Hello,
Say I have a H-250/U military handset I want to build an adapter for to use on regular amateur radio gear...
Mic impedance: 150 Ohm.
Mic output, loud whistle: 2.5mV.
Speaker impedance: 1000 to 1500 Ohm.
How do I match impedance? Audio transformer?
Do I need a mic preamp?
I would love a schematic! Thanks.
Gil.
Gil, do you have Mouser or DigiKey or Radio Shack there in France?
For the speakers, a little audio impedance matching transformer will work. They convert 8 Ohms to 1K Ohms. You don't need an exact match. The transformer's bandwidth is voice only (300Hz to a little over 3KHz), but that's just what you need.
Amazon has some. Can't embed the link so here's the long form link.
http://www.amazon.com/Transformer-3-4KHz-Primary-Resistance-Rating/dp/B00R5C8NQY/ref=pd_sim_sbs_328_1?ie=UTF8&dpID=41cDCALu%2BEL&dpSrc=sims&preST=_AC_UL160_SR160%2C160_&refRID=19E55JNY1XBM6TT5DKNN
Radio Shack used to sell them too, but they look out of stock everywhere.
Caveat emptor: I'm a little weak microphones for ham radio.
There are 2 main types of mics. They need different electronics in the radio.
1) dynamic: basically a loud speaker working backwards.
2) electret: a piezo element generating charge. These need a voltage applied to them but put out 100x more signal than a dynamic.
I just did some digging and it seems some ham radios are designed for electret mics (some Icoms, at least) but it looks like most use dynamic mics. My Yaesu MH-31 mic has a 190 Ohm impedance. Since your mic reads 150 Ohms it's probably a dynamic mic. and the impedance should be fine. The only way to know if you'll need an amplifier is to try it.
In Europe, try Newark / Farnell
Thanks guys, I'll have to give it a try!
Gil