Dual Differential / Balanced?

Kijanki, I have to straighten you out on a few things!

1st- the use of balanced line is lower noise from the cables no matter the length, assuming that the input of the amp is properly designed.

2nd- the even ordered cancellation thing the way you are presenting it is incorrect! How that works is an amplifier that is fully differential and balanced will have even ordered cancellation **from input to output**, not just at the output. What this means is that less distortion will be compounded from stage to stage. If in the case of a tube amplifier, this means the primary distortion product will be the 3rd, which is still considered musical to the human ear. However it will be generated at a much lower level than the 2nd would have been!

At any rate you are not removing even ordered harmonics from the signal- all you are doing is preventing even ordered harmonic generation, which means the circuit will be more neutral.

3rd, the idea that the signal goes through many more stages is a myth. In our amps, the signal goes through exactly one stage of gain, which is less than most entirely single-ended amps. The complexity of the circuit is not the same as the signal path! Our preamps have only 3 stages of gain from the LOMC phono input to the line stage output. Thats a fairly simple signal path compared to almost any preamp. Additionally the output impedance has no bearing on the fact that the amplifier is fully balanced/differential.

Since outputs have no ground reference (current doesn’t flow thru ground) feedback might cause instability. That requires additional third feedback for common mode.

Again, I don’t see how this scheme can help with noise. In addition even connection with XLR cable is not always the best. If you don’t have a lot of noise you might be better of with plain single ended RCA since additional circuitry to make balanced output affects sound.

Essentially the statements in the above quote are false.
@nyhifihead,
if you are looking at solid state amps, the McCormick and Pass amplifiers are quite good and accept a balanced input correctly (many high end audio amps do not, likely because the designers don't know that there is a standard for balanced line operation, defined by AES file 48).

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!

 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.

Ignoring ground and having a floating circuit are two very different things! Our circuits are referenced to ground but are designed to otherwise ignore it.

BTW if a transformer is used at an input or output (for example to allow a balanced input or output for otherwise single-ended operation) the best way to do it is to not have a center tap, with the winding of the transformer simply tied to pins 2 and 3 of the XLR (the signal pins). Pin 1 is then usually tied to chassis ground. This gets the best Common Mode Rejection Ratio (CMRR). The use of a center tap for ground will degrade the CMRR.

dear Ralph,
is your preamps transformer coupled? (or is it in the signal path or stages?)
btw, may i ask if your preamps are designed with absolute no local, no global feedback in power supply & audio circuit

Our preamps are not transformer coupled- instead they have a direct-coupled output that is balanced and differential. Any DC Offsets are controlled by a simple servo circuit (we went through quite a bit of effort to prevent the servo from being a feedback mechanism in its own right). We obtained a patent regarding this operation. We do not use feedback in our audio circuits (loop feedback or degenerative feedback) although the power supply regulation does.

Thanks kirkus for your comments.

Bill Whitlock's paper is excellent. I like the Rane page on balanced operation (http://www.rane.com/note110.html) as a succinct description of the balanced line standard, AES file 48.

^^ The 600 ohm resistor was used in the old days as a termination device that ran double duty. Part of it was to keep the impedance low such that cable artifacts were swamped by the low impedance. The other part was that the output transformers used to drive 600 ohms need to be loaded at 600 ohms to get flat frequency response (many transformers will express the inter-winding capacitance rather than their turns ratio if not loaded sufficiently).

So that 600 ohm standard was around for a while but was not included in AES file 48. For this reason, if one follows only file 48 one might still run into minor differences in the sound of cables, but if the equipment in use has lower output impedances much of that will be swamped. IOW I am saying that AES file 48 does not cover the entire topic (but its got the lion's share of it for sure). Since there is still a lot of equipment in use that supports the older 600 ohm standard, we made sure our gear did too so it would be compatible.

Now I've not looked at the newer Audio Precision stuff, but what they were making in the early 1990s did not conform to AES file 48. I'd be interested to see if the later stuff does. How it did not conform back then had to do with the fact that it referenced its inputs on its balanced connection with respect to ground (pin 1) instead of each other (pins 2 and 3).  I have to say this really surprised me because then as now the Audio Precision equipment represents a high standard of quality in audio test equipment!