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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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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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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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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.
Old 3 weeks ago
  #14
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Quick question: I've determined the cable from a Mackie 1402 to our power amps (70v system) is the culprit for the "buzz" we are experiencing over the system. When that cabling is unplugged the system is silent, when listening to the output of the 1402 on phones the signal is quiet. I am assuming 1402 directly out to an amp w/speaker will be quiet also, but plan to check anyway. The 1402 can do balanced/unbalanced out; the amps are unbalanced in only; roughly 20' cable run to the amps. I want to do this right the first time and it seems that John's wiring suggestion (option 5) is the one I should go with on this. Planning on using Canare star-quad for the heck of it. Is my choice on wiring TRS/XLR to TS correct for my situation? Thanks for any thoughts offered!
Old 3 weeks ago
  #15
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Quote:
Originally Posted by THX1136 View Post
Quick question: I've determined the cable from a Mackie 1402 to our power amps (70v system) is the culprit for the "buzz" we are experiencing over the system. When that cabling is unplugged the system is silent, when listening to the output of the 1402 on phones the signal is quiet. I am assuming 1402 directly out to an amp w/speaker will be quiet also, but plan to check anyway. The 1402 can do balanced/unbalanced out; the amps are unbalanced in only; roughly 20' cable run to the amps. I want to do this right the first time and it seems that John's wiring suggestion (option 5) is the one I should go with on this. Planning on using Canare star-quad for the heck of it. Is my choice on wiring TRS/XLR to TS correct for my situation? Thanks for any thoughts offered!
If the power amp is unbalanced, the star-quad is a moot point. Also, that's likely to hum just like it does now.

However, if you wire the 1402 balanced, TRS, (star-quad if you must, but it won't matter) to XLR-M then go through one of these just before the power amp, your world will be a quiet place.
Old 3 weeks ago
  #16
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Forgot to mention the cable that is currently in place is standard speaker wire - no shield - just 2 conductors. That's why I was thinking a shielded cable would eliminate the noise.

Would that change your thoughts on the situation, Jaddie? Unfortunately the link you included goes nowhere even with my popup blocker off. I'm assuming you might have been suggesting something with an isolation transformer.

Thanks for you reply!
Old 3 weeks ago
  #17
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Quote:
Originally Posted by THX1136 View Post
Forgot to mention the cable that is currently in place is standard speaker wire - no shield - just 2 conductors. That's why I was thinking a shielded cable would eliminate the noise.
Not likely in this case. Perhaps minimally.
Quote:
Originally Posted by THX1136 View Post
Would that change your thoughts on the situation, Jaddie? Unfortunately the link you included goes nowhere even with my popup blocker off. I'm assuming you might have been suggesting something with an isolation transformer.

Thanks for you reply!
Let me try that link again:
https://www.amazon.com/ART-DTI-Trans...s%2C159&sr=8-2
Old 3 weeks ago
  #18
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Thanks for your reply, Jaddie. The link worked fine this time. If I went with the ART-DTI could I just skip the expense of mic cable, use the existing speaker wire (18/22 ga. I think) and use instrument cable (or the existing speaker wire) from the amp patch panel to the ART then to each of the 3 amps? The reason I ask is it looks like the ART-DTI accepts both balanced and unbalanced input. Thank you for your patience and willingness to help.
Old 3 weeks ago
  #19
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Quote:
Originally Posted by THX1136 View Post
Thanks for your reply, Jaddie. The link worked fine this time. If I went with the ART-DTI could I just skip the expense of mic cable, use the existing speaker wire (18/22 ga. I think) and use instrument cable (or the existing speaker wire) from the amp patch panel to the ART then to each of the 3 amps?
Should work, though good practices are to not use speaker wire for anything but speakers.
Quote:
Originally Posted by THX1136 View Post
The reason I ask is it looks like the ART-DTI accepts both balanced and unbalanced input.
Yeah, sort of. The XLR inputs are obvious (though they could be wired unbalanced by shorting 1-3), the 1/4" jacks would be TRS, so to use them unbalanced you'd want to plug in a TS plug or wire R-to S.

This is a transformer box, it won't work if you just feed pin 2 / Tip and ground pin 1/ sleeve. The other leg of the transformer winding must also be grounded or it won't work. You have to tie Ring to Sleeve (which is what happens with a TS plug) or wire the XLR with pint 3 and 1 tied together.
Old 3 weeks ago
  #20
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If you have to use the existing "speaker" cable, you can still wire it balanced. Just use XLRs at both ends, pins 2 and 3, with pin 1 not connected. That will work fine for line level, and RF will almost certainly not be an issue (Hum is the issue now, not RF, right?).

Using paired, shielded cable is the standard way to do this, but best practices dictate that you have a proper balanced receiver (like the ART box) at the end of the run before going into the unbalanced amp input. Since the output side of the ART has your choice of XLR, TRS and phono jacks, just use a [short] standard cable that matches whatever the amplifier has. If the amp has a phono jack input, use a phono-to-phono cable, etc.

Geoff
Old 3 weeks ago
  #21
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This isn't all fancy technical stuff, but hi-fi interconnect cables use two conductor-shaped conductors and a shield grounded on one end doing nothing but shielding. Because conductors designed to be conductors conduct better than using a shield, which isn't particularly designed to be a conductor, as one of the conductors.

Cable deniers will dismiss this. Go ahead.
Old 3 weeks ago
  #22
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Quote:
Originally Posted by lame pseudonym View Post
This isn't all fancy technical stuff, but hi-fi interconnect cables use two conductor-shaped conductors and a shield grounded on one end doing nothing but shielding. Because conductors designed to be conductors conduct better than using a shield, which isn't particularly designed to be a conductor, as one of the conductors.

Cable deniers will dismiss this. Go ahead.
If you’re trying to say that the shield of a shielded cable isn’t as good a conductor as the inner conductors, that’s incorrect, in fact exactly opposite.

As one example, Canare L-4E6S has a conductor resistance of <9.8 ohms per 100m, but the shield has a resistance of <3 ohms per 100M. Lower resistance means better conductor.

THe data is found here:
http://www.canare.com/ProductItemDis...oductItemID=53
Old 3 weeks ago
  #23
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That's not what I'm talking about, but my work is done here.
Old 3 weeks ago
  #24
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Quote:
Originally Posted by lame pseudonym View Post
That's not what I'm talking about, but my work is done here.
And off he goes spreading disinformation on the net. Jaddie is correct, with facts to prove it.
Old 3 weeks ago
  #25
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Quote:
Originally Posted by Radardoug View Post
And off he goes spreading disinformation on the net. Jaddie is correct, with facts to prove it.
No! You mean people actually do that???
Old 3 weeks ago
  #26
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I know that it's a mistake for me to reply to kids who think that they know stuff, but holy cats. Last month there's a guy who thinks he can judge the sound of capacitors by giving them a 1930's harmonic distortion test. Now I'm talking to a guy who thinks that conductivity is what makes a hi-fi cable hi-fi.

It's like generations of work and experience in material science and physical design is getting lost.

There is no more common scenario than that of kids who know their ABCs, and aren't real good at the C's, who think that they know how stuff works. As a kindly old man, let me let you in on something: There's a whole world of knowledge and experience out there that you don't even know exists.
Old 3 weeks ago
  #27
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Quote:
Originally Posted by lame pseudonym View Post
I know that it's a mistake for me to reply to kids who think that they know stuff, but holy cats. Last month there's a guy who thinks he can judge the sound of capacitors by giving them a 1930's harmonic distortion test. Now I'm talking to a guy who thinks that conductivity is what makes a hi-fi cable hi-fi.

It's like generations of work and experience in material science and physical design is getting lost.

There is no more common scenario than that of kids who know their ABCs, and aren't real good at the C's, who think that they know how stuff works. As a kindly old man, let me let you in on something: There's a whole world of knowledge and experience out there that you don't even know exists.
Yeah, these kids today. Idiots. Amateurs. What could they possibly know?

But then, I too might have picked up a few bits and pieces in the last...lemme see....50 years in audio. Might.

If there's something you specifically have to contribute, go for it. Otherwise, all you've done is say the facts are wrong without saying why you think so. If you have vast knowledge to share, blast away. I love to learn new things, even at my age. For example...what do you think actually makes a hi-fi cable hi-fi? You haven't said. If you wan't the kids to absorb your wisdom you first have to share it.
Old 2 weeks ago
  #28
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only 50 years...

JR
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