Balance ??

Not all amplifiers, not even all good modern ones, have balanced inputs. Pass Aleph 3, for example, doesn't. Not all digital sources have balanced outputs. Likewise not all preamps: the CAT's are an example. A balanced circuit involves doubling a lot of signal path components, and the CAT designer thinks money is better spent otherwise. One advantage of balanced is that you can use XLR connectors, where, arguably, the cheapest are better than the most expensive RCA's. Ability to use long interconnects is another virtue of balanced. I'm sure others will tell you more, but these are some fundamentals, as I understand it.

O.K., your audio signal is nothing but a wave (Dude!) with a positive cycle and a negative cycle... with me so far? Anyway, imajine spliting off the positive part and sending it to amplifier "A" and likewise sending the negative part to amp "B" (there are actually two amps for each channel which is why Balanced costs more). Onward... Now that you have the two halves isolated, you compare them to each other (the ground-wire value is used somehow as a reference. ever notice balanced interconects have 3 wires?) with the understanding that they should be complete opposites. If anything is found in common, it is assumed to be hum or noise of some kind and stopped dead in its tracks (does not get out of the amp). The result is a very quiet background for your music (Shazam)!

A balanced interface, whether low-level (interconnect) or high-level (amp output) means that there is no signal-ground reference. When a differential signal swings positive on one leg it is simultaneously swinging negative on the other leg. Ground is used only for shielding & does not carry either leg of the signal. Advantages are noise-immunity esp. with longer interconnects, and 6dB greater signal level than unbalanced. My full-balanced power amp. is dead quiet, compared to the very low-level noise of another amp which is unbalanced. Cost is a factor in balanced designs because there are more electronics involved. No need for any 'special' sources. Digital outputs (from a CD player) may be either unbal. or bal. just as analog connections can be done either way. An industry standard impedance for bal. is 600 ohm, vs. unbal. interfaces which vary.