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.)
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.
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.
- Please note the stereo output Jack socket (J1) connections: only the two inner connections are used, leaving open the external one. In this way the two earpieces are wired in series, allowing mono operation and optimum load impedance to Q4 (64 Ohm).
- 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 into a pocket.
- Gary Pechon from Canada reported that the Amplified Ear is so sensitive that he can hear a whisper 7 meters across the room.
He hooked a small relay coil to the input and was able to locate power lines in his wall. He was also able to hear the neighbor's stereo perfectly: he could pick up the signals sent to the speaker voice coil through a plaster wall.
Gary suggests that this circuit could make also a good electronic stethoscope.