audio hum

Ok, Mark has helped enormously with my charging system wiring! Next task: I have a 175 watt inverter plugged into a cigarette-lighter type outlet. When we play a movie using the laptop plugged into the inverter, the laptop audio is ok. When I take the headphone output of the laptop and plug it into the stereo accessory input of the 12 vdc stereo we get an annoying hum. Same for my POD guitar effects unit, more or less ok (although I seem to remember a bit of "buzz") on it's own but hums when plugged into the stereo. The hum diminishes as my hand gets close to the guitar jack and goes away if I actually touch the jack. Seems like a ground problem, but is it this "squared" sine wave thing of cheap inverters? Peter Bribon, Cal 39

C. Peter Audet wrote: > > Ok, Mark has helped enormously with my charging system wiring! > Next task: I have a 175 watt inverter plugged into a cigarette-lighter > type outlet. When we play a movie using the laptop plugged into the > inverter, the laptop audio is ok. When I take the headphone output of > the laptop and plug it into the stereo accessory input of the 12 vdc > stereo we get an annoying hum. Same for my POD guitar effects > unit, more or less ok (although I seem to remember a bit of "buzz") on > it's own but hums when plugged into the stereo. The hum diminishes as > my hand gets close to the guitar jack and goes away if I actually > touch the jack. Seems like a ground problem, but is it this "squared" > sine wave thing of cheap inverters? I've read Mark's response to this inquiry but I'll add that maybe it's a ground loop problem? You can get hum in audio devices when they are not all connected via one single common ground. In a DC environment like a boat, maybe there is a ground through the negative DC wire, and another via the various connecting signal cables. Ground loop problems are sometimes cured by lifting (disconnecting, cutting) the shield of a shielded audio cable at one end. Chris Campbell