An audio preamplifier is to be developed for use in a small portable public address (PA) system. The preamplifier will have a microphone input, and its output will drive a power amplifier. A block diagram of the complete PA system is shown in the figure (http://www.digikey.com/schemeit/#ign). The dc supply voltages are provided by a battery pack or by an electronic power supply.
A 2-stage audio voltage preamplifier is shown in the figure (http://www.digikey.com/schemeit/#u4k). The first stage is a common-emitter pnp with voltage-divider bias, and the second stage is a common-emitter npn with voltage-divider bias. It has been decided that the amplifier should operate from 30 V dc to get a large enough signal voltage swing to provide a maximum of 6 W to the speaker. Because small IC regulators such as the 78xx and 79xx series are not available above 24 V, dual ±15 V dc supplies are used in this particular system instead of a single supply. The operation is essentially the same as if a single +30 V dc source has been used. The potentiometer at the output provides gain adjustment for volume control. The input to the first stage is from the microphone, and the output of the second stage will drive a power amplifier. The power amplifier will drive the speaker. The preamp is to operate with a peak input signal range of from 25 mV to 50 mV. The minimum range of voltage gain adjustment is from 90 to 170.
The peak is simulated with a peak input signal of 45 mV using partsim (http://www.partsim.com/simulator#13607). As an alternative for a potentiometer, a voltage divider equivalent circuit is used for partsim. The input sine wave signal represents the signal from the microphone and the output resistance (R10) represents the load of the circuit.
The result of the simulation is shown in the figure below.