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.
Assemble the classroom microphone system circuit on a common PCB. A 9 volt PP3 battery can be used to make the unit compact. If the amplifier is used for many hours, a 6 Volt 4.5 Ah rechargeable battery is a nice choice. If it is used, a charger input facility should be provided .Use a small good quality 8 Ohms speaker. If the output is not sufficient, use a high power amplifier IC circuit in the place of LM 380.