So... headphone with built-in speaker adapter. This is what I got out of this.

I would have to agree with Anax here as his being the most likely scenario.

Assuming the efficiency (dB @ 1mW) is the same as that of the HE560, which isn't bad at all at around 90dB/mW, this headphone would only need 1.1V to reach 90dB.
Of course it cannot hurt to reach several dB more  and if your OTL amp can provide 15V the drawn current is just 13mA while reaching 113dB.
You could easily connect it to power amps without any adapters as well.
Even on a 200W/4Ohm amp it would only dissipate 300mW which is not a real problem for thin (lightweight) 'wires'.

In case they managed to reach a higher efficiency (more 'windings') with much thinner wires (= less copper = less weight) the idea isn't bad at all and such a headphone could easily be driven even directly from a 'C'Moy running of +/- 20V rails, let alone a simple OTL tube amp.
the difference between 600 Ohm and 1200 Ohm isn't that big (a factor 2) where the difference between 30 and 300 is a factor 10.
Nothing to worry about here.

resistors are a no go.
35 Ohm + 1165 Ohm in series would seriously lower the usability....
To reach 90dB you would need to apply 6.3V where a 1200 Ohm driver with the same efficiency would need only 1V.

I seem to recall a few old Sennheisers had 2k ohm drivers, so 1.2k doesn't seem that ridiculous.

Charleston?

Quote from: Armaegis on October 12, 2014, 04:37:49 PM

I seem to recall a few old Sennheisers had 2k ohm drivers, so 1.2k doesn't seem that ridiculous.

Charleston?

Maybe? My foggy memory is telling me there was a 4 in the number somewhere.

I'm really intrigued by the Charleston from a collector's standpoint though, though probably also because I'm a swing dancer.

I'm really intrigued by the Charleston from a collector's standpoint though, though probably also because I'm a swing dancer.

Don't bother. They are a rebadged HD440 that goes for a fuck of a lot more, literally. They're not very good.

Not to mention that if you do find one, chances are it's going to be listed for well over $1000; Sennheiser only sold a few of the 1000-product batch at MSRP, and was basically giving the remainder of them away by the end of the event, so... yeah. Don't support that bullshit.