Keep Audio Cables Away from Power Lines
One big factor in avoiding the pickup of power line hum in audio circuits is to keep audio cables away from any power lines, cables or circuits (most notably power transformers). Use good cable management in routing signal and power cables throughout a facility.
Running audio signal cables and branch circuit power line cables in separate steel electrical metallic tubing (EMT conduit), and power feeder cables in rigid steel conduit, greatly reduces the interference between the two types of signals.
Using twisted pair wiring for power line cables as well as audio also helps to reduce interference.
Be especially careful of how cables are laced in a rack where wiring is often tight. Do not run audio cables parallel with and directly adjacent to power line cables and power strips. Keep sensitive low-level audio lines on the opposite side of the rack from the power strips. If line-level audio signals need to be wired in the rack on the same side as the power strips, keep the audio cables as far apart as possible from the strips.
If audio and power lines have to be near each other, make sure they run at right angles to each other to minimize interference, for example, in the placement of power cords from each piece of equipment to the power strip.
Note where the power transformers are located in each piece of gear (or external plug-in power supplies) and keep the audio cabling away from them as well.