Parts :

P1 = 22K
R1 = 10K
R2 = 1M
R3 = 4K7
R4 = 100K
R5 = 3K9
R6 = 1K5
R7 = 100K
R8 = 100R
R9 = 10K
C1 = 100nF 63V
C2 = 100nF 63V
C3 = 1µF 63V
C4 = 10µF 25V
C5 = 470µF 25V
C6 = 1µF 63V
D1 = 1N4148
Q1 = BC547
Q2 = BC547
Q3 = BC547
Q4 = BC337
J1 = Stereo 3mm. Jack socket
B1 = 1.5V Battery (AA or AAA cell etc.)
SW1 = SPST Switch (Ganged with P1)
MIC1 = Miniature electret microphone

---

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:

• 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.