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 1uF 63V Polyester or Ceramic Capacitors
C4 10uF 25V Electrolytic Capacitor
C5 470uF 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.)
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.
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.
Due to the constant-volume control, some users may consider P1 volume control unnecessary. In most cases it can be omitted, connecting C6 to C3. In this case use a SPST slider or toggle switch as SW1.
Note the stereo output Jack socket (J1) connections: only the two inner connections are used, leaving open the external one. In this manner the two earpieces are wired in series, allowing mono operation and optimum load impedance to Q4 (64 Ohms).
Using suitable miniature components, this circuit can be enclosed in a very small box, provided by a clip and hanged on one's clothes or slipped in a pocket.