There are thousands of pre-amplifier circuits. These are three which have interested me and are a little different. This is a simple design circuit of intercom pre-amplifier. This circuit is built by transistor and several low cost components. 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. This is the figure of the circuit.
High quality microphones also tend to be low impedance, typically around 600 ohms. This is low input impedance, high quality pre-amplifier of the sort that could be used in a stage mixing desk. 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.
High quality microphones also tend to be low impedance, typically around 600 ohms. This is low input impedance, high quality pre-amplifier of the sort that could be used in a stage mixing desk. 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.
