Classroom Microphone System Circuit

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.

collar microphone 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

classroom mic audio amplifier diagram

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.

LM380 Pin connection


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  • Ron H

    I can tell you this much. The LM380 and LM384 are basically the
    same as far as internal structure is concerned. I have tried out
    the LM384. The first pair I burned out by my own mistake. I installed a second pair with the right connections connected.
    Here’s how I did mine.Before starting, get a bare Copper Board. Before any connection is made, using a pair of needle nose pliers,carefully bend out pins 1,2,6,8,9,13 and 14. What’s left is pins 3,4,5,7,10,11 And 12. Pins 3,4,5,7,10 11 and 12 will be
    soldered directly to the copper board, because the copper will act like a heat sink.

    The connections as follows: pin 1 is the bypass pin . Connect a 4.7 MFD capacitor. pin 6 is the input
    pin from volume control. pin 2 to ground with a wire.the reason why pin 2 and 6 is open, is for experiment basis. pin 8 is the
    speaker connection through a 470 MFD Capacitor.On the Plus side
    of the capacitor is where your 2.7 ohm resistor and 0.1 MFD go
    to ground. the minus side of the capacitor goes to the + side of
    the speaker with the – wire to the copper board.

    Last connection: connect you B+ wire and 470 MFD Capacitor to
    pin 14. make sure the – Side of the 470 MFD is soldered directly
    to the copper board

    I hope this will answer some questions because that’s how I built mine. this was based on my own work.

  • veereshgouda

    We want design of circuit?? How we can specify resistors values???

  • pee

    Please what can I use in place of IC LM 380…

  • Colpitts

    No question is stupid if it furthers understanding.

  • kiran

    i got a problem in simulating the circuit.I coudn’t find LM380 ic in national instruments so please suggest me another ic that replaces the LM380

  • Ebbe

    Come on guys. all these stupid questions you can’t expect the author to answer.
    Use Your brain instead of demanding spoon feed.



  • Vasudevan

    Hi…Thanks for the circuit…It is working very well and also helped me in the hardware design lab…

  • Student

    Can i assemble this on a bread board. And can u pls give me the detaied explanation for this.

  • student

    what is 220R in R4 and is C1, C2 and C3 in uF? and where the non inverting input in IC connected?

    please reply i need help