This circuit is a circuit diagram of security system. You can use this in the garden and listening to unusual sounds, or maybe just the sounds of wildlife. If you have a car parked at a remote location, the microphone will also vote on anything in this area. Cable can be seen or hidden, screened cable is not required and you can use a cable or speaker bellwire if desired. In principle this circuit there are two parts, a remote microphone preamp, and an audio amplifier based on the National Semiconductor LM386 audio amplifier. Remote microphone preamps using ECM to monitor the vote. A direct coupled amplifier 2 stage built around Q1 and Q2 microphone strengthen weak signals. R2 preset functions as a control, and C1 provides some high frequency roll into the overall audio response. Q1 is run at a low collector current for a high signal to noise ratio, while the Q2 collector biased to around half of the maximum dynamic supply voltage range. Power supply for the preamp is fed through R10 and R6 from the 12V supply. C4 ensures that the preamp power supply are separated and there is no ac voltage is present on the electrical network. Amplified audio output from the collector of Q2 to be fed into the supply line through a 220u capacitor C6. Q2 output impedance is low, the relatively high value of C6. C6 also has a second purpose of allowing the audio signal output through, while blocking dc voltage. The following is a schematic drawing:
In the end, a 10u capacitor C7, bringing home the amplified audio to listen to the location. The first signal further amplifier voltage gain x10 amplified using TL071. C8, a capacitor 22p more a response rolls off high frequencies above 100kHz. This is required as a long wire may take a little radio interference. After amplification by op-amps, audio finally passed to the LM386 audio amplifier. R14 acts as a volume control. R13 and C12 to prevent the possibility of instability in the LM386 and recommended by the manufacturer. Audio output is about 1 watt into 8 ohm loudspeaker, distortion of about 0.2%. If the choice of headphones can be used, although I would recommend a series resistor the same value as the impedance headphones.
