The importance of balanced wiring
Audio cables are prone to pick up interference. Interference is commonly caused by sparks, as produced by electric motors, and the sharp-edged waveforms produced by cheap lighting dimmers.
Interference can also enter through the mains supply. The mains waveform may be dirty - i.e. not the smooth sinewave it should be. Also there is the dreaded 'earth loop', also known as 'ground loop' or 'hum loop'. Earth loops will be covered in detail in another tip.
The way professional audio equipment combats interference is to use balanced wiring. This means that every signal outside of the equipment is carried by two conductors, the signals on which are always equal and opposite in voltage. At the input of the receiving equipment, one 'leg' - as they are sometimes known - is inverted and added to the other.
Any interference will enter both conductors producing identical signals. Inverting one leg and adding to the other cancels out the interference. This is known as 'balancing' and it works very effectively. Also, since the earth conductor of the cable is not used to act as a reference voltage for the audio signal, it doesn't matter if there is some electrical noise present on it. Neither does the earth loop hold any terrors.
Obviously, balanced audio cables must have two signal conductors plus an earth conductor. The very best are known as 'star quad' cables, where there are four signal conductors that are wired as two pairs. The tight winding of these conductors within the cable means that interference is always almost exactly equal, and therefore is cancelled almost entirely.