"What he said..."
Ripper said:
ground loops are a fairly common thing, both with effects and causing noise in amps and electronics circuits. David it might be something as simple as a haloed/cold solder joint on a ground in one of your pedals causing the offending hum. It can also be caused by poor power bars. On my pedal boards I use a switching power supply (from an old laptop, those power supplies work great and most have a pot inside to adjust the voltage. They also put out enough amperage to power alot of pedals). Since going this route I have never had a problem with ground hum.
Faulty guitar cables can also be a culprit in the situation. The easiest place to start is to see if you can isolate it to one offending pedal. If so, then that pedal can be taken a look at. Are your pedals each powered by a separate wall-wart/adapter? One of those could be causing the issue as well.
Ripper is right in his suggestions. I would add a bit, however.
Ground loops causing hum in audio installations have nothing to do with a power supply using negative or positive as ground. They are caused by a hum signal having multiple paths to ground. Hum is an AC signal and you can't think of ground like with a car, where any metal anywhere is a good ground.
The change to 3-prong power wiring was actually the biggest pain in the ass ever given to audio technicians. The problem of ground loops increased a hundred fold.
Consider that any audio device (including pedals run from a wall-wart supply) grounds the outside shield of the in/out patch cords. In the old days you could simply wire all the units with a separate ground wire to a common ground in the system, usually a rack in the studio. All hum was routed to one ground point. The AC supply was 2-wire. True, one side of the power from the wall was grounded but if one unit added some hum you could simply flip the plug over at the wall and that would likely fix it. If you got ground loops you could move the ground wires to run in a "daisy chain" from unit to unit and then grounded at the end.
With a 3-wire system every unit's power cord is a path to ground but hum signals can also "loop" between units by running along the outer ground of the patch cords. There are no ground wires you can move and besides, there's now a ground through the power cords that's "cast in stone" that defeats the purpose of dedicated ground wires.
You should understand that the electricity companies never even thought about ground loops with a 3-prong system causing problems in audio studios or stages. They cared only about power safety. An electric skilsaw can be a shock hazard. Audio hum is somebody else's problem.
So what can one do about it? Well, cleaning up the usual rat's nest of wires and pedals in front of the guitarist is the first start. You can start with one common power bar if each pedal has it's own supply. Sometimes adding the pedal supplies one at a time will identify a specific culprit. Try flipping this one around in the plug. Having just one master supply for all the pedals is a great idea! Most pedal makers will have such a solution but it rarely works for the one different brand you just had to add to the mix!
Using batteries will often quiet things down enormously as well but it can get expensive.
There's more but if you're not a techie then you're not likely to understand it well enough to implement it. Odds are these tips will be enough anyway.
Me, I believe in a Les Paul plugged straight into a 100 watt JMP feeding a stack of vintage Celestions. If you want a boost then use heavier strings and dig in with your pick over the bridge pickup! The only extra hum will be coming from forgetting your RightGuard...
