Hi Folks,
I wrote this a while ago but it may help.
Is Your System Out Of Balance?
One question which keeps coming up over and over is the controversy regarding audio components being "fully balanced" versus what is sometimes referred to as "balanced converting to single ended" at the input of the electronic component (preamp, electronic crossover, amplifier etc). The correct term for this balanced converting to single ended is more accurately referred to as "differential amplifier balancing"
Popular mythology has seen fit to 'bless' the concept of 'fully-balanced' (meaning of course, two completely separate signal paths through a component, with its attendant doubling of parts cost and complexity, and halving of reliability). This approach completely misses the point, which is, of course, to eliminate hum and noise picked up by the audio cables feeding the component.
The reason for this is that a differential amplifier, and this is -REALLY IMPORTANT-
‘rejects any common-mode noise’ which appears at its input, by a factor equal to its common-mode rejection ratio, (normally over 1000:1). A 'fully-balanced' circuit has a common-mode rejection ratio of precisely zero, since all signal, common-mode or not, is simply amplified and passed along via the two signal paths. It then remains up to the following component to attempt to reject that amplified noise, if it has a differential amplifier.
Thus, fully-balanced circuitry is subject to passing along any noise which might be picked up on all the cables. Then it hits the final component in the system, usually the power amp, where the differential amplifier at its input is left to deal with the sum total of the common mode noise in the signal path, (multiplied by all the gain in the system). I don't think this is an ideal scenario. If each component, (source, preamp, electronic crossover, power amp), had its own differential amplifier input, it would cancel any common-mode noise which appeared ahead of it, rather than amplifying it.
james
