classe, bryston fully balanced?

All audiophiles, aspiring or non, should be interested in learning why differentially balanced XLR is high end whereas even the best SE- RCA is not. Alas there is no manula or book, or article. Even the magazines/reviewers are afraid to rock this boat.

I've posted a thread in Audiocircle regarding this issue and this is the only link that James Tanner had provided :-

http://www.bryston.ca/newsletters/31_files/vol3is1.html

Is Your System Out Of Balance?

The 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 uhe 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 amplifer 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 amplifiee.

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 amp]ifier 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 amplyfing 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 hy 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 separale 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 a host of disadvantages from cost to noise overload. to complexity and reduction in reliability. It has no useful advantages in the digital or analog signal chain beyond the mic preamp. Bryston audio components with the exception of our BMP-2 mic preamp, all operate their balanced inputs on differential amplifier technology.

Some of the Classe's are balanced, including the CAM 350 and 400 along with the Omega series. Monoblocks are more likely to be differential because they're often adapted from stereo versions. The Plinius SA series can be bridged to a differential monoblock (about 4X the power) with a turn of a knob and a couple hook-up changes.

Bryston refers to a balanced output for the monoblock 7B's and since a 14B is claimed to be dual 7B's in one chasis, we might assume that as well. I have not read any claims to a fully differential design which seems a bit odd for a product with such pro/commercial usage. The input impedance is quite different for the XLR inputs but that isn't any kind of proof and it's certainly not the 600 ohm "pro standard".