I remember that years ago, a standard was proposed for headphone drivers to have a 300 ohm output impedance, and that headphones should be designed with that in mind. The idea was that integrated amplifiers' headphone outputs were pretty much always done with a resistor of a few hundred ohms in series with the headphones, driven from the power amp's main loudspeaker output. The standard was intended to regularise the de-facto practice. Whether anyone ever took any notice I don't know. Just like the standard that loudspeakers' minimum load impedance should be no less than 80% of nominal. Widely ignored by most? loudspeaker manufacturers.
All the headphones I've measured, which I accept isn't too many, have shown a very even impedance characteristic, with no peaks in the bass, just a gentle rise at HF, reflecting the inductance of the speech coil. I'm inclined to the view that with most headphones, the difference between a low impedance and medium impedance drive is just one of level, as I've not measured any significant change in frequency response. As to distortion, I don't know, but not been conscious of any audible change.
S.
All the headphones I've measured, which I accept isn't too many, have shown a very even impedance characteristic, with no peaks in the bass, just a gentle rise at HF, reflecting the inductance of the speech coil. I'm inclined to the view that with most headphones, the difference between a low impedance and medium impedance drive is just one of level, as I've not measured any significant change in frequency response. As to distortion, I don't know, but not been conscious of any audible change.
S.