I'm in the design phase for a DIY headphone-amplifier project, and am trying to understand the requirements.
In some ways, it seems harder to design a dinky little headphone amplifier than to design a power amp.
Loudspeaker impedances range from 2 Ohms to 16 Ohms, clustering around 8 Ohms. The max is only 8x the min (and the range is more typically 4-10 Ohms).
Headphone impedances range from 12 Ohms to > 600 Ohms, for normal headphones. The max is > 50x the min. (Electrostatics are way higher, but they require special amplifiers and so my questions don't apply to them.)
Headphone sensitivities range from < 80 dB/V to > 140 dB/V, which is a gigantic difference.
For a headphone amplifier to be able to drive high-impedance/low-sensitivity headphones and low-impedance/high-sensitivity headphones, without under-driving the former and without over-driving the latter seems to be pretty much impossible with fixed gain.
Yet most headphone amplifiers have fixed gain. Do they just not drive some headphones, and users select the headphone amps that match their particular headphones? What if they have more than one pair, with different impedances and/or sensitivities?
Some headphone amplifiers have switchable gain, but even the ones I found online with three gain options only cover about 20 dB of variation, well short of the 60 dB needed to cover the range of headphone sensitivities.
What am I missing?
My system? Google for: Martin Logan 420 CLX Descent Stage Summit