I'm an EE, but a bit rusty. I want to make a 600-Ohm dynamic mic work with my Harley Davidson intercom system in my Ultra Classic, for a roll-your-own headset. I have the speakers all worked out, now for the microphone, which runs into a VOX circuit, so there's a thresholding section somewhere in the HD radio. FWIW, I have tried using dynamic mics and condenser mics (there's 8VDC on the mic lines) but they do not trigger the VOX, so I'm assuming the preamp is required (below) to get to the radio's threshold level before they will break VOX. HD headsets are known for horrible performance, so I'm trying to do a better mic. I don't mind using a condenser mic, almost would prefer it for size. I have a great sounding dynamic I use on my Goldwing and other systems, I could use that too.
The Harley headsets have a 200-Ohm dynamic mic running into what appears to be a small surface-mount board (see attached, component side and traces visible images) which is probably a mic preamp. I'm going to run it this afternoon with my Dual Trace scope on the input and output and see what the board is doing and I'll report later. I am pretty sure it's just a small single transistor amp, but I don't know what all the parts are in the image. The 105C No3 1C thing is one I can't identify. Looks like a diode, 2N4401 equivalent on the output side, couple resistors 100k and 2K, and two UFO's, probably lavendar and brown capacitors of unknown values...
What I'm looking for is to buy a design from someone willing to take it on, reverse engineer it or help me come up with a better design. I could probably do this given time, but reverse engineering the SMT board is not easy since some components are not labeled and I want a quality design, which HD does NOT provide
Details to follow...I'm going to determine whatever gain and wave shaping it does today, that's probably all I can do.
Ed