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Topics: Amplifier Circuits / Audio Amplifiers

It has many practical uses. For example, the microphone can be used to pick up the sound of a TV that is set to a very low volume. The circuit will then amplify that sound and feed a loud version to the headphones. this makes it ideal for those cases where it's late at night and you need to be quite, but want to listen to your TV louder that your neighbors would normally like!! It might also be interesting to sit in a quite garden late and night and see what bug noises it picks up that you can't normally hear with just your regular ears.

The circuit uses less that 30 components in total, 4 of which are NPN transistors (3 x BC547, 1 x BC337). There is a log potentiometer used for volume/gain-control... so you can adjust the output volume depending on how loud or quite the sounds are.

Be careful not to blow your ears off!

Parts List:

P1 - 22K Log. Potentiometer (see Notes)

R1,R9 - 10K 1/4W Resistors

R2 - 1M 1/4W Resistor

R3 - 4K7 1/4W Resistor

R4,R7 - 100K 1/4W Resistor

R5 - 3K9 1/4W Resistor

R6 - 1K5 1/4W Resistor

R8 - 100R 1/4W Resistor

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C1,C2 - 100nF 63V Polyester or Ceramic Capacitors

C3,C6 - 1μF 63V Polyester or Ceramic Capacitors

C4 - 10μF 25V Electrolytic Capacitor

C5 - 470μF 25V Electrolytic Capacitor

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D1 - 1N4148 75V 150mA Diode

Q1,Q2,Q3 - BC547 45V 100mA NPN Transistors

Q4 - BC337 45V 800mA NPN Transistor

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MIC1 - Miniature electret microphone

SW1 - SPST Switch (Ganged with P1)

J1 - Stereo 3mm. Jack socket

B1 - 1.5V Battery (AA or AAA cell etc.)