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".

Fully balanced operation is superior. Well built gear will be trouble free for years. I suspect Ryder is stretching his grey matter a bit. Listen and learn, then buy.

it is my understanding that the big brute 28B-SST monoblock from Bryston is the only truly balanced amp in their stable. My 14B-SST is reportedly not "truly" balanced but I'm happy.

You can pretend to understand something that even engineers aren't quite sure how it works or you can listen and learn from a practical standpoint. Reviews, white papers and audio design philosophy are at best, snap shots into the realm of audiophilia! I once trusted what I read...boy was that expensive!!

All of these are fully balanced: Older Classe, including all the x00 and x01 models, all the T series Threshold; all the older Krell models, Mark Levinson, which run in class A; and some A/B amplifiers, some models, Cary, ARC, and Ayre; and many Edge/Maker, Pass, and Boulder, Burmester, Esoteric, BAT ss, there tube amps and VTL sig amps, for example. There are several late models where they are class A/B which are fully differentially balanced, Maker, Pass, Ayre to name a few.
Anthem, NAD, Rotel, and B&K are not, and Bryston seems to using the differential balanced signal, Parasound may also do this in some models, some McIntosh, yet does not output it, which is probably not a bad Idea. If you have to use short pins, it is usually internally balanced, and if it has a switch, it does have the differential balanced interface.

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 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).

There are some real problems with first paragraph above! What is being ignored is that you can have a balanced circuit, and you can have a balanced differential circuit. Both are balanced. The latter is in common use; the former is rare!! The paragraph is written as if the former is simply the way it is and thus I regard it as misleading.

Here's how it really works. Most balanced circuits have a CMRR that is quite high- in our amps its up well over 100db. Any signal that is common to both inputs can't get amplified. The result is less noise.

Now with respect to the Bryston, on a number of them I have seen they have an RFI filter network on the non-inverting input, but nothing on the inverting input. The result is that noise can get amplified with this scenario as the CMRR gets reduced by this practice.

When you are building a differential circuit (which gets its name from the idea that it only amplifies what is different between its inputs) you want to get that Common Mode Rejection Ratio as high as possible! Low numbers usually refer to a design flaw or outright defect.

BTW transformers usually allow for fairly high numbers, so its possible to have single-ended circuitry internally, but if the input and output have transformers you can get really good CMRR numbers. This is how it was done in the old days in studio recording equipment.

However these days differential designs are well understood. A common misunderstanding about balanced differential operation is that the circuit will work better with a balanced input. The fact is that if the CMRR is high, the difference between using the balanced input as opposed to single-ended will not be significant. If you do see big differences between balanced vs single-ended operation, it points to a design problem and nothing else.

Now having said that a lot of our customers report that our amps sound better with balanced inputs and this is true. But its not so much because the balanced input is being used- instead its because balanced cables by and of themselves sound better than single-ended cables so long as they are driven properly.

This brings us to the balanced line standard which is poorly understood in high end audio. So here it is:

1) pin 1 will be ground
2) the source and the input both ignore ground and instead the signal occurs between pin 2 and pin 3.
3) the source will be able to drive 1000 to 2000 ohms with no loss in frequency response.
4) the cable will be a shielded cable with two conductors arranged in a twisted pair within the shield. The shield is ground only and does not carry signal current (see 2 above).
5) The outputs and the inputs of the electronics will operate with similar levels and gains, such that neither input (pin 2 and 3) are favored in any way.

If the equipment adheres to this standard there will be no hum and cable length will be nearly irrelevant. Also, the cost of the cable will be unimportant as one of the goals is to eliminate interconnect cable artifacts. The balanced standard is very effective in this regard. Oddly, it is this final aspect that one would think would be instantly embraced by audiophiles but I've seen a lot of pushback over the years on this point. Ironic, really.

Excellent description !!! 
I love great music and sound... I have played with a lot of equipment over the years and nearly all my pro gear after the 60's had the "balanced" XLR type connectors and cables for low level signal transfer... mics, mixers, signal processors, amps etc.. I found that some brands of cables and connecters were more reliable than others however there was only a minimal  difference in overall sound quality... as usual some were cheap junk and failed on all fronts...
My experiences with home audio gear over the years has been quite different... having worked full time in a high end audio shop for over 13 years, it was an eye (ear actually) opener!
 At first, due to my pro background I found the whole cable comparison thing to be some kind of delusion until I spent some hard core time listening to high end gear like ARC, Apogee, B&W and ML (CAMACS with silver wires? OMG)... The seas parted, I spent time with Mr Kimber (4pr was magic to my ears) and began the journey of the Holy Grail of cable and connectors of which Kimber was one of the forefathers in my mind...  Much has happened since I left that career for another, and I for one, am very happy to see the "Balanced Line" being accepted by the high end home industry and users... that said, most of my gear is single ended... except for a recently purchased ARC preamp which I suspect will create a cascade of change in my system... for the better!

Classe' Omega series is balanced.