Bryston Launches BP-17³ Cubed Preamplifier

[Grit]

Originally Posted by James Tanner - Bryston

Hi Folks,

Here's a little article I did on fully balanced circuits a while back.


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 too 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 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.

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.

Also remember the above supports no need for ‘Fully Balanced’ from a performance standpoint. The 7B, 14B and 28B's are fully balanced because they are designed as ‘SERIES’ amplifiers not because they have a performance advantage.

james

Hey James,

If I interpreted this correctly, you'd recommend single-ended cabling, at least for Bryston components?