From your calculations the 650 is already critically damped?
Overdamped just a little little bit. The damping ratio without electric damping is 1.014 > 1. I didn't expect this since HD 600 is underdamped (0.634).
I think I just like a mellow, bass heavy sound then. Seems less obtrusive and more relaxing on mediocre recordings to me with some series R. I stop noticing the headphones and relax.
Ok. Do you use cross-feed?
How did you extract the lumped elements from the impedance curve?
Step 1: Got the impedance curve from headphone.com
Step 2: Collected the impedance values at 10, 20, 30, 40, 50, 60, 70, 80, 90, 100, 200, 300, 500, 1000, 2000, 5000, 10000 and 20000 Hz.
Step 3: In LibreOffice Calc ("Excel") I copied a calculation tab for HD 600 and replaced the impedance values.
Step 4: Iterated the component values so that the calculated impedance curve matches with the measured one.
Sorry about some of it being in Finnish. This is just to calculate things, not intended to be published to the world. The orange curve is measured and black simulated. Iteration of component values is pretty fast. Rc is basically the minimum value of the curve, R+Rc = maximum value. Lc is easy to get so that the rise at the high frequencies is right. Fo is easy to get and finally L and C dictate the width of the impedance bump around Fo. C is calculated from L using Fo, so actually only L is iterated.
I just use a high NFB output buffer and switchable series R. I built my amp with 10-20-43-78-110-230 Ohms.
Ok. NFB = 10 Ω and then you have 0, 10, 33, 68, 100 and 220 Ω resistors selected with a 2 x 6 rotary switch.
I'm puzzled by your f0 for the HD650. The innerfidelity plots put both resonances at about 90 Hz. Both impedance plots are very similar, with the HD650 having very slightly better damping. Am I missing something?
Maybe the unit innerfidelity measured is very different from the one headphone.com measured? If you list me the impedance values at the frequency point of step 2 above, I can calculate the "alternative facts" for you
Incidentally, there is a page about HF resonances in the electrical curves at
https://diyaudioheaven.wordpress.com/tutorials/how-to-interpret-graphs/impedance/
The results are for a hypothetical headphone though!
Yeah, the high frequency spikes are some high frequency resonances according to this. The membranes aren't perfect.