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Sorry I have not got the hang of quotes either!
My studio is absolutely hum free, but not without some tweeks. Any guitar that gets near a computer or TV screen will hum. Avoid. As mentioned, phase-related dimmers ANYWHERE in the house will cause hum if they are on the same power leg (at the fuse box) as your studio line. Same with any motors; refrigerators, heating motors, flourencent ballasts, etc. Make sure that you're on the other AC leg. You might have to do some wire switching at the breaker box. AC wires in and of themselves won't create a magnetic field problem unless they cross an audio wire at a right angle,
Sorry, they SHOULD cross at right angles for minimum interference.
or unless they're carrying huge current. Any current is huge compared to a mic signal. I know this is voltage and not current but your mains is about +40dB and a mic signal is around -70dB. This is far from subtle!
That's why Romex cable is acceptable. Keep AC and audio wires well seperated. Shielding the entire room is a good suggestion if the field is being generated outside of the studio, and if that's what it takes.. One final thought. Make sure the service has a real good 'earth' ground.
A magnetically shielded room 'conducts' magnetism not strictly voltage so does not really need earthing although doing so will help reduce electrostatic interference and make it 'safe'. Good luck with the 'sweet spot'.
Matt S, with Paul's text mixed!
As I posted earlier, finding out where the field is coming from is an important step. It will take time but is otherwise free. Good hunting, Matt
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