Posted on Feb 5, 2014

The circuit is so simple that there is little to say about it, but it gives very good results. Tr1 is operated in grounded base mode with input to its emitter to give low impedance input. The values shown give correct operation from 9v. If operating from other voltages you may wish to alter the 1K8 resistor to give symmetrical clipping at high lev

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els. If you wish to switch the speaker (to use the same speaker as a speaker and as a mic, then you should arrange the switching to short the speaker out before disconnecting it. This is because the d. c. through Tr1 also flows through the speaker so removing the speaker will upset the d. c. conditions and cause a large thump in the speaker. You could of course fit a 1K resistor in the emitter and use a 100 µF capacitor to couple the speaker. High quality microphones also tend to be low impedance, typically around 600 ohms. This is a low input impedance, high quality pre-amplifier of the sort that could be used in a stage mixing desk. In fact that was the purpose for which it was designed, but never used because I soon after left the company. The circuit uses a dual rail power supply - convenient because there were many op-amps in the machine. Note that Tr1 is a PNP transistor. Theoretically PNP transistors can have lower noise level than NPNs. Tr2 amplifies Tr1`s output. Tr3 is simply a constant current collector load for Tr2, with its current controlled by the 180R emitter resistor. This can be altered to give more current to feed lower impedance output loads. Overall negative feedback is applied via the 100K pot (log is best) which is the gain control. Note that the feedback loop includes the input and output capacitors so the low frequency response is excellent. The 150K resistor is present to charge up the input and...

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