Originally Posted by
lnuss
Nope, can't find anything about specs for those -- good point. So I went to check out my adapter and I'd forgotten that it was a different animal, an Icom HS-61 Switch Box, according to the label, and it incorporates all the needed pieces including plugs for the headset, a push-to-talk switch, a nice coiled cord with adapter plug to fit my IC-A21, and even has adjustments for mic gain and monitor gain. Of course you get sidetone on transmit, too. Anyway, it works well, even in a Super Cub cockpit.
Sorry for being misleading, but I guess that's what I get for depending on memory. Of course there are non-transformer ways of matching impedance, which would potentially be smaller and could still be powered from the voltage on the mic jack, if the radio provides it, but if it's not a single chip it could get expensive.