I found in ebay an old fostex T20 it's an Isodynamic headphone (I bought it more by curiosity).
I received the headphone today as "working perfectly", at first one driver didn't work
I checked the jack connector, it was fine after that the cable with a voltmeter and finally I checked the driver, I thought a soldering default. .. I opened the driver and looks like this:
2 round magnet taking in sandwich the stator (kind of mylar + aluminum flat wire)
(there are not my photos!)
The problem came from a caoutchouc join inside the driver having shrinked, I tightened the screws of both driver and voila both working now but the sound was so-so.
I read some threads about moding the t50rp. I remouved the thick piece of foam at rear of the cup replaced by a piece of felt and tap partially the vent. I was astonished by the sound produced! I didn't expect that whaaou, they don't lake of bass, the treble are extended the medium gorgeous. I don't understand why they have abandoned this technology?
no need any energyser, I can imagine using a bunch of little magnet but the wiring in the mylar
it's another story... if someone have any idea... I know it's not an electrostatic but a cousin