"...there is another option: reduce the size of the hum loop. A hum loop is the same as a coil in a dynamic microphone, although it has only one turn. Any variable magnetic field will induce a signal voltage in it. And there's plenty of magnetic garbage flowing around electrical devices...
btw, i've looked up the formula for induction:
U = - n A B'
where U is the induced voltage in volts that giveth us such great grief;
n is the number of turns (usually one);
A is the size of the loop in square metres;
B' is the amount of change in the magnetic flux density in tesla per second.
the minus sign is cosmetics. it can normally be ignored, but is necessary to ensure the law of conservation of energy works :) .
What does this mean ? To reduce U, reduce any of the factors on the other side of the equation. n and B' are difficult to handle, A is the easiest to go for. it's still worth the effort to keep electromagnetic garbage out of the way, but this may not always be possible.
diagrams:
a) large hum loop, high hum voltage. _____________MIXER..........
| .
| <- power cord . <- audio cable
| .
| .
| .
| .
| ___________COMPUTER.......
| |
O O <- wall socket
b) small hum loop, low hum voltage.
_____________MIXER.
|..................
|.
|.
|.
|.
|.....................
| ___________COMPUTER.
| |
O O <- wall socket
keep in mind that the hum current flows both through audio cable shields and power cable ground wires ! moreover, different earthing points will always have slightly different potentials, so there will always be a little current flowing to and fro, again causing hum. (don't believe anyone who claims to have the definite 0 volts. they lie.) so keep the audio lines close to the power cables, and use a single mains socket for the entire studio (if not, your hum loop may easily be as big as the entire house. great for searching alien signals from outer space, very bad for sound recording :).
this will greatly reduce most hum problems at no extra cost, while avoiding the sound impairment that usually comes with cheap [transformers]. on the other hand, there will be a little amount of capacitive and inductive coupling between the cables that might again introduce hum. should be a lot less, anyway. works for me."
SlinkP interjects: The inductive-coupling hum caused by running the power & audio cables close together is known to cause problems with long cables. Live sound engineers keep their cables well away from power lines whenever possible, but that's because they have to; they are often running cables 100 meters or longer, in which case the induced hum could be huge. They avoid big ground-loop problems by using only balanced cables and leaving one end of each shield (usually pin3) wire disconnected so there's no ground loop.
"summary: generally, the moment an audio circuit is earthed twice, you'll have hum. with asymmetrical wiring in home studios, there are two options: separate the circuits with transformers, or move the earthing points close together and keep power and signal cables together."
"(In theatre), The power cables often include lighting and in a theatre the lighting is regulated through pulse width modulation ... This causes huge spikes on your power, which is disastrous on any audio cable running along them.
I also wouldn't advice running your audio and power cables next to each other in a home setup. A halogen lamp which can be trimmed down often causes spikes on your power too."
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