Differential Balanced Sound Quality

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?

@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.

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.

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.

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.