What is the negative pole in the audio balanced transmission?

Question

So I'm a bit confused when it comes to the difference between unbalanced and balanced. I'll briefly explain how I understand all of this: The balanced cables have three wires inside the plastic casing: two signal wires and a ground wire. The signal wires pass an identical audio signal through each wire, while the surrounding ground wire works the same as in unbalanced cables, shielding the signal wires from external electronic interference. I also understand the rest of the part of this system, but I still have 2 questions.

1. Is the ground cable a requirement that would make it unbalanced without it? I don't think that's a requirement and the ground wire is an option.
2. So if it works without the third ground cable, then the 2nd cable and the first one are responsible for the transmission of the sound. But then what is the negative pole of the two? So can the second cable function as a carrier of the sound and at the same time as a negative pole?

Answer 1

To answer the direct question, there is no negative pole. Balanced means that both signals have the same function and the role.

Balanced signals originally was used with transformers on both output and input. The signal was carried on two wires from the output transformer and connected directly to the input transformer. The shielding, or as we tend to call it ground was only used to create a shield against unwanted electrical interference. Often, the shield was only connected at one end of the cable.

Today, we have changed the way we implementing balanced connections as transformers are expensive whereas IC circuits are cheap. But it will still work with transformer based circuits if you want to connect the boxes you have bought. We call one signal hot and the other cold. What we see, in a correctly designed circuit is that when the hot signal is at plus voltage, the cold signal is at minus voltage. And so on, changing with the signal. An alternative way to describe this is that the two signals our 180 degrees out of phase. The ground signal is still optional, it might be part of the circuit or not.

In a balanced XLR cable ground is connected to pin 1, hot to pin 2 and cold to pin 3. It is almost always possible to connect the shielding to pin 1 on only one side of the cable -- this is sometimes known as ground lift and some boxes has a switch for selecting this. Sometimes the ground can be required between different boxes but this really is rare, the problem solution occurs if the two boxes wants to drive ground towards too different voltage from some reason, outside the range of the input circuits. In real life, ground lift is useful more often than the optional ground connection (connecting the shield to ground at one side is still a good idea).

There is one exception to not needing ground -- when phantom powering microphones. Phantom power, often 48V, is a smart invention going back far in time. Here the signals on hot + cold are still there, but a common voltage of +48V is added to both. Here the ground is the "return" for the power used by the microphone, and no longer optional. (By the way 48V was choosen as it was already common in wired telephone systems at the time). (NOTE: the 48V is protected against short circuit with 6.8 kOhm resistors).