Alternatively, the balanced equipment discussed above separately
amplifies the positive and negative parts of the electrical signal.
This requires double the circuitry because you have to amplify two
signals (0-to-positive and 0-to-negative) instead of one
(negative-to-positive) as in single-ended equipment. If you imagine
music as a sign wave, then balanced equipment is separately amplifying
the upper and lower halves of the wave. The benefit of this added
complexity is that any noise introduced into the signal by your
equipment will be cancelled out when these two "half" signals are
re-combined to make the full-wave signal that drives your speakers. This
is because any injected noise will appear positive going in one
amplification circuit and negative going in the other... add them
together and you get automatic noise-rejection because they'll be in
exact opposition.
@cal3713 The bit about positive and negative parts is problematic. Usually for balanced operation differential circuits are used, and its probably easier to understand that they amplify **opposites** rather than 'halves'. And it does not require double the circuitry- this is because in a differential circuit, the cathode (or emitter, or source) circuit has all the parts used in it in common. As far as noise or distortion goes, the benefit of cancellation accrues with each stage, not just at the loudspeakers. Finally, for a given single-ended gain stage, if done differentially will have theoretically 6dB less noise generated. This can be significant from one end of a circuit (like a preamp) to the other. Two stages, each with 6dB lower noise, that makes 12dB... So you don't need as many gain stages. In our full function preamps there are only three stages of gain from phono input to main output and they can work with LOMC cartridges; contrast that with a typical single-ended preamp which will have at least 4 stages of gain (unless an SUT is employed) or more to do the same thing and you can see that the parts count myth is just that.
