What's the point of balanced inputs?

[niftydog]

more detail;

the input stage is a differential amplifier. It's job is to amplify the DIFFERENCE between the two input signals.

More accurately; it's "differential gain" is much, much, much higher than it's "common mode" gain. (more commonly known as "common mode rejection")

In other words, it has a very low gain for signals that are identical on both inputs; but a very high gain for signals that are different on both inputs.

There's a quick and greasy way of "unbalancing" a balanced line; tie one of the signals to ground and simply use the other signal. The problem is, of course, that you no longer have the difference between the signals. Effectively this halves the voltage at the input and you're down by 6dB (half volume!)

[Gilles C]

I know it's been answered and understood... but I like drawings that shows what's happening  :roll:

http://www.monstercable.com/RobertHarley/ch11_p07.asp

[Samuel]

Peter's note about ethernet / cat5 cable is a good one, and the proof is in the pudding there as far as the effectiveness of balanced signals - each connector in cat5 (of the 8 total) is a fairly thin piece of copper, around 26awg or so, yet you can run a couple hundred feet of cat5 and not be overly concerned about data loss. Admittedly transmission of digital signal will not be as touchy as that of analog, but it's still pretty cool when you think about it.