last decade or 2, I think real grounding/static issues might be a bit overemphasized amongst audiophiles, so before doing anything, see if you think it's going to be a problem with your setup(which sounds great). James

Turntable grounding question...

Reply #2 –
Quote
With normal surroundings and equipment from the last decade or 2, I think real grounding/static issues might be a bit overemphasized amongst audiophiles, so before doing anything, see if you think it's going to be a problem with your setup(which sounds great). James

In my case it isn't... when I detach the current ground wire from the grounding post on the phono stage, the noise floor (not playing any record) rises from ~ -88dB to ~ -60dB and a 60Hz hum is evident.  I don't know why you think these low signal levels (5mV or less) aren't susceptible to EMI/RFI (or that vinyl LPs aren't prone to static), but if you think so then you are wrong.  And BTW, my equipment isn't from the last decade or two.

Anyway, I would welcome any answers to the question -- can I use the outer braided shield on the signal cable (not in the signal path, just a shield) as the ground for the turntable?  And will this help in shielding the signal path from EMI as well?

It's strange... I've asked this question on three audio-related forums now and either the question was misunderstood or nobody could give a straight answer.  HA is my last hope for a definitive answer to this, otherwise I'll just have to experiment.

Turntable grounding question...

Reply #3 –
I had assumed you were planning to ground the turntable as a given, I should have been more explicit--I thought you were looking for alternative methods/enhancements for grounding, which I've heard few success stories about and lots of gripes about the results of mat changes. I've got a plain old Nad 533 with a decent goldring and never had problems, but older/superior setups often have transformer degradation, or in some shadier casesshipped from europe and converted to accept US currents to get an increase in sale price that's in multiples of the homecountry value.

sorry i couldn't help, good luck finding a solution though.(i should have read the message more carefully, I'm still not sure why you'd want to do that over using your traditonal grounding wire)

Turntable grounding question...

Reply #4 –
Quote
sorry i couldn't help, good luck finding a solution though.(i should have read the message more carefully, I'm still not sure why you'd want to do that over using your traditonal grounding wire)

Because the shield is there on the signal interconnects, and it's supposed to shield those interconnects from EMI when used (properly, but I'm not sure the exact way to use it in this application).

Turntable grounding question...

Reply #5 –
I am using the configuration that you wish to use...it works.

Note: 1. You need to consider the capacitance of the cable(Canare will give you the c/m) because Moving magnet cartridges are sensitive to line capacitance(typically 50-150pf). I think the cable you chose won't be suitable...but the method is good. You are correct about using 2 cables..the crosstalk would be terrible!

The Starquad configuration is not needed, look for Twisted pair+ Shield + Low capacitance.

          2. Treat the cartridge as 2 (left & right) coils, each channel then represents a balanced source

e.g for right channel: Red=hot+  Green=cold-
Check the base of your tt that the Arm ground(usually Black) is not terminated to the Green(usually) wire...it should join the Earth wire.

You now have 2 choices:

a) Best practice: The Earth wire is run seperate to shield between the preamp & tt and the shield of the signal wire is soldered at the tt end; at the preamp end is terminated via 0.1microfarad ceramic capacitor(this provides the rf path).

b) Terminate the shields & earth together at the tt and terminate the preamp end.

If you are looking for the 'ultimate', you will need to build a balanced line phono section to take full advantage of the cable system. Bear in mind that your tt may well be the limiting factor (mains tx, motor type etc.)

Still its good fun to play with analog and there is an awful lot of music out there that will never appear commercially and its worthwile doing your best.

Turntable grounding question...

Reply #6 –
Quote
I am using the configuration that you wish to use...it works.

Note: 1. You need to consider the capacitance of the cable(Canare will give you the c/m) because Moving magnet cartridges are sensitive to line capacitance(typically 50-150pf). I think the cable you chose won't be suitable...but the method is good. You are correct about using 2 cables..the crosstalk would be terrible!

Thanks... hopefully the Canare will work, since I already ordered it.  From what I'm aware of, the length matters most re: capacitance and I plan to keep it short (under 1 meter). 

I don't see why ordinary copper would be too high in capacitance for a MM cartridge at a relatively short length -- could you explain it?

Edit -- I found the page at Canare.com, and apparently the capacitance is 162pF/meter for the cable I chose.  What I'll do then is keep the length to about 0.75m, and shorten it further if necessary -- the turntable is sitting right next to the phono preamp, so that won't be a problem.  If necessary, I can bring the length down to 0.5m if treble response suffers.

Edit2 -- it looks like a 1-meter length may work OK... see this message:

http://db.audioasylum.com/cgi/m.pl?forum=v...=vinyl&n=226782

From other reading as well, it looks like 100pF to 250pF is the commonly recommended range with most MM cartridges... in which case, 1 meter of the Canare should probably work fine even given the added capacitance of the tonearm wiring (probably nominal given its short length).

P.S... could you explain this further:
Quote
a) Best practice: The Earth wire is run seperate to shield between the preamp & tt and the shield of the signal wire is soldered at the tt end; at the preamp end is terminated via 0.1microfarad ceramic capacitor(this provides the rf path).

(1) You say the shield of the signal wire is soldered at the tt end -- soldered to what?  The chassis?
(2) Terminated via 0.1uF capacitor to the phono stage grounding post?  And still running a completely separate ground wire in addition to this?
(3) I'm more concerned about EMI from the turntable motor and transformer; RFI is not a problem here.  Would I want to terminate the preamp end of the signal cable shields directly then?  If so, I assume that would make running a separate ground wire unnecessary... correct?

Sorry, that's a little confusing... if you could explain further, it would help.

Turntable grounding question...

Reply #7 –
Quote
I don't see why ordinary copper would be too high in capacitance for a MM cartridge at a relatively short length -- could you explain it?


The twisted pair brings the conductors closer(+ longer) together thus increasing the capacitance(mutual coupling). The material of the conductor doesn't matter(apologies to the Silver Wire People). Remember that the object of a twisted pair is that both conductors 'see' the same external influence.

Quote
From other reading as well, it looks like 100pF to 250pF is the commonly recommended range with most MM cartridges... in which case, 1 meter of the Canare should probably work fine even given the added capacitance of the tonearm wiring (probably nominal given its short length).


Cartridge manufacturers specify optimum capacitance value, there is no need to work to a range. The link is correct in that the affect is in the hf, the problem is that the factors involved are phase, ringing and response linearity, not just roll-off.
To complicate matters, the 47k load is nominal at best (exception of highend preamps). As always, it is easier to start with less where you can then add extra to optimise the performance. Check the circuit of your phono section, sometimes the manufacturers stick a small bypass cap across the input(used as rf bypass)

See if you can locate a test record with white noise, pink noise & impulse signals.You can then get the best result possible out of your phono stage.

Quote
(1) You say the shield of the signal wire is soldered at the tt end -- soldered to what? The chassis?


Sorry, I would use a solder-eyelet & screw onto chassis, arms have varying methods for ground connections, preferably the same point as the Black arm wire.

Quote
(2) Terminated via 0.1uF capacitor to the phono stage grounding post? And still running a completely separate ground wire in addition to this?


Yes, 0.1uf to the post.......Yes, a seperate wire between post & tt (academic anyway as you should use(b) ..see below.

Quote
(3) I'm more concerned about EMI from the turntable motor and transformer; RFI is not a problem here. Would I want to terminate the preamp end of the signal cable shields directly then? If so, I assume that would make running a separate ground wire unnecessary... correct?


Yes, follow (b)

PS: Hope you don't have a Rega 2! the -88db is the best you are going to get out of that( terrible motor sheilding)

Turntable grounding question...

Reply #8 –
Quote
Yes, follow (b)

PS: Hope you don't have a Rega 2! the -88db is the best you are going to get out of that( terrible motor sheilding)

I've gotten a recommendation on another forum to keep the separate (turntable to phono stage) ground wire, and just ground the shield of the signal cables to the tt chassis (leave it floating on the other end) to prevent ground loops.  This would be my preferred choice, if grounding only one end of the shield is enough to bleed off EMI... it'll look less ugly that way, without separate wires coming out of the RCA plug terminations.

I have a Thorens TD-160, which is probably a little better than a Rega 2 (but not much... it uses a synchronous AC motor).  Funny thing is, I get a hum when the motor is off which disappears when the motor is turned ON.  Needless to say that's not a problem, but seems strange nonetheless.

Turntable grounding question...

Reply #9 –
Hum in turntables is something of a black art.

You really have to try all the combinations.  One likely will work.  Don't forget that sometimes just flipping the power cord to the turntable will make a big difference in one polarity versus the other.

I have had some TT's that need a ground wire, some are quieter without one.  Some like the shield floated at the pre-amp, some don't.  Some needed an extra ground wire to the arm.

Turntable grounding question...

Reply #10 –
Quote
I've gotten a recommendation on another forum to keep the separate (turntable to phono stage) ground wire, and just ground the shield of the signal cables to the tt chassis (leave it floating on the other end) to prevent ground loops. This would be my preferred choice, if grounding only one end of the shield is enough to bleed off EMI... it'll look less ugly that way, without separate wires coming out of the RCA plug terminations.


That is my described method 1, without the ceramic cap you may end up with an antenna! Remember that rf manifests itself as hum in some phono stages.

If you are deeply interested in grounding techniques you should chase up the archives of the AES (Audio Engineering Society). There was a series of papers by a Mr.Dilley from Spectrasonics (Salt Lake City, Utah) published in the middle 70s.

From memory, it gave a mathematical analysis of many topologies as implemented in Recording Studios and the Broadcast environment.It doesn't hurt to see how the pros do things.

Good luck

PS: I have a TD125 as well as the Rega, with SME Type III + B&O MMC20CL which I use for voice + piano

Turntable grounding question...

Reply #11 –
I'm even more confused now... someone on another forum recommended I tie the inner conductors together to use as (+), and use the outer braided shield as (-).

Can anyone tell me which would be more likely to reduce EMI and/or RFI at these low signal levels:

(A) Using the outer shield as the (-) in the signal path, or
(B) Using a cable with two inner connectors (+ and -), and grounding the outer shield to the turntable chassis

Thanks to anyone who can help, my head is really spinning now... 

Turntable grounding question...

Reply #12 –
Won't it be possible to try it when you get it?


In my limited experience, testing with the equipment you have is the best thing to do.

I'll give you a for instance: I have one turntable where the best results come from taking the separate earth wire (from the tt chasis) and attaching it to the phono (RAC) plugs at the amp, and not connecting anything to the earth post on the amp. I have another tt which works better when connected as you would expect.


As for: can you use a spare signal or ground connector for that extra earth wire? I don't see why not, as long as it's not in electrical contact (or capacitatively coupled to) a wire you are using. I'd try using an unused inner conductor (if one is available) and you want to dump the extra wire. If it works, it works - if it doesn't, it doesn't!

Cheers,
David.

Turntable grounding question...

Reply #13 –
Quote
I'm even more confused now... someone on another forum recommended I tie the inner conductors together to use as (+), and use the outer braided shield as (-).


= Single core shielded =status quo

Quote
(A) Using the outer shield as the (-) in the signal path,


= as above

Quote
(B) Using a cable with two inner connectors (+ and -), and grounding the outer shield to the turntable chassis


= The method we have been discussing

Draw the suggestions on paper...its much easier to see the differences then.
If all else fails, use a notch filter in your wave editor. IMHO rfi is/was more insidious as it creates unpredictable anomalies.

A long, long time ago there was an audio mag called Audio Amateur..it may be worthwhile ferreting them out..you are covering territory that was nuked in 70's

Turntable grounding question...

Reply #14 –
Shelds work best if there is no voltage across them.  Since the resistance is non-zero,  you don't want it to pass any current other than what comes from it's sheilding function.  Hence the separate ground wire.

Turntable grounding question...

Reply #15 –
Thanks everyone.  I cancelled my order for the Canare Star Quad cable (after realizing it's really intended for balanced sources, and educating myself more on the differences between balanced and unbalanced sources), and am re-thinking how (or if) I'm going to rewire the table.  What I want to do is "get it right" (experimenting will be difficult since soldering is required on both ends), and I have to check on other things also like cable diameter... there's not much clearance at the bottom of the turntable where the cables come out.

Turntable grounding question...

Reply #16 –
Quote
Shelds work best if there is no voltage across them.  Since the resistance is non-zero,  you don't want it to pass any current other than what comes from it's sheilding function.  Hence the separate ground wire.

This is true.  The optimum configuration for a differential measurement has the shield(s) tied at the receiving end only, and a separate wire for grounding the chassis to earth.

For instance:
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