Balanced cables

Wlutke, that’s sort of correct, but I would put it somewhat differently. In a good design an XLR cable will be provided with inverted and non-inverted signals, as you indicated, and those signals will be received a differential receiver circuit which responds to the instantaneous **difference** between the voltages of the two signals. Since the two signal wires in a properly designed XLR cable are twisted together, and the circuits they are connected to cause (or at least should cause) them to have equal impedances relative to ground, induced noise will be essentially identical in the two runs, and therefore there will be little or no difference between them, in terms of noise, for the differential receiver circuit to respond to.

In many designs it is also the case that the amplitude of each of the two signals in a balanced interconnection has the same amplitude as the single signal in an unbalanced RCA interconnection. In such a case the instantaneous difference in the amplitude received by a differential receiver circuit via an XLR connection will be twice the amplitude that would be received from a single-ended signal, resulting in the 6 dB difference in overall system gain that you referred to (assuming everything else is equal, including the setting of the volume control).

There are some lesser designs, however, which provide XLR connectors simply as a convenience feature, and provide that connector with just a single-ended signal and a ground. Also, in cases where a differential pair of signals is provided and received, the quality of the differential driver and receiver circuits may be less than optimal in some cases, and an RCA connection might provide superior results in those cases.

Regarding the grounding scheme, as indicated in the Rane application note which Atmasphere referred to in one of his posts dated 3-1-2018, ideally the shield of an XLR cable should be connected via pin 1 of the connector at each end to the chassis of the connected component. However for whatever reason many designs connect the shield via pin 1 to signal/circuit ground instead, which depending on other aspects of the design can result in some fraction of the signal current being conducted via the shield, in turn increasing sensitivity to cable differences as Ralph explained.

Regards,
-- Al

Atmasphere 2-28-2018
Its probably not the amp that is causing you to hear the cables so much as the preamp. But there is a recent period of ARC amps that had me scratching my head. I’m not sure which models they are. They were out when Kalvin Dahl (with whom I went to school) was still at ARC (about 2-3 years ago). Apparently the amp has a very low CMRR (Common Mode Rejection Ratio) so it only has balanced inputs. Apparently also if you try to run it single-ended the power goes down and the distortion goes up.

I can’t think of a good reason for a low CMRR in a differential amplifier (which is what these amps use). You wind up leaving performance on the table (I’ve been designing differential circuits since the mid 1980s).

Ralph, I’ve never been able to find a schematic for any of ARC’s amps which only provide balanced inputs, such as the recent Reference series amps, but I’ve had the suspicion that instead of using differential stages they basically have a separate signal path through the amp (up to the output transformer primary) for each of the two signals in the balanced signal pair they receive for each channel. That would be consistent with a very dramatic reduction of power capability as well as an increase in distortion if the amp were to be provided with unbalanced inputs via RCA-to-XLR adapters or adapter cables, as was found to be the case with the Ref 150 used by the OP in this thread we had participated in some time ago. I believe it would also be consistent with low CMRR, due to the gain and other characteristics of the two paths not matching precisely.

I have no idea why they might have chosen that kind of approach, and I can’t say for sure that they did, but if I am correct in suspecting that they did so it would seem to explain some or all of the things you mentioned about their amps that you referred to above.

Best regards,
-- Al

Audiozen 2-27-2018
... just about all high end mono amps have both RCA and XLR inputs as well as dual mono stereo designs in a single chassis. So if a balanced dual mono/stereo preamplifier has both XLR and RCA outputs, and both outputs from XLR or RCA or moving signal from a separate mono channel for left and right, then in essence their both balanced cables doing the same thing.

They are both doing the same thing in that they are conveying, or at least attempting to convey, the same information from one component to the other. But they are doing it in very different ways, with results that are unlikely to be identical.

A balanced interface by definition has two signal lines which have the same impedance between each of the signal lines and ground, at least to a **very** close approximation. That equality of the impedances between the two signal lines and ground is a necessary condition for a balanced interface to provide the noise rejection it is known for. It is also frequently the case that those signal lines carry a pair of signals that are of equal amplitude but opposite polarity, which can provide additional advantages such as improved signal-to-noise ratio, reduction of certain forms of distortion, and minimization or elimination of cable effects (if the criteria Ralph has described are met).

Components whose internal signal paths are balanced but which provide RCA connectors in addition to XLR connectors typically convert the unbalanced signal provided to the RCA connector or received from the RCA connector to or from a balanced pair of signals that is processed internally, with the conversion often being accomplished via either an active stage or a transformer. Or in some cases the RCA connector is simply connected to one of the two signals in the balanced signal pair, with the ground shell of the RCA connector being connected to circuit ground.

An interface which conveys a single signal and a ground connection is not balanced, both by definition and as a practical matter.

Regards,
-- Al

Fsonicsmith, thanks. I essentially agree with everything in your post above. There are always myriad tradeoffs in a design, and the net result of those tradeoffs often will not conform to commonly stated paradigms.

For example a major reason, and perhaps the most major reason, for ARCs 20K minimum load recommendation is their use of a coupling capacitor at the outputs of their line stages and preamps, which of course is very commonly done in tube-based line-level components, and which among other things causes a substantial rise in output impedance at deep bass frequencies. And while that effect could be minimized by simply increasing the value of the capacitor (i.e., the number of microfarads), doing so would most likely increase the sonic colorations introduced by the capacitor itself, and/or result in the capacitor being a larger and potentially impractical physical size.

And an alternative approach that is used in some tube designs, using a transformer instead of a capacitor, certainly can have tradeoffs of its own.

Regarding feedback, yes, ARC power amp designs tend to use more of it than many and probably most high quality tube amps from other manufacturers. Consequently they have lower effective output impedances and higher damping factors than those other amps, which in turn can be either an advantage or a disadvantage or neither depending on the particular speaker that is being used. And the downsides feedback can potentially have with respect to the intrinsic sonic character of the amp will depend on numerous other aspects of its design.

As you aptly summarized, "it all depends."

Stringreen, I have no particular knowledge regarding the sensitivity of your Ayre components to cable differences. An experiment that could possibly be informative, though, would be to **temporarily** put a cheater plug on the amp’s power cord, to defeat its safety ground connection. If that results in a perceptible sonic difference it would suggest that ground loop effects may be present, which in turn can result in sensitivity to cable differences especially when the length involved is long. Note Ralph’s criterion no. 2 for minimizing the sensitivity of balanced interfaces to cable differences, in his post that I referenced earlier:

Atmasphere 3-22-2013
... 2) Ground is ignored- the signal occurs only between pin 2 and 3 (this is where most high end audio preamps have a problem- as soon as there are signal currents in the shield of the cable, the construction of the cable becomes critical).

Regards,
-- Al

Fsonicsmith, as a point of information ARC line stages and preamps do not meet the criteria Atmasphere cited in his post that I referenced that would minimize or eliminate sensitivity to differences between balanced cables.  Certainly, at least, with respect to his criterion no. 4:

Atmasphere 3-22-2013
...  4) the output of the preamp should be capable of driving a low impedance load (2000 ohms or less) without loss of voltage, without increase in distortion and without loss of bass (this is the other big area where high end audio preamps have a problem, and also results in cable sensitivity).

ARC's recommended load for nearly all of their line stages and preamps is a minimum of 20K, and in some cases a minimum of 60K is indicated as being optimal.

So when "ARC stresses in their manual that the quality of the XLR going into the Ref 150se is critical," as you indicated, they are indeed "telling the truth."

Regards,
-- Al
 

In many and I suspect the majority of cases, yes. In some cases no. It depends on the designs of the components that are being connected.

For the design characteristics of the interconnected components that are necessary to minimize and quite possibly eliminate the sensitivity of a balanced interconnection to cable differences, and for what I consider to be compelling proof of that contention, see the post by Ralph (Atmasphere) near the beginning of this thread. Also see the follow-up questions I presented to him later in the thread, and his response.

Regards,
-- Al