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The compact and inexpensive condenser microphone audio amplifier described here provides good quality sound from 0.5 watts to 4.5 volts. It can be used in intercoms, walkie-talkies, low-power transmitters and packet radio receivers. Transistors T1 and T2 form the microphone preamplifier. Resistor R1 provides the necessary bias to the condenser microphone while the preset VR1 acts as a gain control to vary its gain.
To increase audio power, the low-level audio output of the preamplifier stage is coupled via coupling capacitor C7 to an audio power amplifier integrated around the BEL1895 integrated circuit. The BEL1895 is a monolithic audio power amplifier integrated circuit designed specifically for sensitive AM radio applications, delivering 1 watt into 4 ohms at 6 V supply voltage. This circuit has low distortion and low noise and operates on supply voltages from 3V to 9V, making it ideal for battery operation. Power-on noise reduction circuit prevents loud noise when power-on.
Coupling capacitor C7 determines the low frequency response of the amplifier. Capacitor C9 acts as an anti-ripple filter. Capacitor C13 couples the available output at pin 1 to the speaker. The R15-C13 combination acts as a damping circuit for the output oscillations. Capacitor C12 provides the starter strapping function. This circuit is suitable for low power HAM radio transmitters to provide the audio power required for modulation. With simple modifications, it can also be used in internal communication circuits.