SonicAlbert said:
You also may be able to lift a ground here or there and get rid of the hum. Just don't blame me if you electrocute yourself!
Hi
Shudders at the thought.....
I agree with the post lower down about buying a test plug from Home depot and testing all the outlets in the room used as a studio. Just because it's a 3 pin outlet doesn't mean that there's a ground connected.
I'm English and over the t'other side of the pond we are a lot more fussy about electrical safety. The three pin wall sockets have screw connections where you poke the wire in a hole and the screw comes down and crushes it tightly in there. Over this side of the pond you wrap the wire around a screw and tighten the screw.
Problem #1 is if you wrap it in the wrong direction, it will actually unwind as you tighten the screw so that little is left to make the connection.
Problem #2 is that with the rigid single core wire, the action of pushing the wall plate back into it's opening can twist the screw undone because the screws are usually on the side of the socket and the wire at a tangent.
The bottom line is that it is very easy for wires to come undone in wall outlets and you need to check this first.
Then, why does something hum?
Generally because there is a difference in ground potential between the two devices and that difference is being superimposed on top of the signal.
Why should there be a difference in potential? Well, you only need a few millivolts to give you an issue and plugging two devices into two separate wall outlets is one way to create this potential difference. Cascaded power strips one after the other (ie plugging a power strip into the wall outlet then plugging another power strip into this power strip) is NOT to be recommended.
Besides the potential differences between two devices connected together, you can also have hum like interference picked up by radiation in the air. The pick ups in a guitar are splendid things for picking up stray ac magnetic fields and need to be kept away from any any ac power lines (in the wall or the floor or the ceiling) to minimise this effect. Simply walking around a room and finding the best spot is a worthwhile exercise.
Plus, a big No No are triac/thyristor controlled light dimmers. If you want quiet power get rid of these evil things. Replace them with either switches or the rotary transformers you see in professional studios. I can detect a lamp dimmer in a studio by just measuring the console noise floor and looking at the oscilloscope trace. Those bloody great spikes that are causing an angry zzzzzzzzz to come out of the monitors.... that's a lamp dimmer in the kitchen. Throw the breaker to take power off the lights and the interference goes away.
Anyway, hopefully I have given you a few places to start looking and please, if the devices have three pin ac plugs and cords... the manufacturer thinks that they need it. Please don't cut the earth connection to attempt a fix...
You would be fixing a symptom and not the fault... and risking electrocution.
Geoff Tanner
www.auroraaudio.net
