[R-390] Sound powered headphones?

[Kenneth G. Gordon]

> Does anyone have any experience using sound powered headphones with a tube
> radio? They're prized by crystal radio builders for their extreme sensitivity. 
> I have a pair of "deck talkers" and have wondered how they would work with an
> R-390/392 or even a one tube regen set.  If I'm not mistaken, the impedence is a
> bit lower, something on the order of 100 ohms, so they would require some sort
> of matching network. Anybody have experience with this? Thanks, Robert Goff
> W7MKA

Hello, Robert:

Yes. I purposely bought a pair of the "deck-talkers" on eBay a 
couple of years ago to build a pair (or two) of good headphones out 
of. The deck-talkers have pretty hefty bandwidth limiting built into 
the interconnect box, probably to eliminate as much "ambient 
noise" from the guns and/or airplane engines as possible.

I removed the mic and filter units and connected the headphones in 
series, properly "phased" so that both diaphrams moved the same 
direction at the same time. I used shrink tube and other means to 
make them as nice as possible.

I have not yet measured their impedance directly, but, as you say, 
they are pretty low. I'll do that and e-mail you back about what I 
discover.

However, their sensitivity is really amazing. They are the best 
'phones I have ever used. I have used them with every receiver 
here and they work fine. All the receivers I have used them with are 
either 600 ohm output or higher. I also use them with my 
Instructograph which has a 600 ohm output and the sound level 
from them is much higher than a speaker with a 600 to 8 ohm 
transformer in its box.

I am not sure that exactly matching impedances would make much 
difference in how they have worked for me, but I intend to find out 
and will post you about it as soon as I get the dope.

Ken Gordon W7EKB