As we all know, headphones, similarly as speakers, have non-linear and frequency dependent impedance. This non-linear impedance creates a divider with headphone amplifier output impedance, so the distortion measured at headphone amplifier output (behind its intrinsic output impedance) would greatly differ from distortion measured into a resistor or linear RLC dummy load. Same applies to frequency response, but this is well known.
Let me demonstrate the distortion issue on example of virtually same headphone amplifier, once with 50 ohm output impedance and second with close to zero output impedance. The headphones used are Sennheiser HD598.
Headphone amplifier with 50 ohm output impedance
Frequency response and distortion into Sennheiser HD598
Distortion in % into HD598
Output impedance close to zero
Frequency response and distortion into Sennheiser HD598
Distortion in % into Sennheiser HD598
Conclusion
Not only frequency response is affected by headphone amplifier output impedance, but also distortion. The distortion depends on headphone impedance non-linearity, on headphone amplifier output impedance and on interaction of headphone non-linear impedance with the headphone amplifier output stage. Measurements performed into resistor or linear RLC load cannot reveal this behavior.
Let me demonstrate the distortion issue on example of virtually same headphone amplifier, once with 50 ohm output impedance and second with close to zero output impedance. The headphones used are Sennheiser HD598.
Headphone amplifier with 50 ohm output impedance
Frequency response and distortion into Sennheiser HD598
Distortion in % into HD598
Output impedance close to zero
Frequency response and distortion into Sennheiser HD598
Distortion in % into Sennheiser HD598
Conclusion
Not only frequency response is affected by headphone amplifier output impedance, but also distortion. The distortion depends on headphone impedance non-linearity, on headphone amplifier output impedance and on interaction of headphone non-linear impedance with the headphone amplifier output stage. Measurements performed into resistor or linear RLC load cannot reveal this behavior.