The need that high impedance headphones have for an amp comes from the fact that high impedance headphones require higher voltage drive to achieve high playback levels. High impedance headphones can be connected to the output of a medium power speaker amp without immediately being turned into smoke and ashes. They are also suitable for use with tube amps, particularly Output TransformerLess, which easily produce high output voltage but have limited current capacity.
Low impedance headphones will draw higher power from a lower drive voltage. This makes them suitable for use with battery powered portable equipment that has limited voltage available. The downside is that low impedance headphones require higher current drive at these lower voltages in order to have the power needed to produce adequate sound levels. This means that low impedance headphones also benefit from being driven from an amp, just that the optimal parameters of the amp’s output are different:
High impedance headphones need an amp with higher output voltage but do not need high current capacity.
Low impedance headphones need an amp with lower output voltage and high current capacity.
In all of our cases of power delivery the output impedance of the source should be significantly lower than the impedance of the load. With high impedance headphones this is easy, and the output impedance of most line level source equipment being typically 50~100 ohms is comfortably able to drive headphones with an impedance of 300 ohms or more. This output impedance not only controls the amount of power that it can deliver, but also the ability of the amp to ‘control’ the drivers. If the output impedance is not significantly lower than the headphone driver impedance then the sound is likely to be unclear and/or flabby.
Some (many?) headphone amps try to meet all requirements by providing high enough voltage output for high impedance headphones and high enough current to feed low impedance headphones. This is a compromise, and rarely works extremely well for all types.
If you are into a little math, the electrical equations are simple:
Voltage (V) = Current (I) times Resistance ®. Various forms of this are:
V=I*R, R=V/I, I=V/R
Power = Voltage times Current. Various useful permutations of this are:
P=VI, P=V²/R, P=I²R