Dual Differential / Balanced?

I can't see how fully balanced system can help with noise reduction. Common mode noise introduced at the input will go thru multiple stages of pre and power amp in both chains of balanced amp. Sum of gains in each chain can be different especially for hight frequencies resulting in amplified noise across speaker terminals.  The more stages in fully balanced configuration the bigger max. unbalance.  It is better, IMHO, to reject common mode noise as soon as possible and use unbalanced configuration.  Fully balanced configuration will reduce a bit harmonic distortion by removing even harmonics.  It is because even harmonics are always positive, even if phase is inverted, resulting in the same signal on both outputs, hence zero difference.  Unfortunately unpleasant odd harmonics, will stay.

sorry, when I said noise I just mean in general anything that will degrade the signal and lower the fidelity

I was under the impression keeping the signal balanced all the way through with true dual differential output/inputs on the pre and power amp would be the best course of action? 

I don’t see benefits of that. Removal of even harmonics will make amp sound "colder" while doubled gain and power stages cost money. You can have much better unbalanced amp for the same price.

kijanki"I can't see how fully balanced system can help with noise reduction."

The reason is common mode rejection of noise, and it's the main reason that balanced operation is standard in professional audio applications.

The OP seems to be confusing electronics that are monoblocks or have balanced inputs and outputs with electronics that are "fully balanced." These are not interchangeable terms. "Fully balanced" usually refers to differential amplifiers. There are a lot of electronics that have balanced inputs and outputs, but that operate internally in single-ended mode. I'm pretty sure that the  Integra Research RDC 7.1 is not "fully balanced."

Cleeds, yes, balanced operation is used in professional audio in form of balanced cables, balanced inputs etc. I’m all for it. I just don’t see how fully balanced amp can have better noise rejection than plain amp with balanced input. Instrumentation amps, like one used in my Rowland amp have 90dB noise rejection at 60Hz. That would imply gain matching of two independent amps to 0.003% to achive the same result. I also don’t see how removal of even harmonics only can help. 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.

kijanki04-22-2016 11:15am " ... yes, balanced operation is used in professional audio in form of balanced cables, balanced inputs etc. I’m all for it. I just don’t see how fully balanced amp can have better noise rejection than plain amp with balanced input. "

A single-ended amplifier with a balanced input and/or output is still a single-ended amplifier and does not offer all the advantages of a differential amplifier, which provides common mode rejection.

" ... Since outputs have no ground reference (current doesn’t flow thru ground) feedback might cause instability. "

That's a red herring. I've never seen instability problems with differential amplifiers.

A single-ended amplifier with a balanced input and/or output is still a single-ended amplifier and does not offer all the advantages of a differential amplifier, which provides common mode rejection. 

This is not true, IMHO.  Single ended amplifier with balanced inputs can provide huge common mode rejection, better in my opinion than fully balanced amp.  Fully balanced amp cancels (at the speakers) common mode input noise, only if gains of both half-amps are identical.  Gains can be matched with discrete components only up to certain point.  Fully balanced amps have some advantages like no output ground current, cancellation of even harmonic, doubled slew rate etc.  but have also many disadvantages.  One of them is the fact that signal goes true many more stages (bad for clarity), output impedance is doubled while the cost (and complexity) is much higher.

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).

@1 - I've never said that it doesn't.  But it will remove cable noise by means of either balanced input in single ended amp or fully balanced amp.
 @2 - again, I did not say it removes even harmonics from the signal. It removes even harmonics produced by the amp at any stage by means of cancellation AT THE OUTPUT.  These even harmonics often make amplifier sounding warm and euphonic.  Yes, the third one is not bad, but higher odd harmonics are quite nasty.  Removing only even harmonics while leaving odd harmonic intact will lower total THD but also will make sound colder, brighter.
@3 - Output impedance will double since there will be two output stages - similar situation to bridged amplifier.  Complexity is much higher no mater how simple the circuit is. If signal goes thru one gain stage only in single ended design, it will go thru two stages in the balanced one  (not to mention two output stages).  You might not see a problem with that, I do.
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.  I'm sure that Al (our technical guru) would agree with me - that balanced output, because of additional circuitry might not always be the best.  I will leave it at that.

kijanki

 " ... I'm sure that Al (our technical guru) would agree with me - that balanced output, because of additional circuitry might not always be the best.  I will leave it at that."

I'd be surprised if anyone here would disagree with you on that, kijanki. It's especially true on devices that use cheap op-amps to process the balanced signal into a single-ended one.

Ralph (atmasphere) responded to your other confusions, so I'll leave it at that.

Nyhifihead
from what I’ve read it only matters if both the preamp and power amp are both truly balanced

That is a belief that is shared by many audiophiles, but it is a misconception IMO. A **well designed** balanced interface may be advantageous regardless of whether or not the internal signal paths of the connected components are balanced or not. And a **well designed** balanced internal signal path within a component may be advantageous regardless of whether or not the internal signal paths of other components in the chain are balanced.

The potential advantages of a well designed balanced interface include reduced sensitivity to cable differences, reduced likelihood of ground loop issues, as well as reduced susceptibility to noise pickup. Of course, as Kijanki mentioned it is not uncommon (especially in modestly priced equipment) for balanced interfaces to perform worse rather than better than single-ended interfaces that may be provided on the same components.

Cleeds
I’m pretty sure that the Integra Research RDC 7.1 is not "fully balanced."

+1. I say that in part because searching the 156 page manual for the terms "differential" and "fully balanced" yields no hits.

Nyhifihead
I was under the impression keeping the signal balanced all the way through with true dual differential output/inputs on the pre and power amp would be the best course of action?

As is usually the case in audio, how good the results are depends less on the design approach that is chosen, and more on how well the chosen approach is implemented. As a general rule of thumb I would expect that monoblock amps, dual mono amps, and fully balanced amps (which mean different things, as Cleeds indicated) would tend to have the greatest likelihood of being preferable choices at price points that are significantly higher than the price range you referred to. Especially given that you need fairly large amounts of power.

Kijanki
...connection with XLR cables is not always the best. Experience of many people on this forum was that RCA connection sounded better than XLR. I’m sure that Al (our technical guru) would agree with me - that balanced output, because of additional circuitry might not always be the best.

Thanks, Kijanki. Per my first paragraph above, yes, I of course agree. It depends on the specific implementation. I’m pretty sure Ralph (Atmasphere) would agree as well. As he said at the end of his most recent post above:

...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).

Regarding balanced outputs having twice the impedance of unbalanced outputs, in the majority of cases components providing both balanced and unbalanced outputs utilize similar output circuits for each of the three signals that are involved (two balanced, one unbalanced), and in fact may often drive the single-ended output with one of the two signals in the balanced signal pair. The usual convention is that the balanced output impedance is defined based on the sum of the output impedances of the two signals in the balanced signal pair, so the output impedance number for the balanced output is often twice the output impedance number for the unbalanced output simply by virtue of that fact. Therefore, each of the two signals in the balanced signal pair will often have the same output impedance as the unbalanced output signal that may be provided by the same component, and the statements about output impedance by Ralph and Kijanki are in a sense both right.

My bottom line suggestion to the OP: Don’t choose based on topology. Although I’ll also say that in your stated price range, and especially given your power requirement, my expectation would be that more often than not you would end up with best results choosing a stereo amp (on a single chassis) that has an unbalanced internal signal path. And if it provides both unbalanced and balanced inputs, try both.

Also, for the OP’s info, the advantages and disadvantages of balanced differential design (that being one form of balanced design) are nicely explained in this paper at Ralph’s site.

Best regards,
-- Al

Al, I was looking at overall output impedance that speaker generated EMF sees, and that would be double. 

I also stated that fully balanced design is not only unnecessary to provide great noise rejection, but in fact might be worse than one achievable in single ended amp with balanced input. High CMRR would require perfect gain matching of both "legs" of the amp and that is not possible at the level of good instrumentation amp (90dB @60Hz in my amp).  It would require 90dB  (0.003%) gain matching with discrete components, calling for <0.0015% resistors, that don't even exist.  It gets even worse at higher frequencies where both multistage amps/legs would have to have identical frequency characteristics.  Same is of course true for instrumentation amp, but to much smaller degree.  Resistors are laser trimmed on the same substrate while amplifier's bandwidth can often be much higher (22MHz in my amp).

Al, I was looking at overall output impedance that speaker generated EMF sees, and that would be double.

Thanks, Kijanki. When I commented about the doubled output impedance question I had lost track of the fact that you were addressing power amplifier outputs. Yes, **everything else being equal** the load applied by an amp to back EMF from a speaker will be numerically doubled (corresponding to half the damping factor) if the outputs are balanced compared to if they are single-ended. Although in the case of most solid state amps, at least, that is unlikely to matter since their damping factors will be sufficiently high for most speakers anyway. Notwithstanding the belief among many audiophiles that, for example, a damping factor of say 500 may result in better bass control than a damping factor of say 50, everything else being equal. I know from many of your past comments about damping factor that you would agree with my disagreement with that popular (mis)conception.

Regarding your last comment, I believe you are envisioning a balanced amp configuration that is used in some designs in which each of the two input signals in the balanced input pair is processed through its own signal path throughout the amp, that is independent of the signal path through which the other input signal is processed. While I believe Ralph is addressing the configuration used in his (and other) designs in which each stage of the amp, including the input stage, is a differential amplifier that responds to the difference between the pair of signals its inputs are provided with. And I believe that your comments and his are each essentially correct in the context of the kinds of designs each of you is envisioning.

Best regards,
-- Al

Kijanki,
Thanks for your interesting and informative comments here. Listening confirms many of your points regarding real life sound quality. I appreciate the theoretical advantages of differential balanced circuits. Coincidence or not some of the very best sounding Amplifiers I've heard were simple unbalanced circuits with RCA  cables. As you and Al  point out  implementation is a very major factor. 
Charles, 

thanks so much for the replies, I'm learning a lot.

sorry for the lack of circuitry knowledge, most of my info is based off of forums like these and it seems the way people describe balanced is tossed inconsistently, I just got schooled though!

I thought the RDC 7.1 had a balanced design because the dacs are dual differential PCM1704, and because BAT had a hand in developing.

The matching Integra research/BAT RDA-7 power amp I know is fully balanced, so I assumed that they would make the pre/pro to match

I've had McCormack/Pass on my mind, how are the classe balanced input?

also does anyone know how good the circuitry is on the emotiva XPA-1? it claims fully balanced and looks impressive

thanks

Al, I agree, that was invalid point - double DF doesn’t make much difference in most cases. As I stated in my older posts speaker’s impedance is mostly resistive and it is in series with back EMF reducing max possible DF to about 1. As long as amplifier doesn’t destroy it too much it is OK. Some of Ralf’s amps had DF<1 sounding fine.

You are right that I assumed two independent amps in differential mode - that’s different from balanced. In order to make identical gain stage in each amp they have to reference each other (like in the first stage of instrumentation amp) instead of being ground referenced. In that case amps aren’t really independent as I thought they should be. Noise rejection in such design should be great but it is not worth extra expense IMHO. Single ended amps with balanced inputs might have great noise rejection as well, while better components can be utilized for the price of fully balanced design.

Charles1dad, Thank you.  I often engage in discussion with Al or Ralph to learn more.  It is difficult to understand why particular architectures are being implemented since it is often fueled by demand.  I like to understand technical merits of design but sound is the only proof.

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

Thank Al, 
So very specific requirements are necessary in order for Ralph's point to be valid,  I appreciate the clarification. This would explain the findings of my friend's Atma-Spheres and his various balanced cables. 
Charles, 

I don't know about advantages (if any) provided by a true differential balanced circuit, but EAR-Yoshino designer Tim de Paravacini (who does a lot of work in the pro recording field) chooses to use single-ended circuits, with balanced inputs and outputs provided by a transformer.

"Checkout VAC SigMKIIa SE and above or Atmasphere MP-1 if looking for transformer coupled preamps " by knghifi

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? if the info is something proprietary to you, then just ignore the questions. Thanks & regards, 
-phil

As is usually the case in audio, how good the results are depends less on the design approach that is chosen, and more on how well the chosen approach is implemented.

Yes, yes, a thousand times yes!  I nominate this as verse 1 in the Audiophile's gospel, to be said aloud, immediately upon arising in the morning, and before retiring at night!  J/K.  Not really... ;-)

 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.

There appears to be quite a bit of overlap in this discussion between the role of balanced equipment interconnections, and circuit topologies that use differential signal paths. This is actually quite understandable -- when one observes many of the design practices in contemporary "balanced" high-end audio gear, it seems that a large percentage of the people who design them are confused about the difference.  But "balanced" interconnects and "balanced" circuit topology are SEPARATE subjects, as they have DIFFERENT reasons for existence.  This post deals with the former; that is, the use of balanced interconnection between equipment.
 
To clarify the basics . . . "balanced connection" (one that usually has an XLR connector in the high-end audio world), what we're talking about is a connection that has two signal conductors, each of which has the same IMPEDANCE to ground, thus the impedance is "balanced" between them.  The fact that there are two signal conductors allows two modes of signal to coexist . . . the mode that pertains to both of the conductors together with respect to ground ("common-mode") and the mode that pertains to the voltage difference between the conductors ("differential-mode").

The overwhelming source of the noise we're trying to reject comes the flow of AC power leakage currents when active electronics are connected together (i.e. preamp to power-amp). With very few exceptions (i.e. Krell CAST), an analog audio signal is defined as the VOLTAGE at the equipment's output.  Of course in the real world, some signal current flows as a function of this signal voltage across the receiving equipment's input impedance and the connecting cable's reactance.  But put simply, in the world of audio interconnection . . . when we say "signal", we're talking about a voltage.

The shortcoming of unbalanced interconnection is primarily the resistance of the shield, as it functions to connect equipment grounds together.  As noise CURRENT flows across the shield resistance, a corresponding noise VOLTAGE appears at the ground of the receiving end.   Very little of this noise current flows through the signal conductor because the signal's input impedance is much higher, and the difference in impedance (hence the term "unbalanced") means this noise current manifests a noise voltage on top of the signal voltage.  In a balanced system, the same noise voltage appears as a result of the shield resistance, but the idea is that it cause identical noise voltage to appear on two signal conductors rather than one, and the signal can be defined as the voltage BETWEEN the conductors rather than the voltage between either conductor and the shield . . . and thus the receiving equipment can tell the difference between the signal and the noise.

But as stated above, in the real world any voltage produced at an input must also result in some current flow, as a result of its input impedance, and as such the balanced connection must have identical impedances between both of its signal conductors and ground for the noise-rejection scheme to work.  Otherwise, a different amount of noise current will flow through one signal line than the other, and the noise voltage will appear as the voltage between the signal conductors, just like the signal.  Put another way . . . the amount to which the impedance is unbalanced is the amount to which it starts to behave like an unbalanced interconnect.

For any balanced interconnect, we can thus predict the exact degree of this behavior from three impedances (common-mode, differential-mode, and differential-mode-imbalance) for each of the source electronics, destination electronics, and interconnecting cable.  The best explanation on this I've found is in Bill Whitlock's paper " Answers to Common Questions about Audio Transformers", available here as AN-002: http://www.jensen-transformers.com/application-notes/.  There are points I'd like to reinforce from this:
1. The balance of signal voltage between the conductors, with respect to ground, DOES NOT MATTER for the noise-rejecting capabilities of a balanced line.  It's the balance of the impedances that's critical.
2. The sensitivity of a system to impedance imbalances is a function of the ratio of the differential-mode (signal) impedance to the common-mode (noise) impedance.  Thus, a balanced input can be made less sensitive to impedance imbalances by increasing the common-mode impedance, or reducing the differential-mode impedance.

A **well designed** balanced interface may be advantageous regardless of whether or not the internal signal paths of the connected components are balanced or not. And a **well designed** balanced internal signal path within a component may be advantageous regardless of whether or not the internal signal paths of other components in the chain are balanced.

The main reason for my diatribe above is that I think many audiophiles would like to have some clarity on what constitutes a "**well designed** balanced interface" . . . so I'll throw my thoughts out there for consideration.

First, an input stage should at least "work well with most stuff people will hook up to it" . . . or better yet, "allow the preceding stuff to work at its best".  For consumer high-end audio, I think this means a signal input impedance of at least 10K through the entire audioband, and for a balanced input, we need to maintain decent noise rejection (maybe min. -40dB) with a source impedance imbalance of at least 5% . . . as this parameter is usually determined by four 1% resistors working in series, plus some cable and connector imbalance.

Second, I think that an input stage must "allow the rest of the unit to work its best under all conditions".  For a balanced input, this means that the input stage shouldn't rely on the rest of the unit (or the equipment that follows) to continue with the task of rejecting the common-mode noise that appears at the input jack.  And if the rest of the unit requires matching differential signal voltages to work properly, then the input stage should be able to provide this regardless of the presence/absence of noise at the input, or whether or not the signal voltage at the input jack is balanced with respect to ground.

It's this second requirement for which an alarming amount of high-end audio gear achieves an epic fail . . . and it's actually much MORE of problem with equipment that advertises itself as being "fully balanced", "differential circuit", or whatever.  I can support Ralph's specific observation on some of ARC's gear, and the fact that they're not alone . . . this is the situation for many, many very well-respected and premium-priced products, both tube and solid-state.

I can only speculate as to why this is the case . . . perhaps many designers become enamored with differential circuit topologies for other reasons, and these circuits require a pair of differential voltages to work.  They then feel that simply wiring the inputs to pins 2 and 3 of an XLR connector has a certain simplicity and elegance about it, but don't fully consider the real-world requirements for a balanced input.  Or they see that other manufacturers have "balanced" designs, and respond in making their own creations by simply stuffing two of everything in the box and thinking that it constitutes an "upgrade".  Or they become smitten with the way their schematics look when they're arranged in a symmetrical fashion about the horizontal axis . . .

Whatever the reason, it's good advice for purchasers of "fully balanced" equipment to do some investigation as to its specific input requirements, and kudos to Al for his time spent in research and assistance on Audiogon forums to this end.

And too you kirkus, for this great tutorial! Thanks.

My thanks to you also, Kirk.  Always great to see you posting here, and for us to have the benefit of your invariably brilliant insights.

Speaking of Bill Whitlock, since the interface-related noise that is being discussed may in many cases be caused or contributed to by ground loop effects, you'll probably find pages 31 through 35 of this paper to be of interest.  To whet your interest, its introduction states that "this finally explains what drives 99% of all ground loops!"

This was called to our attention a while back, btw, by member Jea48 (Jim), who as you may be aware is our resident genius when it comes to electrician-type matters.  I'd welcome any comments you may have on what Mr. Whitlock has to say on those pages.

Best regards,
-- Al
  

There are separate case mono blocks and dual mono two channel designs that share a case but it should have two power supplies per channel, one per channel in the preamplifier section and per channel for each power amplifier section in order to be considered a dual mono amplifier, or "true" dual mono. Who makes these terms up either its a "dual mono" or it is not? I form that in a question because I’m not entirely sure what the rules are or if I’m missing some. I know of some 2 channel power amplifiers that are labelled as being dual mono by the manufacturer yet they have but one power transformer.

Meerzistar 4-25-2016
Who makes these terms up either its a "dual mono" or it is not? I form that in a question because I’m not entirely sure what the rules are or if I’m missing some. I know of some 2 channel power amplifiers that are labelled as being dual mono by the manufacturer yet they have but one power transformer.

As far as I am aware there are no formal rules, and my impression is that different manufacturers use these (and many other) terms with varying degrees of looseness. My single-chassis VAC Renaissance 70/70 amplifier is dual mono even to the extent of having two power cords, as well as completely independent power supplies and multiple power transformers for each channel, while I’m not surprised to see your statement indicating that a so-called "dual mono" amp from some other manufacturer has a single power transformer serving both channels. And certainly there are "dual mono" amps from other manufacturers that fall in between those extremes.

Regards,
-- Al

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.

Speaking of Bill Whitlock, since the interface-related noise that is being discussed may in many cases be caused or contributed to by ground loop effects, you'll probably find pages 31 through 35 of this paper to be of interest. To whet your interest, its introduction states that "this finally explains what drives 99% of all ground loops!"

Excellent link, Al . . . a great comprehensive document that covers many aspects of Whitlock's papers over the years.  The dedication to the late Neil Muncy at the beginning is nice, too . . . his AES paper from the mid-1990s is the definitive work on one of the most prevalent equipment design issues in the audio industry.

I also read Whitlock's AES Paper that's the source for the content of pages 31 through 35 of the presentation paper . . . and this is some brilliant and thorough work as well.  I've long been suspicious of the randomly-arranged conglomerations of THHN that populate the conduit on commercial jobs, even though it's not too clear what's the best route to pursue during construction to avoid it . . . asking union electricians to twist THHN into pairs before each pull seems like a great way to get kicked off the jobsite.  The ideal solution would be a pre-twisted cable with fillers and an overall jacket -- something that could be easily specified on the prints, relatively easy to pull through the conduit, and with established fill tables to make sure the conduit sizing and labor costs are predictable.  In absence of this . . . using multiple smaller conduits or runs of MC where necessary may be the best route, or at least over-size the ground wire(s) for some brute-force reduction may be all that can be achieved in the real world.

I do think that we're lucky in that the "conduit transformer" issue is far lower on the list of worries for a residential single-room audio system than it is for i.e. a large commercial building with high-power active line arrays or digitally-steered columns placed hundreds of feet from their source electronics.  The main susceptibility for high-end audio systems would be where multiple dedicated branch circuits are employed . . . and this can be eliminated as an issue by using Romex, running all circuits to a single multi-gang non-metallic wall outlet box, and connecting all the grounds together at this wall box in addition to the panel.  This eliminates the chance of any voltage differential between the third-prong AC grounds in the system, yet still provides the benefit of fully separating the current flow between the circuits.  Also, I've seen many breaker panels where the connection bar(s) are shared between ground and neutral connections for the various circuits, and this can needlessly impart random voltage differences between the grounds of different outlets, and additional ground-to-neutral noise.

My only slight disagreement with Bill Whitlock would probably be that I don't share the same level of distain for unbalanced interconnection, in the context of the systems that we discuss on Audiogon.  Here, we're usually talking about audio systems that don't share grounds with equipment in other rooms, and can easily be plugged into AC outlets that are all on a single power strip, or ganged together in the wall . . . and the audiophile's idea of a "long" interconnect is . . . maybe 15 feet?  I've just seen so many ill-conceived applications of the 3-pin XLR connector on high-end audio gear that I frequently feel I need to look at a schematic before deciding if it's even usable.   Much of the time I think equipment with poorly-designed "balanced" inputs would work so much better with simpler circuitry, and a plain 'ol RCA jack for the input.

 . . . 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).

Ralph, I'm assuming that by "a standard for balanced line operation" you're talking about the 600-ohm termination resistor . . . and I'm sorry to pull a Kryten here, but AES48 says nothing about signal impedances; rather, it's a culmination of grounding practices derived mainly from the work of Neil Muncy, whom I referenced above.  None of the AES Standards documents mention balanced signal impedances . . . but the Bill Whitlock presentation for which Al provided the link references IEC standard 61938, according to which "all professional and broadcast line amplifier inputs" should be >= 10K ohms.  He also goes on to address the origins of 600 ohms as a specification, and a few reasons why he feels it's inapplicable to modern audio systems . . . and I can't find any reason to disagree with him on this point.

I know you've maintained that the addition of a termination resistor makes cable characteristics non-critical . . . but I'm still unclear as to what electrical mechanism you feel is responsible for this.  In my own experience, while some of my system iterations over the years have seemed more sensitive to cables than others, I haven't noticed a correlation between this and low termination impedances.  I also have 100K, 600 ohm, and 150 ohm termination impedances all a mouse-click away from each other on the Audio Precision system . . . and I've had my share of times where some measureable artifacts that end up being related to cables and interconnection remain completely unfazed by the setting.  So if you have any details on this that I haven't thought of, I'd be very interested to know.

^^ 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!

i don't think that is a critical issue. Other factors are equally important. I happen to have fully balanced, dual mono amps, but other stuff sounds great, too..