Author Topic: Shielding in Instrument, Stereo, and Snake Cables  (Read 160 times)

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Shielding in Instrument, Stereo, and Snake Cables
« on: December 04, 2017, 02:27:32 PM »
Hello All!

I'm developing a specialized guitar/bass practice rig and a pedal prototyping setup for my apartment, utilizing custom I/O "patchbays" (for lack of a better term) for cable management purposes. I had a couple of general questions, mainly regarding proper shielding of cables, that I believe could be a great general resource for understanding why we wire cables the way we do.

The Practice Rig:

My desire is to cut down on the number of cables traversing my floor, so I'm exploring the use of stereo or snake cables. I have dedicated Guitar and Bass modelers mounted on top of my guitar stand, so I won't be plugging directly into them, but a box with an input jack located in a more convenient location for accessibility.

Directly between my instruments and their respective modellers, I have dedicated looper pedals for each the guitar and bass. From the input box described above, I essentially want two effects loops, one per looper pedal. To accomplish this I can either:

1. Run four dedicated mono cables. Guitar Looper In, Guitar Looper Out, Bass Looper In, and Bass Looper out.
2. Two Stereo Cables
3. A Snake cable, with individual TRS cables within.

For option 2 above, do I want to dedicate each cable to each instrument? (i.e. one TRS cable carries my Guitar Looper In and Guitar Looper out). Or do I want to dedicate one TRS cable to my inputs and one for my outputs (i.e. one TRS cable carries my Guitar Looper In and Bass Looper In. The other TRS cable carries the outputs).

What issues, if any, would be created from sharing shields in either of those configurations?

The Prototyping Rig:

I want to develop an I/O box for my pedal prototypes, with the guitar in and amp output in the back, and a few switchable loops to test multiple pedals.

Each "Loop" will have a pair of 1/4" jacks, for input and output. From these jacks, I'd like to have shielded cable that plugs into the jack on one end, and the other end goes to an alligator clip connected to my circuit input or output.

Assuming that my power and ground plug (also coming from the same box, same ground as the in/out jack ground) are connected to the pedal, do I absolutely NEED to ground the shielded I/O cable on both ends?

Or is grounding it at the jack (again, the circuit and the I/O box are at the same ground potential via the power cable) sufficient?

Please let me know if I need to clarify anything from the above.

Thank you all for your help and input!


Re: Shielding in Instrument, Stereo, and Snake Cables
« Reply #1 on: December 04, 2017, 08:31:57 PM »
Hi there!
The general approach to use is "source shielding" - where the audio is carried by a twisted shielded pair, but the shield of that cable is grounded at one end. The source end.  This avoids what (for this discourse is an unfortunate term) what you may know as "ground loops". Ground loop = antenna, under certain circumstances. You only should provide one path to ground in that cable.
It's not hard to maintain throughout a recording studio.  The destination end of the cable is your twisted pair only, going to hot and cold of a balanced differential input, or tip and sleeve in an unbalanced input. You will end up with specific cables doing specific jobs. You should label both ends. "From this to that". Also, metal boxes and patchbay a with grounded connectors can be a problem. You can do a bit of destination shielding if you must. Or do plastic jacks. The shield picks up the garbage and dumps it to ground - better to dump it back at the source gear than get it near the destination, where hungry gain circuits usually await.
I'm talking about balanced twisted-pair shielded cable here. Mic cable. Even for your unbalanced applications. Join the ground and black wire at the unbalanced source, attach only the black and red at the destination. Cut that ends's shield off and shrink wrap so it doesn't peek out and touch.  Snakes are usually independently shielded, insulated, twisted-pair cables.   

Aside from all that, I n a pedal enclosure I usually use shielded cable from jack to switch to pcb. I might abandon source shielding, and simply attach that 1 shield connection to the jack, regardless of signal flow direction. Just out of convenience.
« Last Edit: December 04, 2017, 08:42:43 PM by blackieNYC »
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Re: Shielding in Instrument, Stereo, and Snake Cables
« Reply #2 on: December 05, 2017, 12:01:38 AM »
Don't run mike and line cables together, if you can help it.

Keep speaker and Power cables away from the above.

All your stuff appears to be "guitar cord", an ill-defined level between mike and line. At home scale, I would just bundle them all together fearlessly. For larger situations, guitar-cord generally has to be buffered to limit treble loss, so we don't have to consider that case.