Audio preamplifier circuits

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 levels. 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 100µF capacitor to couple the speaker.
Audio preamplifier circuits - schematic

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