Heya, Ted! You wrote: Run the audio “close” but not too close. Actually, there is no such thing as “not too close” for audio and power. The closer the better. Even if you run a balanced line level cable for fifty feet in close proximity parallel with a twisted pair 20 amp power drawing (typically) no more than a few amps, you’ll get no perceptible induction into the audio and probably no measurable induction either. But the farther away you run the audio from the power, the greater the loop area (difference of potential from external interference) and the greater the potential for external EMI and RFI. In short: The need to keep your audio and your power apart is largely a myth. If you’re drawing oodles of current (say, 50, 100 amps) in a giant power line, and there’s lots of harmonics from nasty power supplies adding power line harmonics, and you coil the audio 100 times around that power line, you MIGHT get some interference.
Try it…. let me know if you EVER get any power interference from proximity of audio runs with power runs.
The reason why it’s good to keep the runs as close as possible is: LOOP AREA. That’s because the audio and the power are part of a circuit. The lower the difference in potential between the two, the lower the loop area for external interference which can cause noise.
HOWEVER, it’s probably a good thing to keep audio cables away from EMI emitters like power transformers. But remember, the inverse square law applies…
P.S. It’s not as if I was always this “enlightened”. Go back 30, 40, 50 years, I, too believed the old myths of “always cross at 90 degrees” (mostly a myth), “don’t run audio and power in parallel next to each other (mostly a myth)” “keep your audio runs away from power lines” (mostly a myth), “heavy duty ground wire back to the ground rod is very important” (definitely a myth: because the longer the cable, the higher the impedance at RF frequencies. When is a ground not a ground? Answer: When it’s more than a few inches long!