This circuit, connected to 32 Ohm impedance mini-earphones, can detect very remote sounds. Useful for theater, film and attending class: every word is heard clearly. You can also listen to your TV to a very low volume to avoid disturbing family and neighbors. Even if you have perfect hearing, you may discover unexpected sounds using this device: a remote bird song will seem very close to you.
The heart of the amplifier circuit is a constant volume control. All signals picked up by the microphone are amplified at a constant level of about 1 volt peak to peak. Thus, the very low amplitude signals are amplified audio and high amplitude are limited. This operation is carried out by Q3, Q1 bias change (hence the AC gain) through R2. A notable feature of this circuit is the operation of 1.5V.
List Component
P1 : 22K Log. Potentiometer
R1,R9 : 10K
R2 : 1M
R3 : 4K7
R4,R7 : 100K
R5 : 3K9
R6 : 1K5
R8 : 100R
C1,C2 : 100nF
C3,C6 : 1µF/63V
C4 : 10µF/25V
C5 : 470µF/25V
D1 : 1N4148
Q1,Q2,Q3 : BC547
Q4 : BC337
MIC1 : electret microphone
SW1 : SPST Switch
J1 : Stereo 3mm. Jack socket
B1 : 1.5V Battery (AA or AAA cell etc.)
