This simple audio preamplifier uses a current diode for an MPSA18's collector load to achieve over 60 dB of gain in one stage for audio frequency. The gain is adjusted to precisely 60 dB (1000) by adding a little emitter resistance, 147 ohms in this case. The input impedance is estimated to be about 150k and the output resistance is about 1k, although the preamp is intended to drive higher resistance loads. (A 1k load will drop the gain to 54 dB.) That 1k in series with the output could be lowered if desired. The output buffer is a self-biasing bipolar/JFET pair that draws about 1 mA. The current is kept low for long battery life and a battery should be able to run the circuit for a week or two of continuous duty. The input-referred noise voltage is probably about 2 nV/root-Hz at 1 kHz.

No LED was added since it would draw significantly more than the circuit. A molded power supply could also be used for continuous duty applications.



Here's the amplifier being driven by a little under 1 mV
at a little over 6 kHz..




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