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Using twisted pair shielded cable for UNbalanced connections
Old 8th February 2012
  #1
Using twisted pair shielded cable for UNbalanced connections

Hi all,

I have a lot of leftover twisted pair shielded cable from when I rewired my composing rig. Really nice stuff from a surplus shop, silver-plated, teflon insulated, low capacitance. It works beautifully for my balanced connections. But now I am wondering if I can use it for unbalanced connections too.

What is the best connection scheme for soldering it to TS plugs in order to minimize the effects of capacitance and maximize the signal integrity?

Option 1) Both conductors to tip, shield to sleeve.

Option 2) 1st conductor to tip, 2nd conductor and shield to sleeve.

Option 3) 1st conductor to tip, 2nd conductor to sleeve, leave the shield disconnected.

Option 4) 1st conductor to tip, 2nd conductor disconnected, shield to sleeve.

Or, should I just forget it and get some decent coax? Thanks!
Old 8th February 2012
  #2
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JohnRoberts's Avatar
 

Option 5-
Lead 1 to Tip
Lead 2 to Sleeve
Shield to Sleeve, but only connected at one end.

For high RF environments consider cap coupling shield at other (open) end.

This way hopefully shield noise will not flow in the audio signal leads. Plug in the hard grounded shield end to the unit with most robust chassis ground.

JR
Old 9th February 2012
  #3
Aha, that's brilliant John! Thanks.
Old 10th February 2012
  #4
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Robo's Avatar
Thanks, I need to do this myself. I just wonder whether Option 3 is still not better than Option 5 assuming you don't use the cap coupling at the floating end technique and/or know which unit would have the most robust chassis ground. If using Option 3, ie leaving the shield unconnected at both ends, the shield would still block some noise wouldn't it, and you also wouldn't be connecting the shield into the audio signal path. But maybe there is an advantage to Option 5 I don't fully understand.

I found another thread in which EveAnna Manley and Monster cable also suggest Option 5 (without cap coupling at one end):

Monster cables unbalanced theory

It's a good read although the OP's posts are a bit redundant.

I checked out a reactance chart (nomograph) and if I'm reading it correct a low impedance cable (75, 100, 150 ohms) even at quite high capacitances doesn't start rolling off until quite high frequency. Like, high enough for anyone recording at 44.1khz not to worry about. I may be wrong though but looks that way from what I can see.

Cheers
Old 10th February 2012
  #5
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Quote:
Originally Posted by Robo View Post
Thanks, I need to do this myself. I just wonder whether Option 3 is still not better than Option 5 assuming you don't use the cap coupling at the floating end technique and/or know which unit would have the most robust chassis ground. If using Option 3, ie leaving the shield unconnected at both ends, the shield would still block some noise wouldn't it, and you also wouldn't be connecting the shield into the audio signal path. But maybe there is an advantage to Option 5 I don't fully understand.
No... If the shield is floating at both ends, any noise will just move the shield like a leaf floating in the breeze. How much wind did the leaf stop? The floating shield will pick up noise and then retransmit it to the internal conductors.
Quote:
I found another thread in which EveAnna Manley and Monster cable also suggest Option 5 (without cap coupling at one end):

Monster cables unbalanced theory

It's a good read although the OP's posts are a bit redundant.
The cap is a hybrid grounding approach to make the shield more robust at stopping RF, while still NOT providing a low impedance path at mains frequency that could cause loops or unwanted ground contamination.

Note: I didn't make this stuff up. I read it in a book about grounding and shielding a few decades ago.
Quote:
I checked out a reactance chart (nomograph) and if I'm reading it correct a low impedance cable (75, 100, 150 ohms) even at quite high capacitances doesn't start rolling off until quite high frequency. Like, high enough for anyone recording at 44.1khz not to worry about. I may be wrong though but looks that way from what I can see.

Cheers
For audio interfaces all (most?) well designed gear will have a finite source impedance (build out resistors), so the dominant cable frequency response effect will be a simple LPF formed by the build out R and the cable C.

Characteristic cable impedance for modest length audio cables is mostly insignificant at audio frequencies. Even DCR is not very significant except for speaker cables where the current can cause IR voltage losses in long speaker runs. For passing RF and Digital signals the wavelengths are short enough wrt cable length, that characteristic cable impedance and proper terminations do matter (reflections inside the cable interfere with signals).

JR
Old 22nd October 2017
  #6
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Quote:
Originally Posted by JohnRoberts View Post
For audio interfaces all (most?) well designed gear will have a finite source impedance (build out resistors), so the dominant cable frequency response effect will be a simple LPF formed by the build out R and the cable C.
Yes. Standard source impedance for audio line level is in the 100 to 600 ohm range, and load impedance is 10K ohms or higher (some older gear as much at 100K).
Quote:
Characteristic cable impedance for modest length audio cables is mostly insignificant at audio frequencies. Even DCR is not very significant except for speaker cables where the current can cause IR voltage losses in long speaker runs. For passing RF and Digital signals the wavelengths are short enough wrt cable length, that characteristic cable impedance and proper terminations do matter (reflections inside the cable interfere with signals).
Yep. This is why we don't "match impedance" for line-level audio. So cable impedance is a non-issue. For more, see http://www.rane.com/note126.html . I think they are a little cavalier on this point; what I learned was that you should start thinking about transmission lines when your cable length was only about 1/10 of the wavelength. (i.e. if a 10th of a cycle or more will fit in your cable.) But that still means you're not worrying about it for audio unless you're working on analog phone lines.

Regarding HF rolloff, here's a real-world example: I'm running 50 feet of Belden 8761 STP between balanced output (ok, it's only impedance-balanced) and balanced input here. Belden shows this stuff to be 47 pf/ft in that configuration. The output impedance of the mixer that's driving it is 120 ohms. If you work the formula from the Rane article, you'll find that the 3dB down point is up around 500 kHz! I'm not losing any sleep over that.

One other point that no one has mentioned: Using twisted pair has a real benefit even for unbalanced connections: The twisting reduces the magnetic loop area of the cable. This reduces inductively-coupled noise pickup. It doesn't do a thing for capacitively-coupled noise (ie electrostatic, ie most RFI); that's what the shield is for. And inductively-coupled noise is usually the lesser problem, unless you have a LOT of current flowing through AC power wiring near your cables. But STP is cheap and readily available. Certainly, if you are having an inductive-noise problem, it's an easy thing to try.

n.b.: The folks over at Blue Jeans Cable say otherwise. See http://www.bluejeanscable.com/articles/balanced.htm . They're correct that twisted-pair gives no common-mode noise rejection when used with an unbalanced input, but then they conclude that it has no benefit at all with an unbalanced input, that coax is superior. They're mistaken. They are also correct that such cable, used that way, will have a higher capacitance than simple coax, but you could add a lot of C to the cable before HF rolloff becomes a problem.

Last edited by RickBrant; 22nd October 2017 at 04:27 AM.. Reason: added detail
Old 22nd October 2017
  #7
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I'm not sure but after 5 years the OP may have moved on... (thread necromancy?)

JR
Old 22nd October 2017
  #8
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Quote:
Originally Posted by JohnRoberts View Post
... after 5 years the OP may have moved on...
Well, it's about telescoping shields. So you'd expect it to have a long reach.
Old 23rd October 2017
  #9
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Quote:
Originally Posted by Jay Rose View Post
Well, it's about telescoping shields. So you'd expect it to have a long reach.
And be diffraction-limited in its ability to provide useful data . . .
Old 3rd July 2020
  #10
This is still the best thread on this topic. There's another on GroupDIY where John reinforces the same concept, which is great. I have a feeling that a lot of people finding this thread are doing something like what I'm up to...

I'm getting ready to do a bunch of in-console unbalanced wiring using twisted pair with a foil shield and a drain wire. This is a 4.0mm 100Ω cable with a very low core-to-core capacitance -- Canford FST. But you could use Belden 9451 the same way. Typically, the cold and the shield will be tied at the source end, and the shield will float at the load end.

I'm wondering if anyone has tried the tie-the-shield-to-the-other-end-with-a-small-capacitor approach in a high RF environment, and determined that it helped. I'm mostly curious what value was used, and if a simple C0G MLCC would be sufficient.

I'm also wondering if there's a major reason to veer towards a spiral or braided shield in this scenario, as it's really quite a pain in the butt to deal with anything other than a foil shield and drain wire for in-console work (no matter how much everyone loooooooves Mogami). I haven't really looked into W2944.

As an alternative to twisted pair, I did find a couple of coaxials that might work: Belden 8417, and Belden 9264. The catalog does give a nod to their audio application. Both conductor and drain are tinned, which is nice. 9264 is...uh..oval shaped. 8417 is grey, sandwiches the drain between two foil shields, and has a paper wrap. Both are in the neighborhood of 3.5mm O.D.
Old 3rd July 2020
  #11
The answer to many of these questions depends on what kind of interference challenge you're trying to mitigate or avoid. A foil shield provides 100% coverage, as opposed to 75-95% for a braided shield. This provides a shielding advantage for very high frequency electric fields, but it comes at the cost of higher shield resistance, which causes a disadvantage for lower frequency interference. Cable manufacturers typically add a drain wire for ease of termination. Although this lowers the shield resistance at low frequencies, it's also known to reduce common-mode rejection of inductive pickup because the current induced in the shield flows asymmetrically (mostly in the drain wire) and this reduces the effective balancing of the twisted pairs you're trying to protect. The other caution about foil shielded cable is that it's only appropriate for stationary installation, because it won't withstand frequent flexing. Spiral "served" shields can cause similar problems current distribution problems, which some manufacturers try to mitigate by making the shield spiral the opposite direction of the pair twists. Such a shield tends to have higher inductance than a braided shield, as well. The relative ease of termination compared to braided shields is a big attraction however. But a disadvantage is that the shield can develop gaps if the cable is abused, say, by rolling a piano over it. For really critical applications, there exist cables available that have both braid and foil shielding. These are absolute murder to terminate, so most people avoid them unless they're truly needed.

My recommendation is to clearly understand the particular RF challenges your installation faces (not forgetting handheld personal devices) and make your cable choice appropriately. How long are the cable runs? Is there low impedance technical grounding to prevent ground-difference currents circulating in the audio shields? The choice of whether to connect one or both ends of the shields rests mostly on this. But where there is potential interference such that the cable length becomes a significant fraction of the interfering wavelength, then capacitive coupling of the "lifted" end is recommended. The exact capacitance value isn't too critical, but it must have low impedance at the challenge frequency, and high impedance at power line frequency. It should also be large compared to the total (i.e. full length) shield to signal wire capacitance.

David L. Rick
Old 4th July 2020
  #12
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jaddie's Avatar
 

A few things to think about:

1. The core to core (conductor to conductor) capacitance is not the same as the core to shield C, which may be lower. It may make more sense to float the unused conductor of a shielded pair for unbalanced wiring in a console.

2. Shielding, even with 100% coverage, is only part of RFI proofing. The other part is good output circuit design and RF immune input circuit design.

3. David noted, a thorough understanding of the interference you are primarily dealing with is essential. If your desk is to be used in a high RF field produced by a broadcast transmitter, you need to remember at what frequencies those operate. Same for mobile devices, etc. And along with frequency, field intensity.

And...my anecdote: many years ago I built a complex array of studio in an office building that faced a major broadcast transmitter center. Before starting construction I measured RF field intensity in the space at all known high power transmitter frequencies. The intensity was greatest in the 50 mHz band, US TV channel 2. In retrospect it was most likely due to some sort of resonant cavity effect, but we anticipated having RFI issues, so the solution was to enclose the entire technical core in RF shielding, and filter every pair of wires passing into and out of that shielded area. It was expensive, and effective. The offending TV station was not even receivable in the completed space. Overboard? Absolutely! Necessary? Not entirely, but you know, you get your one shot, you can't screw up, the company had money, and we, the engineering staff, didn't have enough experience to take a chance on something less intensive.

So we didn't have RFI issues at all, but we did have ground loop issues, resulting in furthering my personal understanding of balanced line receivers, and common mode rejection in the real world. We had two consoles that were completely unbalanced. That worked fine within their own control rooms, but not interconnecting to the rest of the system. We had purchased a flock of power amps that had a poorly designed balanced input with only 40dB of CMRR, which we had to modify to make work with long cable runs.

The lesson learned was that you have to consider the entire picture and not obsess about one small portion of it. The process of choosing the cable to use should include a study of ease of termination, diameter, and flexibility as the total cost of the cable should include installation time. Including of course the electrical properties.
Old 4th July 2020
  #13
Impedance matching asymetrical balanced wiring is another option. Determine the loading resistor value on the unbalanced send. Wire that same value resistor from pin 3 to ground or from ring to ground on TRS jacks. Then you get noise immunity to a balanced destination from an unbalanced source.
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