Electronics > RF, Microwave, Ham Radio

Microphone pre amp for PTT

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nhoj_yelbom:
this is for peltor comtac III's and TCI TABCIII radio ptt headsets. the mic ohms out at 150.

i have made this circuit for the mic to work with my radio
http://www.epanorama.net/circuits/dynamic_to_electretinput.html

i get horrible motorboating noise (digital only, analog fine)
at first i thought it could be the cheap kenwood k1 cable i harvested off a baofeng ptt, but i got a new kenwood oem ptt cable on it now thats shielded

only thing it can be now is the pre amp im using to make the mic work, any suggestions or better circuits to try?
   

nhoj_yelbom:
Bump

orolo:
I think the problem with digital signals might be that this amplifier has disproportionate gain at higher frequencies, if I'm not mistaken. If the collector is biased by 5V via 2.2k, typical for a mic, the base current is about 6uA, and that means that the input impedance of the transistor is over 4K. That, together with the 220n of the cap produces a high pass filter at the input with about 1KHz 3dB point. If your digital data rate is well below 1KHz, you will hear only the step transitions of the square waves. A sinewave of about 100Hz should be greatly attenuated compared with a 2KHz one; that's a good check to know if this is the problem.

If the problem is the high pass filtering, you can try drastically increasing the 220n cap (say, 47u), together with a resistance in series of about 3-4K. That should take the 3db point quite close to 1Hz, without impairing the gain too much. Or you could try a different amplifier.

nhoj_yelbom:
thank you. im open to using a different amplifier, just dont know what would be simple and good for this application. i dont have a scope to check the sinewaves, just increasing the capacitor size and put the resistors where?

orolo:
If I'm right, the difference should be audible. Can you feed the audio, say from audacity with your computer, into the mic, and hear or record the output? The resistor should be in series with the cap, just at the base of the transistor. This is just a hack; I think this preamplifier is a rather crude design. I think a very high input impedance preamplifer, like a FET, would work, if you are trying to convert a low impedance, low voltage signal into a higher voltage, high impedance signal.

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