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.
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.
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