Headphone amp in Benchmark DAC1 was designed to drive headphones with a minimum 30-Ohms impedance without increase in distortion but it didn’t stop you from trying Sony MDR7509 (24-Ohm).
Quote Hugh Robjohns:
I also tried the headphone output, and this proved equally impressive. Driving a pair of low-impedance Sony MDR7509s, the DAC1 had masses of level and drive capability, producing crisp transients with no hint of compression or clipping, even when pushing out ridiculous sound pressure levels.
Anyway, someone else asked the same question on diyaudio forum and the question was answered by a nice guy going by the name panomaniac:
Class-D is a bad idea for headphones for several reasons.
1) More power than you really need. Headphones require very little power compared to normal speakers, so many of the problems of power amps are not much of a concern for headphones. As stated above, Class-A amps would seem a good choice here.
2) Tons of noise. Class-D amps are noisy, by headphone standards. If you plug headphones directly into a class-D amp, even a small one, you'll hear what I mean! This noise is not a problem with normal speakers as they are so much less sensitive than phones.
3) The output filter. Switching amps use an low-pass output filter the is usually designed to work with a 4~8 ohm load. Headphones are often 32 ohms or more, so the filter response will be wrong.
4) H-Bridge design. The small Tripath chips used in the Sonic, Charlize, Fenice 20, AMP6, AMP3, etc. are bridged. Thus the left and right grounds can not be tied together. You would have to rewire your headphones to dual mono.
For the same money, size and trouble, a good Class-A amp, solid state or tube, even opamp, should give you better sound.
Hope that helps.
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