Any link to a schematic for that D104 voice processor?
Bruce's D-104 pre-amp, limiter is an SSM 2166 design with a JFET and an OP amp added for good measure. The SSM 2166 has a lot of stuff in it -- pre-amp, compressor/limiter, downward expander all in a teeny tiny little chip. Steve, WA1QIX, did a design with just the 2166, no JFET, no OP amp that had all that and a two band equalizer. I used to amaze my friends with that design and a three dollar Radio Shack electret.
I'm not sure you need any of this with a D-104, just a big resistor between the microphone element and the amp that's in an amplified D-104 gets you most of the distance you're going to get with a D-104.
Anyway, here is Bruce's design:
http://www.k7dyy.com/images/schemproc.jpgBruce will sell you a board for that design (you supply the parts) that will fit in the base of a D104. It wants +8V or so coming over the microphone cable.
And here is Steve's design:
http://www.classeradio.com/ssm2166_compressor.gifTwo more little tidbits. The 2166 takes a half second or so to settle down so powering on and off with PTT like the traditional D104 amp is not a great idea. However, it does have a quiet mode in which it draws almost no current. I used that. See the datasheet on the Analog Devices website. The other tidbit. I used a surface mount 2166 but I understand somewhere in this great universe of ours the DIP version is still available.
Jon