I voted single-ended. One of the major engineers at Benchmark is on record as saying the benefits as oft' advertised simply don't exist(AMB forum); even as a layperson looking at it it's not clear what you get for doubled output impedance and other tradeoffs.
EDIT: Located the quotes (http://www.amb.org/forum/benchmark-engineer-on-balanced-v-unbalanced-headphone-amps-t326.html#p2942) and reproduced them below:
It is worth noting that AMB contested some of these claims, but I side *very* strongly with the Benchmark engineer on this one.
Initial message expressing opinion on debate:
My name is Elias Gwinn, I'm an engineer at Benchmark Media Systems.
We've been getting a lot of questions lately about balanced headphones. We are interested in the debate, but I can't say we agree with any technical explanations about the benefits of the set up.
So far, there are 4 major points mentioned so far (that I have heard, at least):
1. Unshared common conductor reduces crosstalk
2. Two amps (per channel) increases slew-rate
3. Two amps provide better damping
4. Balanced cabling provides better common-mode rejection
If I may, I'd like to add my thoughts on these points:
1. Most headphones (at least those of decent quality) do not share a common conductor through the length of the cable (as opposed to what was said in 6 Moons). Most headphones have a separate wire from each negative terminal that remain isolated through the length of the cable. In other words, most headphone cables are effectively balanced inherently. If they were sharing a common through the length of the cable, the impedance of the cable may cause some of the signal to show up on opposing channels. However, they are not connected until the plug, and therefore have a minimal impedance to ground.
2. Any headphone amp that is struggling with slew-rate is a poorly designed headphone amplifier. The HPA2 headphone amplifier on the DAC1 has a bandwidth of 55 kHz, and it doesn't even approach any slew-rate limitations even at those high frequencies.
3. Two amps provide WORSE damping. This is why power amplifiers run better in normal mode vs. bridged mode. A balanced (dual-active) headphone amplifier is exactly analogous to a bridged amplifier driving one speaker. The only advantage is increased power, but it comes at an expense of increased distortion, decreased damping, and altered frequency response. This is common knowledge for bridged amplifiers.
4. Headphones don't need any help with common-mode rejection because they inherently will not respond to common mode signal. If, for example, you apply a signal to both terminals of a speaker, it will not move at all. A speaker only responds to differential voltages.
5. There is another cost incurred by dual-active headphone amps that is not addressed. Headphone amps should have as low of a source impedance as possible. If you are using two amps to drive a channel, you are doubling the source impedance. This will cause the headphones to suffer in frequency response, distortion, and ringing.
Please continue the great discussions. It is important to resolve these debates so that product manufactures can respond to provide the best audio solutions possible.
Second response to similar query:
ALL headphones have non-linear mechanical impedances (that is, the mass and shape of a speaker will resonate more at certain frequencies and much less at other frequencies). This means the physical build of the headphones (as well as other physical impedances, like your head and ears!) will try to override the electrical system (amplifier and speaker coil).
To create low-distortion headphone response, one must consider 'damping factor'. A high damping factor will control the response of the speaker, thus preventing the physical impedances from dictating frequency response. Damping factor is the ratio of speaker (load) impedance to amplifier (source) impedance. In other words, the best damping factor will result from a low source impedance. Again, the source impedance from the HPA2 is less then 0.01 ohms...as low as gets!!
Balanced headphone amps will double the source impedance of an unbalanced headphone amp. No matter how low the impedance of a balanced headphone amp, it could be half that much if it was unbalanced. This is one reason balanced headphone amps are not a good idea. (It should also be noted that the balanced output of the DAC1 / USB / PRE is 60 ohms or greater, depending on the attenuator settings).
Not only will the source impedance double with balanced headphone amplifiers, but the total distortion and noise of the amplifier will double as well!! Every output device (opamp, transistor, tube) creates some distortion and some noise. If there are two opamps or transistors or tubes driving each headphone speaker, twice as much distortion and noise will be added!!
The result of balanced headphones is less damping factor, more distortion, and more noise. Also, balanced headphones configurations offer no real benefits, to boot.
Feel free to use the XLR outputs of the DAC1 / USB / PRE for balanced headphone outputs (as mentioned above, the DAC1 USB and DAC1 PRE will do better then the DAC1 at this task, because of the 4562's). It won't damage anything to operate in this configuration. But, for the reasons above, I don't recommend it.
Edited by Willakan - 10/18/11 at 7:00am