I always seem to end up optimizing my headphone amplifier circuits for higher impedance headphones, this mostly happens because I own a pair of 300 ohm HD-600s and it is tedious to design for both
the voltage requirements of high impedance headphones and
the current requirements of low impedance headphones.
Not impossible, just, for the class-A designs I seem to be building recently, increasingly large, heavy, and impractical.
Complimentary transistor circuits, however, offer the opportunity to swap voltage for current at something close to the same design cost. They are therefore a practical topology for efficient class-A power delivery into low impedance headphones. As a design experiment, my aim is to discover how far I can leverage an ultra-low-voltage, unity gain circuit for compactness without sacrificing sound quality.
Ok. Back-of-the-envelope calculations:
A typical 16 ohm in-ear-headphone has a sensitivity of 100-105...