One of the most common points of confusion, and questions raised, relates to the differences between, and meanings of, the terms “single ended”, “balanced” and “differential” operation. And, further to that, what the qualifier “fully” (as in “fully balanced”) means in relation to these terms:
A single-ended configuration uses two signal connections, one for + and one for ground (or signal return). It is the most common connection used to headphones, between components, and in the internals of audio gear, by far.
A balanced configuration still uses two signal connections, but requires that both lines have the same impedance (not nearly the same, actually the same). How this is achieved doesn’t matter (i.e. it could be from differential signal lines off a source or from a single or a single signal line and a resistor on the ground/return line). This, combined with the twisting of the lines, is what’s required to get the noise rejection from balanced operation (CMRR).
A differential configuration, which is still a two-wire connection, drives one line in + phase and the other in - phase. This doubles the potential difference between the signal lines, which is what yields the higher (double) peak-to-peak voltage of differential operation (and thus the higher signal level).
Typically a differential amplifier (balanced or not) requires twice as many amplification circuits (one each, per phase, per channel, so four total) as a non-differential design (one per channel). This means that for an otherwise identical amplification circuit, a differential implementation will require twice the amplification circuitry.
One implication of which is that for a given level of performance, a differential amplifier either has to be more expensive than a non-differential version, or corners have to be cut in terms of the bill-of-materials (or saved elsewhere). So, in general, single-ended amplifiers will tend to offer better “bang for the buck”.
Note: Being balanced does not require differential operation. Nor does being differential in operation require a balanced configuration!
This is why you can have a single-ended amplifier, but still get the benefits (noise rejection) of a balanced connection. It is also why you can have an otherwise single-ended DAC or amplifier offer a balanced output.
Fully Balanced & Balanced Differential
Sometimes we see the term “fully balanced” or “balanced differential”. In theory these terms are actually describing different things.
“Fully balanced” should be taken to mean the entire design, from input to output, is using a balanced configuration.
This isn’t super useful, as the benefits from balanced operation are really in the realm of improved CMRR (common mode rejection ratio) and are most beneficial over long cable runs … not the very short paths on a PCB.
So, more often than not, “fully balanced” actually means “balanced differential from end-to-end”.
Formally, this is a configuration that uses a differential topology from input to output (i.e. no phase splitters on the input, no summers on the output) and uses purely balanced transmission lines from end-to-end.
More commonly it just means something is fully differential, and has balanced inputs and outputs (because balanced transmission lines do not matter as much on the PCB).
It’s not being “balanced” that yields more power vs. “single-ended”, it’s being differential. You can have balanced connections and the same power output as the single-ended connections (see things like the Phonitor X).
However, for the connection to a headphone you need to have a balanced (four-wire, no shared ground) connection to have differential drive (but you can have non-differential drive on a 4-wire connection … the ground wires are separate but are still ground).
With a differential connection you’ll see double the potential difference (voltage) in the signal, which doubles the volume, and then you have twice as much power available from the amplifiers (since there are twice as many of them, with pairs operating in opposite phase), giving a doubling of current capacity. Thus yielding a 4x increase in the output power.
Being “dual mono” says nothing at all about whether something is balanced OR differential. There is no required interplay between those factors.
Balanced headphone drive, whether differential or not, typically yields better stereo separation and the reduced crosstalk often means that low-level details (resolution) is improved too.