Differential Balanced Sound Quality

Thanks everybody I feel like this thread should be a sticky. I'm learning a lot and also being confused by a lot of this high level discussion!

For those of you who might be technically less adroit, I suggest to search for "pints with Ayre" and listen to why balanced is one of their design constructs (along with "no op-amps", zero feedback, etc).

Thanks for all the discussion folks.  It's always nice to read a thread that actually educates, or at least presents the opportunity...

I apologize if you see this as being patronized, that was not my intent!

Cool... no problem.

Take a look at the first hit (this is an images search). The first image shows both what you’ve described **and** what I described in the same image, the latter of which does not have the Johnson noise issue.

Absolutely, agreed... I was talking about the topology shown in the first image.

IME the statement is only true if opamps are used *and* the internal circuitry is single-ended

Yep... op amps are the usual way of doing it and that's what I was talking about.

BTW love the look of your amps and I can see the passion you put into them.

The term is commonly used both in text books and the professional recording environment as shorthand for the inverted signal in a balanced pair..

It is and many people are confused by it! I apologize if you see this as being patronized, that was not my intent! When I am writing anything here, I am assuming that others are reading along. For that reason I try to avoid getting too technical.

If you do a quick google search on differential amplifier you will see that by far the most common use for this term is exactly what I have described and this is what is used in the majority of equipment with balanced inputs.

OK. Here is just exactly that:
https://www.google.com/search?q=differential+amplifier&client=ubuntu&hs=WBe&channel=fs&a...

Take a look at the first hit (this is an images search). The first image shows both what you’ve described **and** what I described in the same image, the latter of which does not have the Johnson noise issue.


If the device has a transformer at its input, this noise won’t occur. If an opamp is used, it might or might not be used that way- depending on if the input is more of the instrumentation variety rather than a single opamp device (and the reason for not doing that would be if the internal circuitry of the preamp or whatever is fully differential; at that point you’ll need opposing outputs from the first stage which a single opamp can’t provide). We recently had a McCormick amplifier come through the shop; its balanced inputs were executed as I described in one of my prior posts (and as you see in that first Google image), using dual matched input transistors, probably similar to a MAT-12
https://www.analog.com/media/en/technical-documentation/data-sheets/MAT12.pdf
with a common emitter circuit.


Our conversation started with you stating that a balanced input would be noisier due to the resistor noise:

The bit that a lot of people are unaware of is that balanced inputs are actually noisier than single ended (RCAs etc.) in that they utilise relatively high value resistors that introduce their own (johnson) noise into the signal.

IME the statement is only true if opamps are used *and* the internal circuitry is single-ended (in this case, if succeeding opamps are used but with the signal only applied to one input, I would regard that as single-ended. Another way to look at that is look at the volume control; if it has one deck for each channel then its single-ended. This thread is about ’Differential Sound Quality’ so I have to assume that circuits/products that are in fact fully differential are included.).

There's no such thing as 'cold signal'.

The term is commonly used both in text books and the professional recording environment as shorthand for the inverted signal in a balanced pair... It doesn't confuse me and I don't need to be patronised.

If you do a quick google search on differential amplifier you will see that by far the most common use for this term is exactly what I have described and this is what is used in the majority of equipment with balanced inputs. 

The conversation flow of this thread is about the pro's and con's of balanced connections and what I have said is correct.

So if I start with a conventional amplifier block with an inverting and non-inverting input and a single output. A single ended amplifier input might be a 100Ω series resistor followed by an RF filter & DC blocking capacitor, in non-inverting mode the input impedance is set by the resistors to ground at the input so it’s not difficult to maintain a high input impedance alongside a low thermal noise from the series resistor. The actual impedance of the non-inverting input is so large that it can be pretty much ignored.

If you take the above example and feed the cold signal into the inverting input, the series resistor on the cold input will dictate the maximum input impedance as the current will be flowing into the virtual ground at the summing point. So 100Ω is now out of the question. You might for example choose to go with 10kΩ series resistors on both inputs, that’s 20dB more thermal noise than 100Ω.

@pragmasi 


There's no such thing as 'cold signal'. There's non-inverting and inverting. What you're describing (as seen in your link) has to do with an opamp which has differential inputs but a single-ended output. A differential amplifier always has dual outputs. Look again at the diagram I linked:
http://www.atma-sphere.com/en/resources-understanding-our-circuits.html

When I look at your amplifier I see that you have two outputs and I suspect that is the source of confusion... at what point does the cold signal get inverted?.. or does it connect to the negative speaker terminal?

Really, I think this 'cold signal' thing is confusing you. Both inverted and non-inverted signals are 'hot'. They must both be treated the same way. If you are referring to the inverted signal (for example the minus output of a phono cartridge) it gets inverted at the output of the device to which it was applied as an input.

A related question that has been bothering me. If your circuit isn't perfectly duplicated after signal splitting, doesn't this introduce timing errors upon recombination?

@cal3713 

No, at least not at audio frequencies and beyond. Timing becomes an issue at radio frequencies but we have bandwidth to 400KHz in our line stages and it does not seem to be a problem there.

I really take issue with the term 'signal splitting'. That's probably because I don't see that happening. A differential amplifier does have two halves; these are intimately coupled together in a tube circuit at the cathodes, in a transistor circuit, at the emitters, and in an FET circuit, the sources.

I'm going to use the term 'emitter' in place of 'cathode' or 'source' in the following explanation:


In all cases, since the current for both halves is flowing thru the common emitter circuit, if one side of the differential amplifier is turned on, all the current goes thru that side so the other half is forced off and vice versa. It important to understand that this process occurs in real time; there's no 'slight delay'; for one side to turn on the other side **absolutely is** being turned off in perfect tandem.


If both halves are turned half-way on their outputs will be at the same level. At all times the current through the emitter circuit is constant. Because the devices aren't perfect, its advantageous to put a current regulator in the emitter circuit called a 'Constant Current Source' (CCS). The more constant the current in the emitter circuit, the more theoretically perfect the differential effect. To this end the quality of the CCS is arguably as important than the gain of the devices used in the differential amplifier.


If you drove only one half of the differential amp, if it had perfect differential effect, both outputs would be equal and opposite. In practice there are slight differences. But if you have a succeeding differential gain stage these differences go away- they are not exacerbated.


Because there are slight differences when driven single ended, when you drive them balanced the distortion is slightly lower. The higher the CMRR (Common Mode Rejection Ratio, measured in dB) the less this is so.

Differential amplifiers get their name from a simple fact: They amplify what is different between their inputs. If one input is at ground, then they amplify the side that has the signal (single-ended). If both sides have the *same* signal they won't amplify (because that signal is Common to both sides). If the signals applied are opposite phase of each other, then they get amplified. It doesn't matter so much if the two inputs aren't exactly equal; what matters is that they are opposite- the outputs of the differential amplifier will even things out. There's no 'recombient distortion' or any such nonsense.


The variable here is the Common Mode Rejection! If its poor (less than 80dB) what I said in the paragraph above starts to go out the window. If its very high (140dB) it really won't be measurable whether the input is single-ended or balanced.


Achieving a good CMRR value isn't hard. We can do it with 6SN7s.

@atmasphere Happy to try to explain, it’s a shame that there’s no way of posting a picture...

So if I start with a conventional amplifier block with an inverting and non-inverting input and a single output. A single ended amplifier input might be a 100Ω series resistor followed by an RF filter & DC blocking capacitor, in non-inverting mode the input impedance is set by the resistors to ground at the input so it’s not difficult to maintain a high input impedance alongside a low thermal noise from the series resistor. The actual impedance of the non-inverting input is so large that it can be pretty much ignored.

If you take the above example and feed the cold signal into the inverting input, the series resistor on the cold input will dictate the maximum input impedance as the current will be flowing into the virtual ground at the summing point. So 100Ω is now out of the question. You might for example choose to go with 10kΩ series resistors on both inputs, that’s 20dB more thermal noise than 100Ω.

When I look at your amplifier I see that you have two outputs and I suspect that is the source of confusion... at what point does the cold signal get inverted?.. or does it connect to the negative speaker terminal?

Edit: I just did a quick google search to find a picture... I know nothing about the site and I've not read the content but the schematic in the header is what I'm talking about. The cold input current flows into the summing point (where the Va label is), so R1 sets the cold input impedance... in fact the impedance will be lower than R1s value but that's beside the point. If we change this to single ended with a gain of 1/1, R1 becomes open circuit and R3 is a dead short. The input impedance is R2 + R4, which means R2 can be low and R4 can be higher and the thermal noise is calculated from the voltage divider.

There is a reason: In most consumer (and professional) amplifiers the balanced signal is dealt with at the input by a differential amplifier necessitating the need for a series resistor on the inverting input. In the worst case the diff amp is the load seen by the source so the resitor needs to be large enough to present a reasonable input impedance (that’s the ’relatively high’ part).

@pragmasi

Any gain stage (differential, balanced or not) might need stopping resistors at its inputs, but I suspect this isn’t what you’re talking about.


We’ve been building differential amplifiers for balanced inputs longer than anyone else in home audio (IOW we’ve introduced balanced operation to home audio with our MA-1 amplifier in 1986 and followed up with the first fully differential balanced preamp in 1989), and using vacuum tubes have nevertheless gotten fairly good CMMR values, in excess of 100dB. Each input (pin 2 or pin 3 of the XLR) sees the same input impedance, which is what you would expect of a balanced input, and both have the same resistance between the XLR connection and the actual grids of the input tube. So far we’ve not seen any such need for a resistor as you describe. For what are you thinking this resistor is needed/what’s its function?


You can see a simplified example of one of our input circuits in the article at this link:

http://www.atma-sphere.com/en/resources-understanding-our-circuits.html

As you can see, the diagram is a textbook example of a differential circuit. I really am mystified by what resistor you’re talking about! Can you explain in greater detail?

The bit that a lot of people are unaware of is that balanced inputs are actually noisier than single ended (RCAs etc.) in that they utilise relatively high value resistors that introduce their own (johnson) noise into the signal.

This statement is problematic. There’s no reason why any such ’high value resistors’ be used in a balanced circuit that aren’t also in a single-ended circuit.

There is a reason: In most consumer (and professional) amplifiers the balanced signal is dealt with at the input by a differential amplifier necessitating the need for a series resistor on the inverting input. In the worst case the diff amp is the load seen by the source so the resitor needs to be large enough to present a reasonable input impedance (that's the 'relatively high' part).

A better solution is to use an instrumentation amplifier which buffers the input so all you need to worry about is the ability of the buffer to supply current to the diff amp in which case the series resistor can be smaller. But in both cases there needs to be a series resistor that is larger than that required for a single ended input fed into a FET for example. 
A transformer input is obviously different and more of a rarity but very beneficial for noise reduction and does a better job at dealing with RF/EMC.  However transformers are far from perfect, don't have great LF linearity and HF response can be iffy, so no panacea.

Another option and perhaps the one that the OP was alluding to is feeding the balanced signal right through to the speakers using a bridged amplifier. I am less aware of the pro's and con's of doing it this way but I'd be interested to know what the measured CMR is of such systems as the gains of the two halves of the bridge would need to be very well matched.

My point was that balanced isn't 'better' by nature, just different and if you have a noise free environment with no ground loops it could be slightly worse.

In most cases my advice would be to go for balanced as there are likely to be more people who would benefit from the removal of noise at the input than would notice the additional noise from the circuit.

I believe the only benefit is that you can connect the circuit to other balanced equipment.  

(Unless you really like the sound of a signal passed through a transformer of op-amp as is used to create a balanced output from a single-ended device.  And some people do swear by the sound of going through some iron.)

I have a question. My Bryston 2.5 SST2 has balanced connections and years ago I owned Rotel amps and preamps w/balanced connections but they weren't differential balanced. In fact you can find a number of amps and preamps that don't have the differential circuit. What is the benefit of a balanced connection in these units??

@atmasphere Yes, indeed... the half terminology could lead people astray in that way, thanks.  

A related question that has been bothering me.  If your circuit isn't perfectly duplicated after signal splitting, doesn't this introduce timing errors upon recombination? 

I see a lot of people building balanced designs over on diyaudio, but don't see much talk about controlling wire lengths, and otherwise perfectly matching the two amplification circuits.  Obviously it's not a deal-breaker given the experienced success, but I can't help but imagine it's introducing error if there are any post-splitting differences...

Thanks.

Alternatively, the balanced equipment discussed above separately amplifies the positive and negative parts of the electrical signal. This requires double the circuitry because you have to amplify two signals (0-to-positive and 0-to-negative) instead of one (negative-to-positive) as in single-ended equipment.  If you imagine music as a sign wave, then balanced equipment is separately amplifying the upper and lower halves of the wave. The benefit of this added complexity is that any noise introduced into the signal by your equipment will be cancelled out when these two "half" signals are re-combined to make the full-wave signal that drives your speakers. This is because any injected noise will appear positive going in one amplification circuit and negative going in the other... add them together and you get automatic noise-rejection because they'll be in exact opposition.

@cal3713 The bit about positive and negative parts is problematic. Usually for balanced operation differential circuits are used, and its probably easier to understand that they amplify **opposites** rather than 'halves'. And it does not require double the circuitry- this is because in a differential circuit, the cathode (or emitter, or source) circuit has all the parts used in it in common. As far as noise or distortion goes, the benefit of cancellation accrues with each stage, not just at the loudspeakers. Finally, for a given single-ended gain stage, if done differentially will have theoretically 6dB less noise generated. This can be significant from one end of a circuit (like a preamp) to the other. Two stages, each with 6dB lower noise, that makes 12dB... So you don't need as many gain stages. In our full function preamps there are only three stages of gain from phono input to main output and they can work with LOMC cartridges; contrast that with a typical single-ended preamp which will have at least 4 stages of gain (unless an SUT is employed) or more to do the same thing and you can see that the parts count myth is just that.

@oldears Great quote.  There's someone over on diyaudio who's posting tag is "The power supply of my power supply is my FRIEND!"

Here is a quote from the late Charlie Hansen in The Stereophile, talking about their Ayre Codex, their cheapest electronic component having a linear power supply: "remember that every circuit is a modulated power supply....When you run balanced, it is like making the best power supply 1,000 times better."

I'm just a newbie compared to most here if not all. I recently converted to what I believe are true AES48 supported pre-amp and amp (wyred4sound both units) and can say that the noise floor and transparency have never been greater in any system of mine in 30 years. The noise floor is basically inaudible. I am currently using "simple" balanced cables with Neutrik ends and mogami wire between the gear and quite happy. I have other weak links in the system but this is not one of them.  

Thanks to the knowledge freely shared here. 

Also, I took the opportunity to read the article I linked. It has lots of good information about the 2nd harmonic, but none about the 3rd. Here’s some more Pass writing that discusses both:

https://www.passdiy.com/project/articles/audio-distortion-and-feedback

And here in the introduction (on p3): https://www.firstwatt.com/pdf/art_ba_3.pdf

@atmasphere - Ralph - another excellent explanation. Thanks. 

I've been a big fan of balanced circuitry and cables since I discovered what a difference they could make in my system in the late 80s. Never looked back.

@atmasphere Ralph, apparently we posted at the exact same time.  Any facts of operation I got wrong in mine?  As always, thanks for your contributions...

The bit that a lot of people are unaware of is that balanced inputs are actually noisier than single ended (RCAs etc.) in that they utilise relatively high value resistors that introduce their own (johnson) noise into the signal.

This statement is problematic. There’s no reason why any such ’high value resistors’ be used in a balanced circuit that aren’t also in a single-ended circuit.

Because it is so obviously the wrong way to go. The number one factor in quality sound is quality parts. Going balanced requires twice as many parts. A whole duplicate circuit.

@millercarbon This statement is false and is a common myth: it ignores the bit about ’differential’. If the circuit employs differential amplifiers, then the parts count is about 50% higher rather than double. There is a distinct advantage to differential amplifiers (which are often used in amps with single-ended inputs, as they are often used as phase splitters); they have much more power supply noise rejection and if the circuit is fully differential, then even ordered harmonic distortion is cancelled at each stage throughout the circuit, rather than compounded (which can increase higher and odd-ordered content).


There is an advantage to this- when the circuit is fully differential, the primary distortion harmonic is the 3rd, which is treated by the ear the same as the 2nd in that it contributes to ’bloom’ and ’warmth’. But in such a circuit, the 3rd will often be at a 10th of the value that is seen in a circuit that has a 2nd harmonic as the primary distortion component. In mathematical terms, the former has a cubic non-linearity whereas the latter has a quadratic non-linearity. When there is a cubic non-linearity, succeeding harmonics drop off at a much higher rate than they do if the non-linearity is quadratic. In a nutshell, fully differential circuits are much lower distortion and so sound more neutral. This is not subtle either- its easy to hear. So there is a distinct advantage to all those extra parts if the designer took advantage of all the aspects of differential operation!

Balanced is totally designed for professional use. Runs of hundreds of meters. Not tens of feet.

This statement is also false. The benefits of balanced line connections are there even if the connection is only 6 inches. I can go into it further if you like; FWIW this idea of ’only for long distances’ is also a common myth. Think about this- you don’t have to know the technical side; the simple fact that you can run such long distances without coloration is a tip off that those colorations won’t be in a 6 foot run either! If you’ve heard differences between single-ended cables, then you know what I’m talking about when I use the word ’colorations’.

I’m not a fan of preamp to amp balanced connections. I’ve always preferred single ended overall. YMMV

@geof3 The reason balanced operation might not seem to bring home the bacon in high end audio is that there is a standard for balanced line operation, which most high end audio manufacturers do not support (and don’t seem to realize how this degrades the sound). When the standard isn’t supported, the cable construction becomes audible like it does when running single-ended. Balanced operation isn’t two single-ended signals running together in a cable! Its a single signal traveling in a twisted pair with both sides ignoring ground. Please Note that I am not arguing with you here- I’m sure that what you’ve experienced is very real. But what you experienced was not how balanced operation is supposed to work. BTW, the balanced line standard is known as AES48. You can always ask the manufacturer if their equipment supports AES48; if they don’t know or don’t know what it is, then its a very safe bet their gear doesn’t support the standard and so will not demonstrate all the advantages of balanced operation.


Those advantages are:
1) lower noise (blacker backgrounds)
2) neutral, transparent interconnect operation regardless of length (more detailed, less colorations resulting in no need for expensive interconnections)
3) no ground loop noise or intermodulations (not as harsh)

But if the standard isn’t supported these things go away and as you’ve heard, might even be worse.

@dentdog Do note that the technology that makes power "balanced" is different from the technology that makes amplification equipment (i.e., sources, preamps, and amplifiers) balanced. 

If you care for an explanation, read on. 

Apologies for length, but I'm just now starting to understand all this and wanted an excuse to type it out so that I could consolidate the learning.  Please do offer corrections anyone...

--Balanced Power--

Traditionally, AC power is delivered with 120v on the "hot" line and 0v on the neutral return.  In balanced power equipment, the hot line is run at +60v and the neutral is run at -60v.  

--Balanced/Differential Equipment--

Alternatively, the balanced equipment discussed above separately amplifies the positive and negative parts of the electrical signal.  This requires double the circuitry because you have to amplify two signals (0-to-positive and 0-to-negative) instead of one (negative-to-positive) as in single-ended equipment.   If you imagine music as a sign wave, then balanced equipment is separately amplifying the upper and lower halves of the wave.  The benefit of this added complexity is that any noise introduced into the signal by your equipment will be cancelled out when these two "half" signals are re-combined to make the full-wave signal that drives your speakers. This is because any injected noise will appear positive going in one amplification circuit and negative going in the other... add them together and you get automatic noise-rejection because they'll be in exact opposition. 

--Distortion & Preferences for Balanced vs Single Ended Equipment--

Interestingly, in addition to external noise sources, every amplification circuit also creates its own "ringing distortion." If, for example, you play a sign wave at 1000Hz, the circuit will also produce a signal of reduced amplitude at different harmonics. Thus, you'll see small spikes at 2000Hz, 3000Hz, 4000Hz, 9000Hz, etc. These are called even (2000, 4000, etc.) and odd harmonics (3000, 9000, etc.). The balanced circuitry also eliminates all even harmonics when the signal is combined to re-create the music. This is because their peaks line up directly with the peaks of the original signal and they get cancelled just like any other noise/distortion that affects both the positive and negative amplification circuits at the same time.  

While this is a good thing in principle because your music has less distortion, about 1/3rd of human ears actually find the second harmonic distortion pleasing.  It seems that this harmonic ringing creates the perception of increased warmth and 3-dimensionality in stereo listening.  Alternatively, signals with a dominance of 3rd harmonic ringing is often perceived to have increased clarity and detail... and again is preferred by about 1/3 of listeners.  For more discussion from Nelson Pass, read this link:  https://positive-feedback.com/audio-discourse/the-pass-h2-harmonic-generator/

I believe that variances in preference for these different distortion profiles is likely one reason why you see such different (and strongly held) recommendations about the importance of balanced vs. single-ended gear from different posters. 

While my training as a scientist initially biased me towards a "balanced is better" perspective (why wouldn't you want less distortion???), I have subsequently found that I have a consistent preference for gear with a 2nd harmonic dominant profile.  At this point I'm confident enough in that preference that I'll only buy single-ended gear.  

Sorry for all the typing folks.  I'm bored.  Thanks.

Have had both and I must say that while the balanced front end was not of the quality that my present unbalanced front end now exhibits, there existed a depth in the music that was at times uncanny. Reducing the noise floor in my opinion is a critical aspect of a system when attempting to hear into the music. 
I still use balanced power with both an EQUI=TECH wall unit and Balanced Power Technologies  AC treatment. Still I have found that "spooky" sound quality a bit less prevalent in the current system. 
Using a BAT VK P 10SE phono stage fed my system a very deep and quiet signal. Alas the unit at 20 years old began to have problems. 
Running high efficiency speakers brings out the charm of a balanced system. I say it's worth it.

     My preamp, amps and universal disc player offer both balanced and unbalanced connections.  I use balanced/xlr cables for all connections since they are objectively more secure and I subjectively perceive them as quieter and a heck of a lot cooler than unbalanced/rca cables.  My dog and 2 cats also can't knock anything loose when they manage to get back there and start messing around.

Tim 

You’ll find the thoughts on this very polarizing, and in reality, it just depends. My current system is all single ended, with the exception of my phono pre out. I run that balanced for several reasons. I’m not a fan of preamp to amp balanced connections. I’ve always preferred single ended overall. YMMV

I've read where running a true balanced (differential) amplifier as such sounds much better than running it single ended (I'm assuming the same amp has both balanced and single ended inputs here).

That's the story. The one time I was able to do a valid comparison that is sure not the way it went. Dealer wanted to show off his Krell and how great it was balanced. So I made him let me hear it with the same interconnect both ways. Obviously not the same RCA/XLR but the same brand and level interconnect. As close as close can get.  

With balanced there definitely was a blacker background and...  that was it. The sound had slightly more of a hard edge to it. Just the kind of thing a lot of guys confuse with detail. 

Anyway, it is all beside the point. Because it is so obviously the wrong way to go. The number one factor in quality sound is quality parts. Going balanced requires twice as many parts. A whole duplicate circuit. So even if there is some advantage, it would have to be twice as good. And it's not. Nowhere even close. At best a slight improvement. Which you paid a hefty price tag to achieve. Bad move.

Balanced is in my opinion a misuse of technology. Balanced is totally designed for professional use. Runs of hundreds of meters. Not tens of feet. Components with hundreds of connections frequently being moved around. Home systems have a few and they tend to not be moved around much. On and on it goes. Every design attribute of balanced has the professional in mind. Home systems need locking connections? I don't think so. 

Good choice though if your system is the Kingdome. 

Balanced connections are all about eliminating external noise that gets picked up in cables etc. They also pretty much deal with ground loops as well. So if either of those things are a problem in your system then using balanced connections is effective way of dealing with them. You also get a 'free' 6dB of gain when the balanced signals are mixed back to one signal... but I don't think gain is in short supply with modern equipment.
The bit that a lot of people are unaware of is that balanced inputs are actually noisier than single ended (RCAs etc.) in that they utilise relatively high value resistors that introduce their own (johnson) noise into the signal.
So if you don't have problems with interference or ground loops then single ended will be quieter. It's really down to your own system as to which is better.

When I read your first sentence I thought you might have been talking about bridged amplifiers vs single ended... that's another subject if you did.

You'll get higher gain and improved S/N when running a differential amplifier in balanced mode, and you'll also get the benefit of CMR.