BALANCED & UNBALANCED AUDIO SIGNALS

There’s a lot of confusion about the terms ‘balanced’ and ‘un-balanced’ in the analogue audio world. There needn’t be, as this is pretty simple. So here goes.
This balanced/unbalanced thing only applies to microphone level and line level signals… not to speaker level signals. (and what we’re saying here only applies to analogue, not digital, signals.)
When a sound is turned into an electrical signal, in an audio system, the signal shows up as some variations in the voltage between ground (earth) and the signal core of the cable. These voltages wiggle up and down. The faster they wiggle, the higher the frequency of the audio signal… and the bigger the wiggles the louder the signal. Now there are various ways of turning the original audio signal into this electrical signal…but the most commonly used is to rely on electrical induction. What happens here is that the sound is mechanically turned into the movement of a coil of wire in a magnetic field which causes an induced electrical current. (this may also be done by moving the magnet in a coil of wire…which results in the same effect) Anyway this principle is a function of the close connection between magnetism and electricity. It’s going on inside your headphones, speakers etc in reverse, turning electricity and magnetism back into sound.
These little electrical wiggles are really very tiny. When they reach an amplifier they get increased in power. The basic contraption that does this is called a differential op-amp, and it amplifies (makes bigger) the difference between the signal cable and something else. In unbalanced systems the something else will be earth. This is all well and good, except for the difficulties caused by the real world. Because, you see, the real world is full of lots of electromagnetic crud. Everything around you is chucking out a horrid mish-mash of electromagnetic noise. The PC you’re sitting at is doing it, your mobile phone is doing it, the telly is doing it… even the mains cables around your building are doing it. And into this stew of noise we go and dangle a skinny little cable with our microscopic signal running down it. The induction principle we talked about above comes into play here and induction causes the undesirable noise to get superimposed on top of our audio signal. So if we do nothing about this our nice audio signal comes out at the other end of our system accompanied by hum and crackle.
The cable manufacturers help as best they can by wrapping the signal core of the cable in a protective screen, which is connected to earth. Unfortunately this is where the first real problem occurs. The screen, connected to earth, doesn’t pick up the induced electromagnetic junk, but the signal core does pick up what junk leaks through the screen. When we get to our differential op-amp there’s a difference between the signal core and the screen, which is amplified.
Wouldn’t it be nice if the screen and the signal core picked up the crud at the same level? Then when the amplifier received the signal there would be no ‘difference’ so nothing to amplify, just the original, clean, audio signal!
We can’t get the screen and the signal core to act the same, but we can get two identical cores inside the screen to do so. So in a balanced system we put the signal between these two cores, and wrap them in a cosy jacket of screen bonded to earth.
Now, as both cores pick up the same induced grot, the differential op-amp ‘sees’ no difference and effectively filters out the interference. Cleaner sound!
You’ll realise, of course, that balanced systems become more expensive. There’s the extra core in the cable. And all the plugs and sockets have to have three connections, one for the ‘hot’ signal, one for the signal return, and one for the screen. Usually balanced signals will go via 3 pin XLR connectors (XLR indicates screen (X), live (L), return (R). Incidentally the pins and numbered this way too. ie. X=pin 1, L=pin 2, R=pin 3. Occasionally manufacturers will use ring-tip-sleeve (sometimes known as stereo) jack plugs. If the equipment you are dealing with has connectors using phono (RCA) plugs or mono jack plugs it is running “un-balanced”. And that means that you may experience unwanted noise on your cables!