Why Single-Ended?

Phase has to be inverted somewhere, either for TRUE balanced (XLR cable) or FULLY balanced design.  It involves extra circuitry that does not make sound more transparent.  For short connection, in electrically quiet environment, single ended design might be a better choice.  As for the FULLY balance design - it offers slightly* better common mode electrical noise rejection at very high frequencies and cancellation of even harmonics produced by the amps.  I don't care for both since my connections are short, while shields and twisted pair, in my XLR cable, work really well.  Cancellation of even harmonics, produced by the amp, makes it sounding colder, while odd harmonics, responsible for brightness, are left intact.  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").

* In order to provide good common mode rejection two halves of FULLY balanced amp cannot be independent.  Negative cross-feedback has to be used to equalize gains of each half and that might be far from perfect.

Hello Kijanki,
That was a very interesting and well reasoned post describing some of the inherent advantages of single ended topology. Just as Ralph presents a compelling case for fully differential balanced circuit.   It is quite clear why both topologies  can result in excellent sounding audio components. 
Charles 

+1 Charles.

Regarding this comment in Kijanki’s post:

In order to provide good common mode rejection two halves of FULLY balanced amp cannot be independent. Negative cross-feedback has to be used to equalize gains of each half and that might be far from perfect.

With some fully balanced architectures the need for negative cross-feedback, or even any feedback, can be minimized or avoided. For example, Ralph’s designs employ an architecture based on differential stages, and are typically spec’d as using just 1 or 2 db of feedback. And in the solid state domain many fully balanced Ayre amps employ zero feedback, and I believe most or all fully balanced Pass amps use very minimal amounts of feedback.

Also, as stated in one of the papers at Ralph's site, "for a given number of stages of gain, differential amplifiers have about 50% more parts," rather than being closer to the equivalent of two single-ended amps that are otherwise comparable. Although with some other balanced architectures the parts count may indeed be close to double.

Best regards,
-- Al

almarg, thanks for the advice.

I have a pair of ISO-MAX PI-2xR converters I have used in the past, but it's a bit a kluge.  I hadn't thought to try converting just one side of the KUBE link to balanced -- a half kluge.

An alternative in the manual for the KEF 107/2s is to insert the KUBE in the tape loop of the preamp, but it's not obvious to me how that works.  The JC 2 BP does have a fixed output for recording but no monitoring capability.  I suppose that output could go to KUBE, but where would the output of the KUBE go?  If to one of the JC 2 BP inputs it would become the source.  

The manual for the Ayre phono stage (I use a JC 3) includes diagrams that seem to suggest converting an RCA plug from a turntable to a balanced input, but I may be misreading that.

Finally, the noise level of the system is low enough to be inaudible even with an ear pressed to the mid/HF driver.  My concern is maximizing sound quality of an excellent sounding setup, so maybe it's wild goose chase.

db  

Al, since I don’t see removal of even harmonics (while leaving odd harmonics intact) as an advantage, then the only thing that speaks for Fully balanced amp is better noise rejection in comparison to amp with True balanced input. 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. 

Perhaps it is achievable using differential stages, but unless gain setting resistors are not on the same substrate stable 90dB does not seem possible.  Perhaps he is using very expensive matched resistors, but price of his amps justifies the cost of components.

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. In any case, it is valid for both True balanced and Fully balanced.
Again, I would buy an amp if it sounds good and not because it is Fully balanced. If anything, I would avoid fully balanced amps as too complex (too many parts to fail).