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Old 22nd August 2008, 03:30 PM   #1
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Default Re-wiring to balanced (tonearm)

Quote:
"My JVC L-A11 has no ground wire either. But due to it not humming, it must be using the RCA jacks sleeve as a ground."
...got me thinking about my re-wire project on an MG-1 airbearing.

Technically a phonograph cart is balanced; +/- for both left and right as you discuss in one of your FAQs.

Quote:
"A phono cartridge's electrical output is inherently balanced!!! Balanced means "plus and minus," not "signal and ground" (called single-ended or unbalanced). Most turntables convert this to single-ended (the "minus" wire is connected to the cable shield), unnecessarily throwing away the noise rejection and all other benefits of balanced wiring! Such turntables, however, will have "standard" shielded cables that will have to be replaced with balanced wiring."
My last tonearm was wired into an RCA box directly with the litz wire, and I needed to ground the arm with another 5th litz wire.

My current table terminates the leads with a connector (like an S-video plug) on the chassis, and then a new set of wires goes to the RCA box; this pair is terminated like any unbalanced RCA with L- to outer and L+ to center, etc.

So the question I have is the signal & ground of unbalanced a true Earth ground if you have one piece of equipment in the chain earth grounded? IIRC this unbalanced "signal and ground" daisy chains the units with regard to ground? Or maybe that's balanced because pin 1 is a dedicated ground between units, hence ground loop and floated shields sometimes used in pro-audio as a remedy...?

I'm going to use a flat pre that has balanced inputs. I was planning on terminating my existing wires (not the tonearm wire which terminates at S-video plug) with XLRs and connecting L-/+ to pin3/2 and R-/+ to pin 3/2 and using some 89259 copper braid shield which would terminate on pin 1 of each XLR. I had planned on floating the shields on both balanced cables at the tonearm chassis end (I could later ground them to the chassis if need be).

Am I go?

best,
DC
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Old 26th August 2008, 02:47 PM   #2
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Regarding a suggested resistor addition:

"The only remaining (and minor) issue is cartridge loading. Fortunately, moving-coil cartridges are relatively insensitive to loading. Most microphone preamplifiers have an input impedance of 600 ohms - close enough to the typical 100 ohm load. If this seems bothersome (it won't hurt anything), it's relatively easy for anyone even remotely handy with a soldering iron to incorporate a (1 percent or better tolerance metal film type) 100 ohm resistor across the signal leads, inside the cable connector (one resistor each for the right and left channels). (Okay, with a 600 ohm preamp impedance the correct value resistor would be 120 ohms; but this small difference in resistance won't make much difference in the sound - provided the load resistance is the same for each channel, of course.)"

I have read that MC (or MI from Grado) can actually work fine (and often perform with less ultra-sonic ringing) at 1K. I have a flat gain pre with 36K. This 120ohm mod seems like it is simply for when a a lower (i.e. 600ohm input impedance) is encountered?

Also, do you know if current travels down both wires (pin 2 & 3_ when a tonearm is wired balanced? Some have argued that it doesn't work this way. I have used wiring that is not strictly balanced geometry in that it is a coax with shield and covered center conductor INSIDE the first wire; they are nota twisted pair. This is a short run and has a braided shield over the entire thing which is grounded to pin1 and the chassis of the tonearm. I have yet to hook up my flat pre, but do you think this wire geometry would be detrimental?
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Old 26th August 2008, 04:04 PM   #3
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Doc, a twisted pair inside a braided shield is much closer to optimum. I found that shielded CAT5 with all of the pairs paralleled worked beautifully.

Each wire of the twisted pair does carry AC current. The voltages are anti-symmetrical with respect to ground, i.e., if the instantaneous voltage on conductor 1 with respect to ground is v, the voltage at the same instant for conductor 2 is -v. And the currents must be equal and opposite by symmetry.
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Old 11th October 2010, 11:51 AM   #4
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Default best balanced tonearm wiring

I have decided to try balanced tonearm configuration as my phono amp has balanced inputs, is it necessary to use screened cable even if the length is short, 400mm? I have made up a pair of leads using microphone cable, ie: +/- and screen to ground, but I would prefer to use 3 separate teflon coated silver without screening for ease of manufacture. Any thoughts on this. Thanks in advance.
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Old 11th October 2010, 12:50 PM   #5
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Quote:
Originally Posted by Hank Bakker View Post
. . . I would prefer to use 3 separate teflon coated silver without screening for ease of manufacture. Any thoughts on this.
Iíve been using teflon-coated silver-plated wires with no screening to connect an arm to a balanced-input RIAA pre-amp for a dozen years or more. (The wires are lightly twisted.)

If there is any hum, I canít hear it. Well worth a try - it's not expensive.

HTH
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Old 12th October 2010, 09:25 AM   #6
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Thanks Rylands, I'll give it a try; I guess ditto for balanced from pre to power amp.
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