Thank you, I will jump across the regulator, and see if I can make it work. If all else fails, I can bypass the entire preamp and use it as a straight desk mic.
I agree that you could bypass the voltage regulator (78L08), but I wouldn't eliminate the preamp entirely.
If it was my microphone, I would remove the 78L08 from the circuit and then add a jumper from input to output. The 8 volts from your transceiver should have more than enough current to run the preamp.
Together with the 0.01 uF capacitor on the input, the 100 Ohm resistor forms a RC filter. This helps keep any high frequency noise, that might be coming in on the power lines, from getting through to the preamp. The preamp operates at about 6 ma, nominally, so a voltage drop through the 100 Ohm resistor would only be about 0.6 volts. If you were driving it with 13.5 volts, the input voltage to the regulator would still be satisfied.
A typical dynamic microphone element has a very low impedance. While it can drive high impedance circuits, the sensitivity is quite low so amplification is essential. The transistor used in the preamp (2SC2458L) has high current gain, good linearity, and low noise. It will take the low output from the dynamic microphone element and bring it up to usable levels. The 680 Ohm resistor on the collector effectively defines the output impedance, which is about normal for most transceivers.
You should also be able to run the preamp with a 9 Volt battery, with the 78L08 removed. A standard 9 Volt battery should give you about 10 to 20 hours of continuous operation before the voltage droops to low to be useful. But for initial test purposes, it should be sufficient.
Martin - K7MEM
