The circuit is designed to detect audio signals on a remote area for security purposes.The LM386 has been designed to work with low voltages with the current being consumed from a power source or a battery is quite minimal.
The preamplifier on the remote side utilizes an electret condenser microphone (ECM) to identify the audio signals. The ECM can be very durable, very sensitive, extremely compact in size and has low power requirements. To amplify the weak input signal from the microphone, a 2 stage amplifier is constructed around Q1 and Q2. These amplifiers TL071 are designed as low noise versions with low input bias and offset currents, low harmonic distortion and fast slew rate ideally suited for high fidelity and audio preamplifier applications. The capacitor C1 provides high frequency roll off while resistor R2 acts as a gain control to the overall audio response. For a high signal to noise ratio, the Q1 operates at low collector current. For a maximum dynamic range, the Q2 collector is biased around half the supply voltage.
The remote preamp uses an ECM microphone to monitor sound. A direct coupled 2 stage amplifier built around Q1 and Q2 amplify the weak microphone signal. Preset resistor R2 acts as a gain control, and C1 provides some high frequency roll off to the overall audio response. Q1 is run at a low collector current for a high signal to noise ratio, whilst Q2 collector is biased to around half the supply voltage for maximum dynamic range. The power supply for this preamp is fed via R10 and R6 from the 12V supply. C4 ensures that the preamp power supply is decoupled and no ac voltages are present on the power lines. The amplified audio output from Q2 collector is fed onto the supply lines via C6 a 220u capacitor. The output impedance of Q2 is low, hence the relatively high value of C6. C6 also has a second purpose of letting the output audio signals pass, whilst blocking the dc voltage of the power supply
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