The term balanced audio can be a little misleading to someone getting into hifi. It seems like a perfectly rational goal that the sound of a stereo system should be balanced, both tonally and as the term relates to the volume level of the left and right channels.
Yet, as with many different things in audio, the word “balanced” means several different things. And it’s the third meaning that has the most bearing on the way some electronics are designed, and produces a unique effect of noise cancellation in a home audio system.
Balanced audio describes how the ground wire interacts - or is absent from - the audio signal. In line level signal cables, this means the signal is split between positive and negative cables, and doesn’t travel along the ground cable at all. The result is 3 wires in each cable, using XLR connectors at the end.
In amplifiers and source equipment, balanced means that separate, identical parts of each channel circuit carry the negative and positive elements of the sound, and like the cables, leave the ground free of music.
The benefit in both cases of balanced audio is essentially this: when you have noise or hum enter both the negative phase and the positive phase of music at the same time, and the two are summed together at the amplifier (one is inverted to accomplish this) the noise that was in positive phase going into both phases of the music gets inverted with the negative phase of the music and added to itself, and thus cancels itself out. And the volume of the music doubles, further distancing it from noise.
This is how noise-cancelling headphones work as well. A small microphone outside these headphones captures external noise, flips its phase and adds that to the music you’re listening to. When both the positive phase noise from outside the headphones plus the negative phase noise in the music reach your ears, all you hear is the music. Noise is shushed.
In this episode of The Hifi Podcast with Darren and Duncan, the guys do a much better job of explaining how this works and why it’s important. Have a listen below, or learn more at www.thehifipodcast.net.