Amp Hum

[Drusillus]

Hey guys,

I'm pretty much an idiot when it comes to the inner workings of amplifiers, wattage, impedance, all that stuff I slept through in physics class... I've got a problem with my rig hopefully someone here can help me diagnose. Here's the situation:

1. I plug in my amp (Carvin KB1010), and I can crank all my channels and main volume to full blast and the amp is completely silent, no hum or hiss or anything, nice and quiet.

2. I plug in one of my keyboards (JP8000) and I instantly get a fairly quiet low-frequency hum.

3. I plug in my other synth (Fantom-S) and I get an even bigger hum coming off of that keyboard.

With both keys plugged in and amp set to performance level, the hum is really bad :( I've tried changing cables and that doesn't help, so since the amp is silent by itself I'm left to believe that it's a problem with my synths. Does it indicate a problem with the power supplies in my synths? Or could it be a problem with the power outlet I'm plugging my keys into?

Any advice you guys could give would help, this is one of those stupid problems I should understand and know how to solve by now :)

[Omega Monkey]

Sounds like a ground loop. Im guessing that either your 2 keyboards, or 1 keyboard and the amp or some similar combination are plugged into different circuits (ie with different grounds). This has the electricity trying to ground in 2 places which causes a loop (or something like that). Solution, get a power strip and plug everything into that, OR lift the ground on anything that is plugged into a different circuit from the amp.

Considering both your keyboards seem find by themselves, its obviously not a problem with them. So I would say theres a 99% chance its a ground loop problem, which as described is easy to fix. Just make sure the circuits in your house (or wheverer) can put out as much power as you need for all the equipment.

[Cary]

I agree with Omega Monkey, sounds like a ground loop*. Make sure everything is plugged in on the same circuit. Two boards and an amp shouldn't tax a 10A circuit too bad, no worries there. If any of your power cords/wallwarts are not polarized (one blade bigger than the other) try flipping it around in the outlet. Have you tried plugging in elsewhere (like someone else's house?) Anything looping on any circuit you're on could be giving you grief.
Also make sure none of your audio cords run parallel with any power cords...there's a slight chance you could be picking up some EMI there.
I suppose there's a smaller chance of a cap starting to "leak" in one of the power supplies, but less likely. Most of the time when they go, they'll crackle, not hum.



* a ground loop is caused when there's a difference in potential between two grouding points. If something...say an amplifier...is between those two grounds, a small current will flow between them causing the noise. http://www.epanorama.net/documents/groundloop/

[Drusillus]

Hey guys, thanks for the replies... in both of my practice locations, my keyboards are plugged into one outlet and my amp into another, so it sounds like that might be the cause of the problems... I'll try them all in the same outlet/power bar and see if that helps, thanks again!