I have a 3 conductor wire that are all wrapped in a foil shield with wire conductor, for a total of 4 wires.

I am connecting the 3 inside wires to audio left, right, and ground. This is an unbalanced setup, right?
In this case, should I be shielding the left from the right as well? what about audio ground?

Where/how do I connect the shielding?
Do I connect both sides (i think no?)? Which side? To where?
My choices for where to connect the shield: audio ground, chassis, nothing

I would like to avoid all ground loops and noise.

If I felt like putting on some EMI protection and have some braided EMI shielding roll, should I bind the EMI shielding to the foil shield to make one shielding conductive layer or should I separate the shield layers with insulation? any pros and cons to each method?
Do I connect both sides (i think no)? Which side? To where?
My choices for where to connect the shield: audio ground, chassis, shield (wherever that may be connected to), nothing

I feel like I've seen some manufacturers with multi-layer shielded wires that have separated layers, others that are together, and some that have a combination. What are the benefits?

Please make the distinction between EMI & RFI, since most of the colloquial speech involves using EMI as a catchall for both EMI & RFI, but as I understand it, they are two different things with two different requirements and it can be confusing to me.
If you believe foil is "better" than braided, please provide facts as to why and how, as it is my belief/understanding that they are each designed for different purposes and would like to understand your reasoning.

I have no electrical engineering degree, so I could be wrong.

  • \$\begingroup\$ Rane Note 151 would be a good starting place for you. \$\endgroup\$ – Matt Young Feb 14 '15 at 20:36
  • \$\begingroup\$ i have seen it (and saved it) but not really read it. i will read it in-depth. \$\endgroup\$ – Zero Feb 14 '15 at 20:38
  • \$\begingroup\$ You're using the wrong cable. You should be using twin shielded pairs, not for ground loop reasons but to minimise crosstalk. \$\endgroup\$ – user207421 Feb 15 '15 at 0:36
  • \$\begingroup\$ @EJP Are you proposing that I should be changing my setup from unbalanced to balanced and then back to unbalanced again? \$\endgroup\$ – Zero Feb 15 '15 at 3:10
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    \$\begingroup\$ No. What EJP is saying is that with the L and R wires right next to each other, they can induce cross talk. If you're listening to music you might not notice but if you're really trying to minimize noise, you would want to use separate shields for L and R. And if there are significant currents like for headphones, you probably should just use separate wires to use the jacket to add distance between the L and R cores. That is why a lot of headphone wires are actually two separate wires. Cut open some old headphone wire and I think you'll fine they have separate shields. \$\endgroup\$ – squarewav Feb 15 '15 at 16:38

Your cable can work well if you don't have any ground loops. That is: you are passing your audio and their ground wire inside your shielded cable. If the two pieces of equipment are otherwise floating with respect to each other, this will work well.

However, if there are other ground connections between the two pieces of equipment in addition to this cable, you run the risk of a ground loop.

Note that if you do run into a ground-loop problem, it can be fixed by adding isolation transformers in-between the equipment. I do this regularly when dealing with ground problems in professional broadcast intercom systems.

The shield should be grounded at one end only. There is on-going discussion as to the best place to ground the shield: at the source or at the destination. There are valid reasons for both. My preference is to ground the shield at the destination. This ensure that the cable is still shielded should it be unplugged from the source.

Note that when I say 'ground the shield', you would either connect it to the chassis ground connection of the equipment OR connect it to the ground pin of the audio signal you are passing. FWIW - I rarely connect the shield to the chassis for audio signals.

When I'm working with XLR connectors, I almost always ground the shield to pin 1 of the connector rather than the shell because some equipment doesn't allow for grounded connector shells. RTS intercom BP-325 belt packs are a perfect example of this: the connector shell is a part of the molded plastic enclosure and there is no shell connection.

On the other hand, RF signals are the opposite. The shield is almost always connected to the chassis or shell for cables, connectors, and signals.

In other words, it depends on what you are doing.

Regarding adding a braid over top of the foil shield: you are wasting your time. Most braid shielding has anywhere from 60% to 90% effective shield coverage whereas foil shield is a solid 100% coverage.

Finally, some coax cables have multiple shields. Television Video camera cables known as "Triax" are a good example: the two shields are insulated from each other. The outer shield is Ground, the inner shield is DC Power to operate the camera, the center conductor carries the signal.

Another coax cable that has two shields is the soldering wands (RM3E) for Metcal MX-500 solder stations. There, the double shield is used to minimize RF leakage from the cable. Both shields are grounded at one end and I'm pretty sure that the outer shield is floating at the other end. I think.

  • \$\begingroup\$ if I connect the shield to the ground, would that not introduce noise? also, my research has told me that foil shield is only good for RFI, whereas braided shielding doesnt NEED to have solid coverage since it's designed to block longer wavelengths, in the EMI range. EMI also is more affected by thickness much more so than RFI and so the foil is even less effective for EMI. So I will want to put braided shielding even if it doesnt have 100% coverage since I am trying to block EMI. I also want that foil to block all RFI since braided is horrible at blocking RFI, as you allude. i could be wrong \$\endgroup\$ – Zero Feb 14 '15 at 23:10
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    \$\begingroup\$ The easiest way for you to learn about shielding and how effective different shield materials are (braid vs foil) is to actually try it. A standard balanced (professional) cord-type microphone (XLR connector) and suitable mixer is all you need. Try listening to the mic with the shield disconnected. Make 10 foot cables with XLR connectors on both ends (M+F) and listen to the mic as you lay the mic cable next to a fluorescent tube fixture. I used to use a 2-tube 4' fixture and just lay the cable between the tubes. What you learn will help you in the future. \$\endgroup\$ – Dwayne Reid Feb 15 '15 at 3:23
  • \$\begingroup\$ thats true. but i do not have a balanced audio connection. I am trying to figure out what best i can do for this particular setup, but thanks for the info \$\endgroup\$ – Zero Feb 15 '15 at 4:02
  • \$\begingroup\$ What I have suggested should work well for you. If you do run into a ground loop, your only choices are to either eliminate the other ground connection (if possible) or use isolation transformers (or some form of differential amplifier). I do a lot of Broadcast TV and Radio audio stuff and deal with these kinds of issues on a regular basis. I use a lot of transformers because they are passive and don't require power. Mind you: I use high quality transformers. \$\endgroup\$ – Dwayne Reid Feb 19 '15 at 6:20
  • \$\begingroup\$ Agreed. I've been looking into just building my own isolation transformers... but finding specific information online about this is not working out for me as easily as i hoped. If I can build one for relatively cheap and it does a decent job, then I would be satisfied. \$\endgroup\$ – Zero Feb 20 '15 at 3:35

Left, right and ground on 3 conductors would be unbalanced. Balanced means that 2 conductors are used for each signal (meaning you would have 2 wires just for the left channel) where one signal is phase inverted so that noise picked up by the cable will be canceled out when the receiving end unbalances the signal.

You should not connect the shield at both ends. The shield should be connected to whichever end has lower impedance. This is almost always just audio ground and usually grounds fan out from a center point so that is probably enough information to identify the correct place to ground the shield.

  • \$\begingroup\$ What is the benefit of grounding the shield to audio ground vs chassis? or why would chassis (on either end) not work? \$\endgroup\$ – Zero Feb 15 '15 at 20:20

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