Differential Balanced Sound Quality

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??

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.)

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.

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.