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I have dozens and dozens of cables. XLR, 1/4", a few TRS cables, and some adapters and hybrids. Some are less than a few years old, some are decades old, a few may be older than I am. And every so often, I notice that one of them is a bit noisy, and I replace it or, if possible, repair it. (Sometimes the jack connections are the problem, and those can be resoldered.)

However, I'd prefer that I not make these discoveries in the middle of a gig, or a recording session.

How can I go about testing cables? What exactly causes line noise in cables? Is there a way to find out what part of the cable is producing it? (I'd rather cut off the bad bit and have a good-quality, if shorter, cable.) Are certain kinds of cabling more susceptible to degradation?

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2 Answers

Alternately, if you can't afford a cable tester, you can buy or build a cheap continuity tester or borrow a volt meter (digital or analog).

Example of testing cables with a cable tester and a meter: https://www.youtube.com/watch?v=98e4JdPvMbE

(one problem with this example is that he does not test for shorts. Remedy: take the same probe to the remaining pins to be sure it does not zero on pins the signal is not suppose to be present on.)

Example of home made continuity tester: https://www.youtube.com/watch?v=LFdvxasvLb4

If you know that your cable has continuity (no opens) and no shorts, then you might still be scratching your head about noise issues. Line noise can have many sources:

1) ground loops, there are a ton of ways to have ground loops, here are few that come to mind: multiple and inconsistent ground sources, outlets not grounded properly, faulty ground connection inside your equipment...

2) proximity issues, placing a cable too close to a power field especially if the power is unbalanced and the audio cable does not have very good shielding will make things worse.

3) cable integrity, compare a good shielded cable with a poor one, this should convince you to test and compare cables when you are purchasing replacements.

4) utility line noise: Sometimes it's not your equipment or your cables. If you have an O'scope take a look at the AC coming out of the wall at the gig. If you see huge spikes moving through the AC it may be from that guy welding next door, or some other source in the neighborhood, or even the fluorescent lights at the gig or a bad ballast, or light dimmers that use SCR's. You might consider a line filter/power conditioner to clean this up.

Power Conditioning: http://www.isa.org/InTechTemplate.cfm?template=/ContentManagement/ContentDisplay.cfm&ContentID=82300

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You can buy a cable tester, just search for "audio cable tester" and lots of results will pop up.

As far as what causes cable failures, it is mostly mechanical stress. Unplugging a cable by the cable rather than the jack will transmit force to the relatively weak solder joints. Repeatedly doing this will cause the solder joints to fail. Mechanical stress can also sometimes cause bits of shielding wire to make contact with one of the signal wires, causing a short. Sometimes just opening and carefully closing the housing of a connector will fix that failure. Other times you need to trim and re-solder.

DANGER DO NOT TRY THIS

An interesting but definitely not recommended solution for a faulty cable is described here - a sealed 12v battery is connected to the tip and shield of a two-conductor cable. Any faults will be rapidly vaporized. Note that this is very dangerous. Car and motorcycle batteries are typically lead-acid, and can build up hydrogen gas. A spark near a battery can result in an explosion which will cause serious harm! Further, if the cable in question has a big short, it could get hot and burn you. If your cable is made of flammable material, it could catch fire and burn you. DON'T TRY THIS

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