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 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 (Ganged with P1) J1_____________Stereo 3mm. Jack socket B1_____________1.5V Battery (AA or AAA cell etc.)
Device purpose:
This circuit, connected with 32 Ohms 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 neighbours. 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.
Circuit operation:
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.
Notes: