I have a 15 foot audio extension cable. For months the cable gave significant noise. Through happenstance I came to realize it was the cable and not other factors. With one end of the cable shorted the noise was extremely significant. After cutting out the middle section of the cable and solder the other two ends back together there is no noise.
How could this be? My best guess is somehow the cable was acting like an antenna. There is significant mains hum floating around in the room and I believe it was probably picking it up(it was 60hz(or 120hz) hum that it was creating).
Here were the steps for those that don't believe this is possible:
- I was moving the device that the cable connects to around and noticed that with the cable disconnect both sides of the devices there was no noise.
- I plugged the cable in on the main device and the got noise
- I shorted one end together and still had hum.
- I used a different cable and no hum.
- I cut the middle section out and used one end shorted. No hum. (slightly more complex but that?s the idea... I didn't just cut the cable randomly but made an estimation where the likely problem was)
- Using virtually identical placement of the cable as before but virtually no noise(it doesn?t even show up on the meter compared to several bars(10 bar meter and the original noise was 3 to 4).
This almost proves conclusively that it was that section of the cable. I tested it with a meter and there seems to be no open connections and all the wires have < 1Ohm resistance. The noise was always on the wire. I wiggled the wire to try to locate it and that is how I determined it was in the middle section somewhere(since sometimes the noise would seem to decrease).
I'm really interested in what could cause this phenomena that produces such a huge amount of noise? Is it simply an antenna effect with two or more wires making a high impedance connection?