First of all, it's not a weird idea at all. It's more common for handheld radios, but not unheard of (ha, a pun!
) for desktop or mobile radios. Personally, for desktop use, I prefer a set of lightweight headphones like the Presonus HD7 and a desk microphone, but to each his own.
Second, does anyone think this is really hard or mysterious? Do you want to build an adapter or buy an adapter? Buying one might be hard as they are generally built for handheld radios. It would not be hard to get it mostly built:
1. Google "peltor y cable adapter".
Here's a shitty airsoft one (a good one will cost you a mint).
2. Next, get a shitty airsoft PTT adapter for some random radio.
Here's another example. Note the TP-120 pinout given in this Amazon listing, it might be important
. The "U94" is the designation for the jack that the TP-120 plug goes into.
3. Crack open the PTT and rewire as required.
4. Chop off the radio plug and wire in an RJ45 connector for the FT891.
The advantages of this approach are twofold: you get nice molded cables and the mechanical design is all done for you.
Or you can go completely from scratch.
1. The two pin connectors on the Peltor are, for the male, U-173/U (aka MP-101, NSN 5935-00-888-5351) and the female, U-172/U (also mates with U-179/U, MP-101, and JJ-055).
Buy them lot's of places, or
here and
here.
RJ45's are, of course available anywhere. Cut up an Ethernet cable.
The microphone for your FT891 is a dynamic mic. No worries about electret mic's. Spec's and wiring diagram (click the "wiring" link) are
here.
That's the most important information right there, it tells you how to wire that RJ45 jack for PTT and mic connections. Easy peasy.
Note the impedance of 190 ohms and sensitivity of -78dB. The spec's for the standard Peltor boom mic's are
here. Those mic's are actually a little more sensitive, which is good, and the 150 ohm impedance should be fully compatible.
Important question: exactly which boom mic do you have? Make and model? Legit Peltor or weak sauce airsoft?
eta: or just get a nice pair of Presonus HD7's and a Yaesu desk mic