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This audio amplifier circuit is useful in classrooms to reduce the strain of lecturing ,if the surrounding environment is noisy. It uses the power amplifier IC LM380 that gives 2 watts output which is sufficient in a confined area. The amplifier is portable and the whole circuit and the battery can be enclosed in the Speaker box itself. The circuit uses a preamplifier stage around NPN transistor BC 548 and a power amplifier around LM380.An external Collar MIC can be given to the input using an audio jack .The voice signals are amplified by T1 and fed to IC1 input through the volume control VR. Capacitor C3 reduce the undesirable howling from the speaker. LM380  is the general purpose audio amplifier and its gain is internally fixed at  34 dB . Its output is short circuit proof with internal thermal limiting. The IC works between 5 to 22 volts DC and is dual in line version. Classroom Microphone System Circuit Schematic Audio Amplifier Classroom Microphone System Circuit Assemb...

This is a microphone pre-amplifier circuit that use discrete components. The gain of this circuit is 100 and should be enough for most microphones. It is set by the ratio of the 10k resistor to the 100R. As common-emitter amplifiers, This circuit uses two directly coupled transistors. Here is the schematic diagram of circuit: Please note that this circuit has no bias voltage source, so you can’t use with condenser type microphones. You can use this circuit with dynamic type microphone, or condenser electret module with active circuit inside (any microphone which is equipped with battery inside can be used).