1,5V Earphones Audio Amplifier

This audio amplifier circuit, connected to 32 Ohm impedance mini-earphones, can detect very remote sounds. Useful for theatre, cinema and lecture goers: every word will be clearly heard. You can also listen to your television set at a very low volume, avoiding to bother relatives and neighbors. Even if you have a faultless hearing, you may discover unexpected sounds using this device: a remote bird twittering will seem very close to you.
1,5V Earphones Audio Amplifier - schematic

The heart of the circuit is a constant-volume control amplifier. All the signals picked-up by the microphone are amplified at a constant level of about 1 Volt peak to peak. In this manner very low amplitude audio signals are highly amplified and high amplitude ones are limited. This operation is accomplished by Q3, modifying the bias of Q1 (hence its AC gain) by means of R2. A noteworthy feature of this circuit is 1,5V battery operation. Typical current drawing: 7,5mA. List Component P1: 22K Log. Potentiometer 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 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 D1: 1N4148 75V 150mA Diode Q1,Q2,Q3: BC547 45V 100mA NPN Transistors Q4: BC337 45V 800mA NPN Transistor MIC1: Miniature electret microphone SW1: SPST Switch J1: Stereo 3mm. Jack socket B1: 1.5V Battery

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