Dual Differential / Balanced?

Al, I was looking at overall output impedance that speaker generated EMF sees, and that would be double. 

I also stated that fully balanced design is not only unnecessary to provide great noise rejection, but in fact might be worse than one achievable in single ended amp with balanced input. High CMRR would require perfect gain matching of both "legs" of the amp and that is not possible at the level of good instrumentation amp (90dB @60Hz in my amp).  It would require 90dB  (0.003%) gain matching with discrete components, calling for <0.0015% resistors, that don't even exist.  It gets even worse at higher frequencies where both multistage amps/legs would have to have identical frequency characteristics.  Same is of course true for instrumentation amp, but to much smaller degree.  Resistors are laser trimmed on the same substrate while amplifier's bandwidth can often be much higher (22MHz in my amp).

Al, I was looking at overall output impedance that speaker generated EMF sees, and that would be double.

Thanks, Kijanki. When I commented about the doubled output impedance question I had lost track of the fact that you were addressing power amplifier outputs. Yes, **everything else being equal** the load applied by an amp to back EMF from a speaker will be numerically doubled (corresponding to half the damping factor) if the outputs are balanced compared to if they are single-ended. Although in the case of most solid state amps, at least, that is unlikely to matter since their damping factors will be sufficiently high for most speakers anyway. Notwithstanding the belief among many audiophiles that, for example, a damping factor of say 500 may result in better bass control than a damping factor of say 50, everything else being equal. I know from many of your past comments about damping factor that you would agree with my disagreement with that popular (mis)conception.

Regarding your last comment, I believe you are envisioning a balanced amp configuration that is used in some designs in which each of the two input signals in the balanced input pair is processed through its own signal path throughout the amp, that is independent of the signal path through which the other input signal is processed. While I believe Ralph is addressing the configuration used in his (and other) designs in which each stage of the amp, including the input stage, is a differential amplifier that responds to the difference between the pair of signals its inputs are provided with. And I believe that your comments and his are each essentially correct in the context of the kinds of designs each of you is envisioning.

Best regards,
-- Al

Kijanki,
Thanks for your interesting and informative comments here. Listening confirms many of your points regarding real life sound quality. I appreciate the theoretical advantages of differential balanced circuits. Coincidence or not some of the very best sounding Amplifiers I've heard were simple unbalanced circuits with RCA  cables. As you and Al  point out  implementation is a very major factor. 
Charles, 

thanks so much for the replies, I'm learning a lot.

sorry for the lack of circuitry knowledge, most of my info is based off of forums like these and it seems the way people describe balanced is tossed inconsistently, I just got schooled though!

I thought the RDC 7.1 had a balanced design because the dacs are dual differential PCM1704, and because BAT had a hand in developing.

The matching Integra research/BAT RDA-7 power amp I know is fully balanced, so I assumed that they would make the pre/pro to match

I've had McCormack/Pass on my mind, how are the classe balanced input?

also does anyone know how good the circuitry is on the emotiva XPA-1? it claims fully balanced and looks impressive

thanks

Al, I agree, that was invalid point - double DF doesn’t make much difference in most cases. As I stated in my older posts speaker’s impedance is mostly resistive and it is in series with back EMF reducing max possible DF to about 1. As long as amplifier doesn’t destroy it too much it is OK. Some of Ralf’s amps had DF<1 sounding fine.

You are right that I assumed two independent amps in differential mode - that’s different from balanced. In order to make identical gain stage in each amp they have to reference each other (like in the first stage of instrumentation amp) instead of being ground referenced. In that case amps aren’t really independent as I thought they should be. Noise rejection in such design should be great but it is not worth extra expense IMHO. Single ended amps with balanced inputs might have great noise rejection as well, while better components can be utilized for the price of fully balanced design.