humming speakers on iMac

[John T]

Curing an earth loop can be a most frustrating problem if you don't know the circuit layout of the various components.

I would start, for the time being, by getting rid of the UPS. It's only confusing things for the moment and it may be causing some.

It's a case of a system of elimination....

You say you had no problem when the speakers were used via your PC. Did your PC have a two or three pin power plug? I'm not familiar with your speakers but assume they are of the powered variety i.e. they have their own inbuilt amplifier. Do they have a two or three pin power plug? Not all appliances have an earth connection at their power plug (they have a two pin plug) They are either completely isolated from the mains or pick up their earth connection via the appliance they are connected to.

Earthing problems with audio equipment often arise when two appliances, when fitted with their own earth connections are connected together - this forms a "loop" which results in a mains hum.

Therefore, should your speakers have a three pin plug this is where the problem could be. In order to check it, you'll need to disconnect the earth lead from this plug. They will still be grounded via the neutral lead to your iMac.

Hope this helps. As I said, locating an earth loop can be a tricky problem!

[slam5]

Come to think of it, maybe the problem with the iMac and the speakers is because they are on different UPS. I will keep on trying until I find a solution. I will post results once I found it. Thanks!

[slam5]

It definitely sounds like a grounding issue - as has been pointed out. Have you tried a different outlet - for instance - the one that the PC was in that didn't exhibit the problem? Turn off lights - and any other appliance/electronics that may share that outlet's circuit.

It just wouldn't make any sense for the PC to have "better" grounding, not as a general statement, unless you happen to have a grounding issue with your particular iMac for some reason - or perhaps just your audio jack. Maybe your UPS is introducing the hum?? Try it without.

hi, i finally fixed the problem. i plug the iMac and the speakers into the same UPS and the problem went away. Or at least, I can't hear it. Very happy man now!