I use a David Clark Helicopter headset with my Yahoo 857.
Replaced the 4 conductor phone plug with a 1/4" TRS.

Earphones work fine with whatever impedance match/mismatch there might be.

The 5 ohm dynamic mic is a little whimpy. I got it to work well with a RatShack
audio transformer which is normally 1000 ohm CT, 8 ohm secondary. I reversed
it and used half of the CT to allow the 8 ohm side to marry the 5 ohm dynamic
element, and the 500 ohm side (1000 ohm CT) to marry the 600 ohm input to the radio.

Even though that worked great, I put a cheap elec condensor mic element
on the surface of the existing headset mic and ran that to a little preamp
to give the condensor some bias. That setup works fabulously. The mic
element is simply held in place by the foam and leather bits on the "Phoenix"
noise reduction parts. And instead of that heavily compressed, close
mic "aviation sound" I get a great, full range, audio Hi Fi sound on
HF and VHF.

Photos on my website -


Craig 'Lumpy' Lemke
www.n0eq.com