The mike preamp itself is based on the LM837 Low-noise audio opamp from National Semiconductor. The first stage is a noninverting 26dB gain stage (fixed). Note the use of the 33pF capacitor in the feedback loop, this reduces the tendency of the LM837 to oscillate (it has a large bandwidth of 17MHz). Two 5.1V Zener diodes are connected back-to-back across the input, to protect the amplifier from large voltage transients at the input.
The next stage is an inverting variable gain stage (-âˆž to 60dB), again with a 47pF capacitor across the gain-control resistor. The pot used here should be of high-quality to prevent crackling, etc. Also, the wires should be tightly twisted and shielded to prevent noise and mains-hum pickup. The last stage is an inverting buffer (unity gain), to correct the inversion caused by the variable gain stage.
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