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.