[goobicii]

lets say you have HD800 with its dynamic driver,image time slowing down as you observe the driver moves creating music.Now the driver have certain Xmax,the maximum excursion,the maximum distance it can travel before it runs into some limits.Lets make imagine that limit is 100db at that happens when driver travels 1mm from centre,this would be 2mm peak to peak xmax.
 
My question is,does the impedance,or resistance of the dynamic driver inside HD800 changes depending how far it is from middle/centre? Could it be made that the amplifier will measure the changing impedance tens of thousands times a second to get high precision feedback on where exactly the dynamic driver is on the path from centre to peak excursion?  

 

[MindsMirror]

It's an interesting question. Fundamentally your question seems to be related to phase distortion. The position of the driver from center or peak excursion, as well as its direction of travel, describes its phase. When the resistance of the driver varies across its phase, that results in a phase shift, where the current leads or follows the voltage by some degree as in and RLC circuit. When there is a phase shift, the proportion of current to voltage varies depending on the phase angle, so the apparent instantaneous resistance does vary with the position of the driver.
 
You can see measurements of the electrical phase of the HD800 here:
http://www.innerfidelity.com/images/SennheiserHD800.pdf
As you can see, there is a phase shift of up to +/-11 degrees at certain frequencies.
 
There may also be a non-linearity in the resistance in the resistance when the driver moves far from the center, but I believe this effect would be very small, as the driver does not really move very far. If there were a significant non-linearity of the resistance with driver excursion, it would manifest itself as harmonic distortion. As long as it stays within the normal operational range, the driver will not produce excessive distortion, and therefor there must not be a significant amount of non-linearity at the maximum excursion of the driver.
 
An amplifier is not a measurement device. It is not the job of the amplifier to directly regulate the position of the driver. The amplifier's job is to regulate the voltage at its output, and the headphone's job is to respond linearly to the voltage. As long as the headphone does respond linearly to the voltage, which should be the goal of any good headphone, there is no need for the amp to know the position of the driver, it only need to be concerned about the voltage. Many amplifiers do already have continuous feedback to regulate their output voltage.

 

[Roseval]

An amplifier is not a measurement device.

I can only second that.
It is possible to add some additional circuitry to measure actual cone movement and compensate for "errors' in its position compared with the signal
An example: http://www.psiaudio.com/en/wp-content/uploads/sites/2/2015/01/psi_audio_descr_aoi.pdf