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[soundian]

Let's look at an unbalanced signal first:



It's easy and cheap, but it comes with a catch.
Because of signal loss in a long cable and noise picked up along the way, unbalanced cables should only be used for short distances, a few metres is normally fine.

However, when we want to transmit the signal further and we're in a potentially "noisy" environment we have to overcome these two problems.
This is where a balancing circuit comes in.
Every piece of balanced equipment has circuits which also deal with a "reversed" signal.
It works like this:
Pin 1 is always earth
Pin 2 (hot/positive) transmits the unbalanced signal, (see above)
Pin 3 (cold/negative) transmits the same signal, 180 degrees out of phase.

Like this:



When these signals reach the other end, the reversed signal is reversed again, so it becomes identical to the original, and then added to the original, giving:



(note the scale change on the Y-axis)

That overcomes problem number one, signal loss, so what about noise.

Here’s a positive signal with a noise spike picked up on the way:



And the same signal reversed with the same noise spike:



Notice what happens to the noise spike when we flip the reversed one before adding though:



Once that is added to the original signal, the two noise spikes will cancel each other out.

This is what DI boxes are for basically, preparing the unbalanced signal to be sent longer distances.

On a more practical note: If either pin 2 or pin 3 is disconnected on your cable or mic, it will still work, but you'll need to increase the gain to get the signal back to the required level. So if you have any unexpected signal drops, especially around 6 dB, that's the first place to look.