Differential Balanced Sound Quality

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.

@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.

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.