Shielded speaker cable, how does the shielding work?

[Cal Weldon]

I remember years ago when we used to run wires through auditorium floors, that you always used shielded speaker cable because it was in proximity of power cables. So I understand what it is used for and why but it's the how that has me confused.

I thought that for a shielding to be effective it required a ground loop to be created. So, if I used the shielded cable, I can hook the amp end up to a ground but I'm not sure what to do at the speaker end. Am I on the right track or does the hook up at the amp do this for me. Again, I thought there had to be a loop. It's so far back, I simply can't remember.

Please help, the neuro-transmitter synapse thing isn't at top speed today.

Cheers,
Cal

[Nappylady]

No, in fact, a ground loop is a bad thing. There will be, in this instance, three conductors--the red and black wires form the circuit, and the shield is simply connected to ground, but ONLY on one end. If you connect it on both ends, it could actually make the AC hum even worse.

You should be aware that the shielded lines that were run before were probably line-level or microphone-level, and so they needed sheilding. Amp-level signals usually don't require shielding because the mains hum from adjacent wires is usually miniscule.

If you sheild wire that will be used by a power amplifier to drive speakers, you should probably NOT shield it, as it can cause the amplifier's load to be overly capacitative, hurting high frequency performance, and potentially causing oscillation. This oscillation can destroy the amplifier.

If the wires are running next to AC power for more than a foot or so, then perhaps some shielding is called for; I'd say try it without, and only shield it if you find it necessary.

[markp]

Don't shield high level signal wires like speaker wires. In addition to the capacitance problem there are other issues. Very high signals can generate a field larger than the diameter of the shield.

[lucpes]

Actually, it would be best to make sure that your cables don't mix up too much with power line ones, and note that at these impedances & currents (amplifier close to 0 ohms, speaker ~4-8Ohms, some serious amps) shielding won't matter anyway.

[SHiFTY]

In old auditoriums, and some modern PA equipment, the amplifier has a large output transformer which steps the voltage up to 100V line. Each speaker has a small transformer which steps down the voltage. This is done so you can have 100 speakers on a single cable without too much resistance losses.

It also makes the speaker more prone to interference, as spikes cause excitation in the transformers. So that might be why you used shielded cable. Or maybe your boss just got a sweet deal on some shielded cable from his friend?