That's incorrect, the line driver for an XLR output is simply a polarity inverter that exposes both the original and the inverted signal at the same magnitude. The trick is that both lines are equally affected by the interference, you invert the inverted signal again on the receiving end resulting in 2x the signal and 0x the interference.
That's just one of the possibilities. You can either do the cheapo one-line-driven-by-inverter trick, or use a transformer or a differential line driver (like 8903A output stage); basically the opposite of an instrumentation amp. These behave pretty much the same as a transformer, i.e. they have high common mode output impedance. Available in chip form as well, e.g. DRV135.
Problem is the circuit needs very accurate resistors and needs to be carefully balanced, just like an instrumentation amp.
Sorry, what’s different here? The second line is there so that its signal can be subtracted (inverted + added) from the first line, no? I guess I didn’t realize it also had the the inverted audio signal on it, though, thanks.
