Let's do the science bit first. (Strangely enough, I vividly remember studying the theory of this at Uni. Unfortunately, the Professor failed to cite probably the most commonly used application of a balanced signal in the world - the humble microphone. Had he done so I think I may have paid a little more attention during those lectures...)
Unbalanced signals are easy to explain: they are normally transmitted along a pair of wires (aka 2-pole) - the signal, and the signal ground (aka common). Thus, the signal consists of a continually varying electrical voltage, with respect to it's ground. Typical examples are the connection from guitar to amplifier, or the interconnects of a domestic hi-fi system.
The big problem with unbalanced signals
The world would be a lot simpler if we could use unbalanced signals for everything. However, they have one big, no, massive drawback: interference. As you can imagine, unbalanced signals pick up interference from electrical devices in the vicinity. Over short distances (under 10 metres) you might not notice, but over longer distances, not only does the interference become more noticeable, but signal degradation will occur.
So, unbalanced signals are transmitted along a pair of wires, but are susceptible to interference. The answer? Go balanced! In a nutshell, a balanced signal is the same as an unbalanced signal, but with the addition transmission of a anti-phase signal (think mirror image). That is, a balanced signal is transmitted along three wires (aka 3-pole) - the signal (known as +ve or in-phase), the signal ground, and an anti-phase (-ve) signal. Typical examples are microphones, outputs of mixing desks, DMX transmission lines, etc.
Now the science bit...Let's think of how electrical interference affects an unbalanced signal: put simply, it adds electrical noise to the signal which can be usually heard as a buzzing noise. Now, a balanced signal is affected by the same interference, but (and it's a big 'but'), because the balanced signal has a 'phase' and 'anti-phase' version of the signal, the interference affects both signals equally. The clever bit is done by the balanced input stage of your equipment, where the phase and anti-phase signals are processed by a differential amplifier circuit that is able to reject the electrical interference (known in the trade as common mode noise rejection).
A balanced signal path consists of three parts...
The most important thing to remember when dealing with balanced signals is that you need three parts: the balanced OUTPUT (eg microphone), the balanced cable, and the balanced INPUT (eg mixing desk). If any of these three parts is missing then you will (at best) default to unbalanced mode. If you connect a balanced output to an unbalanced input, you will always have an unbalanced signal path. If you use an unbalanced cable with a balanced input and output you will always be unbalanced. This is VERY IMPORTANT! You can convert an unbalanced signal to a balanced signal using a D.I. box, but we'll save those for another time...
What does this mean for cables and connectors?To recap, unbalanced signals require two wires to transmit a signal, whereas balanced signals require three wires. Starting to ring any bells? Yes, this is the reason why your microphone cables use 3-pin XLR connectors. Conversely, your guitar leads use mono (2 pole - Tip and sleeve) jacks. It's not just the connectors, however. A balanced signal needs three wires - +ve signal, -ve signal and screen - hence the reason why your microphone cable has two cores, plus a screen. And yep, you've guessed it - your guitar cable has one core, plus a screen.
Nearly every XLR connector manufactured is a 3-pin connector - that is, designed for use with balanced signals.
This is where things start to get a little more complicated. You've probably noticed that jack plugs come in two varieties - mono and stereo. These are also commonly known as TS (Tip/Sleeve) and TRS (Tip, Ring, Sleeve) which describe the electrical contacts of the plug. It won't take much to realise that a TS jack is an unbalanced connector, whereas a TRS jack is a balanced connector.
RCA Phono plugs
These connectors consist of a central tip, and a sleeve. They are unbalanced. If you have any Semi-Pro or Pro playback equipment (CD players, mini disc players, etc), you might well have two sets of outputs on the back - one set on phono plugs, and the other set on XLRs - making sense now? (phono=unbalanced / XLR=balanced)
The XLR to Phono cable
Of all of the cables I sell, the XLR to phono cable is probably the most popular. This cable has an XLR connector (balanced) at one end and a phono plug (unbalanced) at the other. I've already stated that a balanced signal path requires all three parts to be balanced. So, what's going on here?
The cable is actually transmitting an unbalanced signal (this is always the case as it is using the 2-pole phono connector). At the XLR end, Pin 3 (which would be the anti-phase signal) is connected to Pin 1 (signal ground). This is done simply to ensure that it is not left 'floating'. The circuitry of 99% of balanced inputs and outputs has been designed to co-operate with this wiring scheme. Hence you are able to connect the unbalanced outputs of your CD player to the balanced inputs of your mixer. Please note that the signal is NOT BALANCED! In certain circumstances (especially with old equipment) the input/output circuitry is not designed to work with Pins 1 & 3 connected. In this case, it is normal practice to leave Pin 3 (the anti-phase signal) unconnected (floating).
Thanks for reading!
I think that just about sums up the rudiments of balanced and unbalanced signals. If you have any further queries, or if you dispute anything I have written here I'd be pleased to hear from you.