Dual Differential / Balanced?

I won’t even comment on the issue of instability because you can find it in any FAQ/primer on fully balanced design, but it appears that you question my notion that connection with XLR cables is not always the best. Experience of many people on this forum was that RCA connection sounded better than XLR.

That is true, please consider though that there is a standard for balanced operation, of which about 95% of high end audio ignores.

The result of not adhering to the standard is that you may well get inferior operation. When balanced line is done properly (and can be done with single-ended circuits, the Ampex 351 tape machine being an excellent example) the result is that the interconnect cable ceases to be a part of the system sound. A common example of an audio company not adhering to the standard is any ARC balanced preamp. The standard calls for ground to be ignored at the input and output- it is for shielding only. But the ARC preamps put signal current through the shield of the interconnect, which causes the construction of the cable to suddenly become critical. The result is that the cable might have an audible fingerprint when used with such a preamp (and I’m not picking on ARC beyond example, this is a very common situation with many balanced products in high end).

So suddenly something that is supposed to eliminate cable interaction doesn’t; IOW the balanced cable is going to editorialize just like a single-ended cable, which IMO makes me wonder ’why bother?’ As you pointed out, others do too.

IME, audiophiles obsess over interconnects and for good reason- they often sound different and often one sounds better than another in their system. I’ve experienced this a lot; I’m not one of those engineers that thinks this is all some sort of audiophile fantasy!

But the fact is, if you can get the interconnect cable to not editorialize, it will result in better sound. So far the balanced line system is that solution. That is why I mentioned the AES file 48; even though I’m not a particular fan of the AES in general (lots of the types of engineers I just mentioned tend to hang out with that otherwise august group), the definition that is file 48 is rather handy and succinct. Unfortunately unknown or misunderstood by the majority of high end audio.

So yes, there are many on this forum that think RCA connections sound better out of experience, I’m just pointing out that is happening due to the balanced system not being executed correctly (which BTW, also means that the length of the cable is irreverent). That is why there is even a debate in high end audio about the benefits of balanced!

Well Ralph your  explanation of misuse /improper application of "balanced " circuits would explain why I don't usually find them better sounding in many audio systems. Nonetheless single ended /unbalanced components can  often sound fabulous. Superior implementation of either circuit type is mandatory. 
Charles, 

Thank you Ralph, I will keep reading more.  I use fully balanced stages in low level amps without any ground reference.  It has bandwidth of few kHz only, but full scale of the signal is in single millivolts.  At the end A/D converter with differential input provides its own ground reference.  I can see a problem with fully balanced audio amp design that has no reference to ground.  Both outputs can be floating together since without output current or voltage difference feedback won't react to that.  It needs ground reference somewhere or some kind of servo on common mode. 

Shield should never be used to carry signal but it was unfortunately common practice long time ago.  Scope's coax is a good example of that.  Scope with shorted leads, touching circuit under test, shows phony signal - since ground return path (possibly thru supply) causes current flow thru the shield, that input amp (referenced to scope's BNC GND) sees as a signal (voltage drop on the shield).  Shield converts common mode to fake normal mode signal.

My small Rowland has only XLR inputs - perhaps mature decision in class D amp, so I was not able to compare it with single ended RCA cable.  I still can see substantial sound quality difference between decent XLR cable (AQ King Cobra) and very good one (AZ Absolute). 

I have to read AES48 standard, you mentioned.  There was wonderful EDN magazine issue on that many years ago. Grounding and shielding is considered by many as black magic.

Kkijanki,
A friend of mine has the Atma-Sphere  MP-1 preamplifier and the MA-1 amplifiers. With this fully differential balanced  signal path he says he can hear  differences between various brands of balanced  interconnects.  By the way his MP-1 sounds terrific mated to my Coincident 300b SET amplifier. 
Charles, 

Charles & Kijanki, see Ralph’s first post dated 3-22-2013 in this thread for a summary of what is necessary to make a balanced line-level interface insensitive to cable differences. And also for what I consider to be a compelling proof Ralph offers for his contention.

But also note a question I posed a few posts later in that thread, and the answer Ralph provided:

Almarg 3-27-2013
... to eliminate interconnect cable differences is it necessary that the component output actually BE driving a low load impedance (2000 ohms or less, to use your figure), or is it just necessary that it be CAPABLE of doing that?

Atmasphere 3-27-2013
You touched on an interesting point about the load vs the capability to drive that load. I have found that the capacity to drive the load plays a huge role, about 80% but for that last bit of cable artifact to go away the load must be there as well. Flipping the coin over- if the capacity to drive the load is absent, then its moot and there will be cable colorations.

I see that the MA-1 amplifier which Charles mentioned has a specified balanced input impedance of 200K. I’d imagine that one reason Ralph would have chosen to not provide a very low input impedance, such as the 600 ohm standard he refers to, is that most preamps made by other manufacturers would not be able to drive such a load with good results.

Best regards,
-- Al