General tech questions

How would a bad cable cause hum?


I have a digital reverb pedal that was apparently causing unbearable hum. It only occurred when the pedal was engaged, otherwise the hum was gone. My first thought was that it was the pedal itself - seemed pretty obvious. However, after a while of troubleshooting, I determined (I think) that it was the cable that leaves the reverb pedal and returns to the amp. Otherwise, the cable works fine. I swapped it with a different cable and the problem seemed to disappear. A cable is pretty simple - two conductors. What could possibly cause an otherwise good cable to inject hum into an effects chain, and only when the one pedal is engaged? I'm stumped.


Coaxial cable has only one conductor in the center of a dielectric and is then surrounded by a woven shield (also called the ground) that is the second conductor. Different types of coax have different impedance a as well. Bad dielectric or one stray piece of shielding wire touching the center conductor can cause hum. A broken spot in the dielectric insulation could also be the problem. As always, bad connectors or a bad solder job could be the culprit. Wrapping the cable around your elbow and hand can break the internal insulation if done often enough. Wrap cables in loose loops, and don't tie knots in them. Use string ties or Velcro. Works with everything from string to wire to waterhoses.

Cable with two wires inside of a braided shield are either stereo cables---ring, tip, sleeve 1/4" phone connector; or, are for XLR Mike cables.

Never use signal cables for speakers or or speaker cable for mic level inputs. Speakers want twisted pairs of the same gauge or parallel "zip" cord for short runs. Don't use AC extension cords for speakers because they already have cool connectors on them. I've seen too many cases where the speaker played a 60Hz note for a couple of seconds before the smoke came out. Always use the right cable and connectors for the right purpose. I've been working with wire for over fifty years. I've dealt with everything from 42 gauge phonograph tone arm hookup wire to microphone cables and sound snakes to multi conductor lighting cables to 0000 three phase feeder cables. Cables and connectors are ALWAYS the first place to troubleshoot a problem.


I agree with slim. I want to add that a shield that is poorly connected or broken can act like an antenna. Radio noise connected right to your amp.


Locally, we had a CBer running a 3KW linnear amplifier with rewired HAM gear that really screwed with guitar amps, wireless phones, radios, and all kinds of things. It took the FCC a couple of months to track him down.

Most guitars do not have enough shielding for their wiring. The best I've ever seen was Danelectro. All coaxial cable inside of a copper foil box---nobody does it like that anymore.

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