definition of balanced mode vs. single ended

Equipment that is "fully balanced" inside has duplicate amplifiers such that the + & - legs of the circuit are phase-inverted and referenced separately to a common ground. When the inverted phases are summed together at the end of circuit, noise from one side cancels noise from the other (achieving "common mode noise rejection"). This offers greater signal-to-noise ratio relative to a single-ended circuit utilizing one amplifier of comparable quality. The fully balanced circuit with twice the components is of course also more expensive. The cable has three pins & a shield, not five as mentioned by Ehart.

I suppose it's possible to carry two balanced channels on a single 5-pin connector, but in audio one usually finds an XLR connector with three pins (hot, cold, ground) plus the barrel. The barrel can be connected to a shield or directly to the ground pin, but typically the barrel is left floating. BTW, I would doubt that a balanced cable makes much difference (even on long runs) when attached to a component that isn't full-balanced. Such components often add a phase-inverter to derive a balanced output. The additional electronics at the balanced output may degrade the sound relative to the RCA outputs. An external XLR/RCA converter plug uses just the hot/ground pins of the XLR junction to derive a single-ended signal for the RCA, discarding the inverted signal on the cold/ground pin side. In this scenario, half the circuitry in your expensive full-balanced component goes to waste.

S7horton & Bob Reynolds,

There are some quiddities with pseudo-balanced designs that compromise rather than take the "full-balanced" approach. For example, like Bob I also use transformers in my CDP (Sony SCD-1) to create a balanced signal. The stock SCD-1 is a strange beast-- particularly for a statement product-- insofar as it has fully differentially balanced DAC, but converts to single-ended in the analog gain stage, and back again to balanced using an op-amp based module at the XLR output. This is doubtless a compromise to cut costs in the analog stage, while adding additional electronics that degrade sonics. A passive step-up transformer replacing the entire output stage is a big sonic improvement. The output of the secondaries can be considered balanced insofar as the (-) signal is lifted from ground. But I'm not sure I'd called what the transformer does "true-balanced", in that it does not invert signal so as to provide CMMR.