This article covers how (and why) balanced lines work, and their benefits over unbalanced connections in professional audio equipment.

If you've ever heard people saying how "great" balanced audio connections are compared to unbalanced, but don't really understand why, this article might help shed some light on the issue.

To begin with, it's important to understand that balanced connections rely on the use of the correct type of cable. Unlike unbalanced connections which simply have a centre "hot" core, and an outer shield, balanced connections have two centre cores that are twisted together and covered by an outer shield. The fact that the two inner cores are twisted together means that noise should be picked up equally on both conductors, and is important in allowing the receiving device to properly reject noise. These two cores carry the �hot' and �cold' phase, the only difference being that the cold line is 180 degrees out of phase from hot.

The graph above shows how we might expect a sine wave to look if we used an oscilloscope to trace both the hot and cold phase at the same time. This "imaginary" sine wave has picked up some interference down the cable, this is circled in grey. Remember that our two centre cores in the cable have been twisted together, so the interference has been picked up equally on both phases.

At the receiving device (whether it be a mixer, amp, effects processor, etc) both phases are combined. Exactly how this is done is beyond the scope of this article, but to put it simply, we invert the cold phase back to its original state. In an ideal world with no interference, both the hot and cold phase would now look exactly the same, although in practice things are very different. You can see from the graph above that our original interference is still there, but in inverting the audio signal carried by the cold phase we have also inverted any interference that we picked up on the way. If we take the middle point between the difference of the hot and (inverted) cold signal, as indicated by the dotted line on the graph above, we can get back the original signal without any interference!

In a perfect situation, our signal would now look similar to the one above, exactly as it was at the source device. Of course this does rely on the fact that interference is picked up equally by both the hot and cold lines, which sadly isn't always going to be the case.