.jpg)
This circuit is connected to a mini headset with an impedance of 32 Ohm, helping to detect sounds that are far away. Useful for theater, film and presentation lovers: every word will be heard clearly. You can also listen to TV at a very low volume to avoid disturbing relatives and neighbors. Even if you have perfect hearing, you can still detect unexpected sounds with this device: a bird chirping in the distance will seem very close to you. At the heart of the circuit is a constant volume control amplifier.
All signals picked up by the microphone are amplified to a constant level of about 1 Volt peak-to-peak. In this way, very low amplitude audio signals are strongly amplified and high amplitude audio signals are limited. This is done by Q3, changing the offset of Q1 (hence its AC gain) by R2. A notable feature of this circuit is operation on a 1.5 V battery. Typical current consumption: 7.5 mA.
Due to the constant volume control, some users may find the P1 volume control unnecessary. In most cases this can be bypassed by connecting C6 to C3. In this case, use the SPST slider or toggle switch like SW1.
Pay attention to the connections of the jack socket (J1) of the stereo output: use only two internal connections, to open the external connection. In this way, the two headphones are connected in series, allowing for mono operation and optimal load impedance at Q4 (64 Ohm).
Thanks to tweaked miniature components, the circuit can be placed in a very small box, fitted with a clip, and hung on clothing or tucked in a pocket.