Originally Posted by Fast_Fingers
Scotty's technical explanation was solid.
Er... actually it was quite wide of the mark and repeats the usual misunderstandings about the balanced audio interface.
Let me try to explain the salient points...
The term 'balanced' in this context refers to the impedance of the signal wires with respect to ground. They have to be equal. That's the only thing that matters and the only thing that makes a balanced interface balanced.
Not the equal and opposite polarity signal concept -- and not all balanced interfaces use equal and opposte signals as I'll explain further in a moment.
Let's take a step back and look at how the unbalanced interface works.
To convey any signal voltage from one place to another we need two wires. In the unbalanced interface one wire carries the variable signal voltage and the second wire carries the reference voltage from which that varying signal voltage must be measured. This is normally zero volts -- audio ground.
For convenience, that audio ground reference wire is usually configured as an overall screen, because that brings the advantage of capturing radio frequency interference and grounding it where it can do no harm to the wanted audio signal.
However, if that interference manages to get through the screen, it generates a voltage relative to ground and appears on the signal wire in exactly the same form as the wanted signal. The interference and the wanted signal therefore combine to form a new composite signal and the interference can't be detected or extracted as a separate entity by the receiving amplifier stage. Once the interference gets in, you're stuck with it.
Moving forward again to the balanced interface, here we usually have three wires -- two for the signal, serving basically the exact same roles as for the unbalanced arrangement in so far as one (or both) carries the wanted signal voltage while the other (or both) acts as the voltage reference.
The third wire is entirely optional but if present is configured again as an overall screen. It takes no part in conveying the signal or any voltage reference. It is purely there to help protect against radio frequency interference, and in many cases is not used or required at all (patch bay cords being a common example where the screen is often omitted).
Now, should the unwanted interference break through the screen it will try to induce a voltage in both of the two signal wires, in the same way as it did in the unbalanced example. Here's the critical thing, though: if those two wires have exactly the same impedances to ground, the induced voltage will also be exactly the same on both wires. On the other hand, if they have different impedances to ground (ie, they have unbalanced impedances), the induced interference voltages will be different on the two signal wires -- and we are completely skuppered!
Whereas the signal receiving input stage in an unbalanced system simply detects the signal voltage (wanted signal plus interference) on the signal wire, as measured relative to the ground reference carried on the cable screen, the balanced input stage is much more cunning.
The balanced input stage is a 'differential' input, which means that it has two input ports and it is only interested in the voltage difference between those two ports. If both ports see exactly the same voltage there will be zero output. No difference, no output! However, if they carry different voltages the output will be the difference between them.
If you have +1V on one input and -1V on the other, you'll get a 2V output signal. Equally, if you have +2V on one input and 0V on the other, you'll still get a 2V output. But if you have +1V on both inputs, you'll get 0V at the output.
So... because we have a balanced interface in which the two signal wires have identical impedances to ground, any interference signal will produce the same voltage on each wire. This is called a 'common mode' signal and it will be completely ignored (rejected) by the differential input stage -- and hence any interference is removed from the wanted signal.
As I've mentioned, the wanted signal has to be conveyed in such a way that it produces a voltage difference between the two wires. One way -- the 'traditional way' is to send exactly the same signal voltage on each wire, but with opposite polarities. (Not usually at different amplitudes, as was suggested in Scotty's post -- although actually that would still work, just with a lower output signal). This 'equal and opposite' format is what you would normally see from an output balancing transformer, or from a dual active output stage.
Because each wire essentially acts as the reference voltage for the other, with this 'equal and opposite' signal format the differential input stage sees a signal which is twice the size of either signal individually -- and this gives rise to the often talked about 6dB level hike it produces when compared to connecting the balanced output to an unbalanced destination.
Alternatively, there is no reason at all why one wire shouldn't be held at zero volts (audio ground) while the other carries the entire signal voltage on its own, as I illustrated earlier.
The difference in voltage between the two wires in this case, as detected by the differential input stage, is still the wanted signal. And provided the 'cold' side exhibits the correct impedance to ground necessary to balance that of the driven 'hot' side, the interference rejecting capabilities of the system remain unaffected and identical to the conventional 'equal and opposite' arrangement.
This configuration is often refererd to as 'impedance balanced' to differentiate it from the 'equal and opposite format', and it is becoming increasingly popular with manufacturers for several reasons. Firstly, it can be configured so that the signal level remains the same whether the balanced output is connected to a balanced input or an unbalanced one.
Secondly, it exhibits 3dB less inherent noise than the dual active arrangement (because there is only one gain stage involved instead of two).
And thirdly it is cheaper to implement than the dual active arrangement (because it requires only half the components).
So to sum up, the advantage of the balanced interface is that the wanted signal can always be separated from the unwanted interference signal -- but the whole thing relies NOT on the fact that the two signal wires carry equal and opposite signal voltages (because they don't have to be formatted that way, as I have shown) BUT because the impedances to ground for each signal wire are identical or balanced.
The theory is based on the Wheatstone Bridge if you want to investigate further.
Finally, it's probably worth mentioning that the signal wires in a balanced cable are twisted so that electromagnetic induction from nearby magnetic fields (such as from mains cables or transformers) cancels out over a short length of cable. Starquad cables manage better EMI rejection because the twist is tighter and there are four cores wired in opposite pairs to imporve the cancellation effect. The cable screen has little effect on EM interference, while the signal wire twisting has little effect on RF interference. Both are needed and serve different roles.
To answer the original question, "Does plugging a mic into a balanced ruin the mic, or does plugging it into unbalanced ruin it, or does it not matter" The answer is, it doesn't matter. However, phantom powered mics can only be used with a balanced connection because that's the only way phantom power can work, and without phantom power the mic won't work at all. Plugging the mic into an unbalanced input won't harm the mic, but it won't work either.
Plugging a non-powered mic into an unbalanced input will simply unbalance the output of the mic. No harm done and the mic will work... maybe it won't sound as good as it could, but it will still work.
Hope that helps -- sorry for the verbose reply, but it is inherently a slightly more complicated story than most people know or understand.
Just remember that 'balanced' means balanced impedance, not 'equal and opposite' signals. The latter might not exist but the system still works to reject interference. If the impedances aren't balanced it simply won't reject interference.