I couldn't find an impedance vs frequency plot for the AKG K601 headphones, so I did a quick test. I connected the K601 to the headphone output of a NAD 320BEE integrated amp, the line out from a soundcard to the line in of the NAD, and the headphone output, in parallel with the K601, back to the line in of the soundcard. I then ran a loopback frequency response test, and I have attached the result. The NAD has, purportedly, a 220 ohm output, and my understanding is that a more typical high impedance output would be 120 ohms. (and I also understand that very low output impedance amps are much more common than 120 ohm amps). The K601 has a nominal impedance of 120 ohms, and when I loaded the NAD down with the K601, the output did drop by roughly 9dB (assuming the notches on my soundcard level meter are in 3dB steps - they are not labelled!), which tallies.
I wanted to know how much the amp's output impedance would affect the frequency response of my K601s, and it seems that the answer is "not very much". (I certainly haven't been able to hear any difference, switching back and forth between my V-Can, which has a 5 ohm output, and the NAD).
So, here is the amplitude vs frequency graph, in case anyone else finds it useful. (I haven't bothered converting it to impedance):
Greg.
