This circuit is very useful in classrooms where strength of students is more or size of classroom is big and where surrounding atmosphere is noisy, in that case students sitting at the back face problem that they cannot hear the voice of the lecturer properly. As a result lecturer has to speak in loud voice which can create problem in the throat like pain in throat, swelling etc. For this you can use collar Mic which are available in market but they are very costly, so to solve this problem you can make this simple circuit and use it as collar Mic. Main advantage of this circuit is it is portable as it uses very less and easily available components.
This circuit is build around a single IC that is LM380 with few more components like Mic, transistor, resistor and capacitor. LM380 is a general purpose audio amplifier, its gain is internally set to 34 dB. Operating range of LM380 varies from 10V to 22V. It comes in standard dual inline package.
Working of this circuit is very simple. Provide power supply when you want to use classroom Mic. Now adjust the condenser Mic properly near to your mouth. So when you speak in front of Mic this signals are pre amplified with the help of transistor T1 and this signal are now fed to IC1 that is LM380 pin number 6. For getting the clear voice vary the variable resistor VR1. Be careful while choosing the variable resistor it should be variable log resistor otherwise your circuit will not work. Now these signals are again amplified by IC1 and we will get output at pin 8 of IC1 which are then feed to speaker and amplified signals can be heard.
For powering the circuit you can use PP3 batteries but if you are going using this circuit for long duration of time than use rechargeable batteries of 6V/4.5Ah. Use good quality and small size speaker to make the unit compact.