Why Single-Ended?

Thanks Al,  Third harmonic, that supposed to be euphonic, will be the same in fully balanced or single ended design.  Removal of the even harmonics cannot make amplifier sound "warmer".  It is achieved by reducing higher order odd harmonics produced by the amp.  It is likely related to excellent design and not topology itself.

Also, FULLY balanced amp has practically two amps inside and costs much more.  I could buy much better single ended amp instead. That's why I would never buy FULLY balanced amp ("Fully" is not always "Better").

This statement is false. A fully differential amplifier does not have twice the parts and is not nearly two amps inside! This is a very popular myth.

Our amps tend to be less expensive than SET amps with a fraction of the power, and yet we have more bandwidth with lower distortion (without using feedback). As any transformer designer can tell you, a good SET Output transformer is a bit of a trick! Its very hard to make them in higher power levels- for this reason you can easily build push-pull amps that outperform them for less money.

Of course, we have the added benefit in the cost department of no output transformer...

Balanced input of my amp goes to instrumentation amp (THAT1200) that has CMRR of 90dB@60Hz and 85dB@20kHz. I don’t believe that it can be achieved without cross-feedback in Fully balanced amp. 90dB would be equivalent to overall gain setting (all stages) resistors matching to 0.003%. This cannot be done (be stable) even with multiturn trimpots, etc. Perhaps Fully balanced design offers better rejection at higher frequencies, but I doubt it - frequency response of both halves would have to be identical.

We tend to get about 87-92db CMRR without feedback of any sort. The key is proper Constant Current Source design and I can safely say that most CCS circuits I see in most amps are terrible. Hint: a good CCS design will employ two stages. It simply isn't possible for a CCS with one stage to work right.

I can hear the difference between different XLR ICs - perhaps Ralph was talking about matched 600ohm input/output designs that I’m not familiar with.

If this is the case its probable that your gear does not support the balanced standard. Let me guess- you can run the signal single-ended just by disconnecting pin 3, right? IOW what I am saying is that in your gear, the non-inverted signal occurs between pin 2 and ground (pin1) and the inverted signal is pin 3 and ground. If that is true, then the gear does not support the standard as ground is not ignored, and all of a sudden the cable becomes audible. This is a very common design error and to give you an idea of how common, I saw that mistake being made in a piece of Audio Precision test equipment 20 years ago! I don't know if they ever fixed that...

Third harmonic, that supposed to be euphonic, will be the same in fully balanced or single ended design.  Removal of the even harmonics cannot make amplifier sound "warmer".  It is achieved by reducing higher order odd harmonics produced by the amp.  It is likely related to excellent design and not topology itself.

This harmonic structure thing is another myth. How it works is, if the circuit is fully differential and balanced, the primary distortion product will be the 3rd harmonic, at a diminished level (IOW much less than you would see of the 2nd harmonic in a tube amp). This is true whether the amp is tube or solid state! The implication here is that the topology in fact plays a huge role. In a tube amp the higher orders will be absent given proper design.

When you think about it this makes sense. After all, triodes are quite linear so why should a tube amp make more distortion than a solid state amp? The answer has a lot to do with topology and how much feedback is applied. We don't use much in the way of feedback as we are trying to avoid higher ordered harmonics (and its the feedback that contributes to that in most designs).

One thing you are not taking into account is how distortion compounds from stage to stage. If the gain stage just does not make the distortion, it can't be compounded by the distortion of the next gain stage. We only have one stage of gain in our amps, so higher ordered harmonics really don't play a role. This allows the amps to be very relaxed.

This statement is false. A fully differential amplifier does not have twice the parts and is not nearly two amps inside! This is a very popular myth.

 
I don't know your designs, but usually output stage, at least in SS amp has to be doubled, as well as stage driving it.  You can place differential amps in front of it, but most of the expense is already there.

If this is the case its probable that your gear does not support the balanced standard. Let me guess- you can run the signal single-ended just by disconnecting pin 3, right? 

Not sure of that.  Perhaps, since it is done without input transformer there is ground reference for instrumentation amp that is usually connected with higher value resistor (and small cap) to a chassis. Signal is still differential and I cannot understand why would it reduce effect that cable brings.  Cable capacitance, inductance and dielectric absorption are still there.  Do you think that extremely dirty copper would sound wonderful in balanced cable?

This harmonic structure thing is another myth. How it works is, if the circuit is fully differential and balanced, the primary distortion product will be the 3rd harmonic, at a diminished level 

It does not make sense.  I understand how even harmonics are eliminated but don't know of any mechanism that would remove odd harmonics in fully balanced amp.  Remember we're talking about Fully balanced amps in general - not only your designs.

One thing you are not taking into account is how distortion compounds from stage to stage. If the gain stage just does not make the distortion, it can't be compounded by the distortion of the next gain stage. We only have one stage of gain in our amps, so higher ordered harmonics really don't play a role. This allows the amps to be very relaxed

That's great, but it will be also true for well designed single ended amps.   There is no gain difference between topologies that would explain any additional distortions.

I'm not sure why Nelson Pass designs Fully balanced amps.  Perhaps it is market demand, that the tone of OP question suggests.  For the same reason designers still use unregulated linear power supplies instead of line and load regulated extremely quiet SMPS  (Jeff Rowland uses them).  I suspect that in your case objective, since there is no output transformer, might be to get more output power.  Once you have double output stage front is just a small addition.

This is the good side of an open forum discussion. Two obviously qualified people who can clearly (and politely) present strong positions for the merits of opposing technologies. Kijanki, you have made one of the most reasonable cases for the sonic advantages of the single ended circuit and its simpler design. This positive perspective is rarely seen. Well done gentlemen!
Charles

I totally agree Charles. I've been following and enjoying all of these well informed ideas and learning quite a bit in the process. Sounds as if both topologies can offer great sound if properly designed and implemented. Kudo's to all the gents that have contributed their vast knowledge in such a civil fashion. I'm looking at purchasing a new pre and amp that are OTL but don't know if they are single ended or fully balanced.....on one hand this discussion makes me want to find out but on the other hand I guess it doesn't really matter since either approach has its merits.