Hi Guys,
The fully balanced concept does have some concerns for me. Below is an article I did about 10 years ago on the subject and I guess my concern is whether the decision to produce a fully balanced preamp has more to do with marketing than performance.
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 place, 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 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 pricisely 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.
Bryston makes a product which operates in the fully-balanced mode a microphone preamp (BMP 2), but this unit has an input transformer which rejects common-mode noise by a factor of over 250,000:1. The reason it operates on two separate signal paths is to expand its dynamic range beyond what digital storage media can accommodate. Since the next step in the signal path is into digital storage media (CD, DVD etc.) from there, this separate signal path is obviously not a concern in any following signal-processing on its way to your living room, and your ears.
All the above simply points out that what has been called fully balanced circuitry has some issues from cost to noise overload, to complexity and reduction in reliability. It can be asked if there is any useful advantage in the signal chain beyond the mic preamp?
james
