The importance of using shielded wire

[franklinh]

This thead will probably be over in about two replies. However, I've searched quite a bit and not really found the answer. The scenario is:

I'm building a (active) stereo mixer. The panpot is feeding multiple bussing switches (at line level) , about 3.5" between farthest point to hit on switches with signal and panpot lugs, traveling over the first two to get to the last one. There's room to keep the left and right output wires away from one another, and room to "fly" above the back of the switch. Better than an inch between adjacent channels. The whole assembly is out of the way of everything else and on it's own lexan panel. Do I need to use shielded wire from the panpot to the switches? I'm sure I'd find out the hard way if I could listen to it as I went, i'm just at a point in building where knowing would save me a chunk of time and labor either way.

The only example to work from is a y@maha module where they did use shielded wire for this but is was also tightly bundled with other signals.

I'd imagine this is part of why layout is so crucial. Are there minimum distances to be observed? Is crosstalk the big concern here? What year is it?

Thanks,
Frank

 

[NewYorkDave]

Level and impedance have a lot to do with it. You did mention "line level" (which is nebulous, but we can take that to mean "well above mic level"). But what are the source and load impedances involved?

High-impedance, low-level circuits need to be shielded more than do low-impedance, high-level circuits (but you gotta watch that cable capacitance!). And unbalanced circuits require shielding more than do balanced circuits. And shielding is always in order when circuits with a large level differential are located near one another.

The best any of us can do, via the internet, is to take a guess at the relative need for shielding in what you're building.

Me? I'd probably shield it.

 

[CJ]

And only ground one end of the shield.
Which end?
Flip a coin.
Whatever the coin tells you to do, try the opposite and have a listen.

 

[mikka]

Nyuk Nyuk :green:

 

[PRR]

> about 3.5" between farthest point to hit on switches

Don't shield.

> it's own lexan panel.

Do shield. Wires, pots, switches, all wrapped in metal. Otherwise every time you bring your hand near, you will inject buzz.

Or do what everybody does: use metal panels.

If you really hate the sight of your own blood on the 3/8" metal-drill, get some aluminum foil tape (sold for sealing heating ducts; NOT cloth but real metal) and put it on back of the lexan.

 

[franklinh]

Hmm...

NYD:
I apologize for my lack of good info - i'm using the "seat of pants" method for what I don't fully understand here. The modules feeding the 10k panpots are supposedly able to drive a 600 ohm load, so I guess this is a low impedance / high level situation here. Once through the panpot and the assign switch, we're heading into a 10k input impedance of an electrodyne ACN (a post you may recall). BTW did you get my pm that tacoland in Kingston NY is still existing?

CJ: Can you tell me WHY I should only ground one end of the shielded cable? Ground loop?

PRR - thanks for the tip on the perils of not using metal panels! I would shed some blood now but I can't bear the thought of lining up that many holes again. I suppose some of that fancy shielding paint meant for electric guitars might work also.

Thanks,
Frank

 

[dayvel]

When I'm running multiple signals like that, I like to use flat rainbow cable with every other wire grounded at one end. Keeps things neat, and the color coding prevents confusion.

Being a musician who doesn't like putting his hands at risk, I make a lot of my cases out of acrylic. The foil tape works well (run your thumbnail along the overlaps to put a crease in the foil to insure continuity.) Heavy duty aluminum foil and spray adhesive works well for large areas.

 

[franklinh]

Can anyone tell me WHY I should only ground one end of the shielded cable? Is there any reason other than ground loop?

Frank

 

[Flatpicker]

[quote author="franklinh"]Can anyone tell me WHY I should only ground one end of the shielded cable?...[/quote]Grounding both ends can cause a ground loop.

[quote author="franklinh"]...Is there any reason other than ground loop?...[/quote]Not to my knowledge.

 

[CJ]

http://www.rane.com/note151.html

 

[franklinh]

Ahhh - I forgot about the rane notes. Good stuff in there. Now I will go forth.

Thanks everyone!

Frank