#1507 Ramsey 2m Microphone Amplifier (part 4 of 12)

Episode 1507
microphone preamplifier

[brainndamage]

U4B is a 2nd order lowpass sallen-key filter

[davidv]

I wonder if PL might be for injecting "private line" tones, very low audio frequency tones used for activating repeaters and other receivers. It is after the microphone amplification and filter circuit so that would be the correct point to inject PL tones.. Regards, David

[rickwise]

I'm wondering about one thing: the circuit seems to knock down the incoming amplitude a lot. C83 impedance will be about 160k at 1kHz. And it sees an input impedance of 50k (divider R40/R59). So the signal at the op amp input will be 1/4 of the mic input voltage.

[flarkel]

Interesting that the output of the 2nd OpAmp stage is cut down by a factor of 10 on the 10K pot being fed by a 100K resistor. It seems a waste to have all the gain the first stage. Is that to reduce noise?

[rhp]

Is the 2k2 resistors near the microphone jack used to bias the microphone?