Amplified Ear

Useful to listen in faint sounds

1.5V Battery operation

Circuit diagram:

Amplified Ear


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 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.

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.