If you are looking for the sensitive sound pick-up circuit then here is exactly what you are looking for. The circuit given in this site can be used as a different devices. It can be used as a simple microphone as well as with more exotic device as a sound operated alarm. This circuit is equally effective for a bugging device.
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Here in the circuit diagram you can see that microphone is employs as the transducer. The output of the condenser microphone is quite low it has to connect with an FET amplifier. This amplifier circuit is power by the R1,R2 resistor network. The output of condenser microphone is fed to a two-stage amplifier. Transistor T1 (BC149C) utilise current series feedback the first stage. The second stage comprising transistor T2 is connected in the voltage shunt feedback configuration. These two stages provide sufficient gain to pick up even the slightest whisper. The amplifier circuit in the site requires 4.2 volt supply which can be obtained with a resistor R9 [1k]. The value of this resistor may be altered to suit a supply voltage other than 6 volts. Output of the microphone amplifier can be made variable by connecting a 10k potentiometer as shown in the circuit. Circuit‘s gain can be increased by reducing the value of R6 to 47 ohms or 22 ohms. depending on the input sensitivity of the main amplifier system. Increase in gain was also observed by using 3V supply and eliminating R9 altogether. The microphone should be housed in a small round enclosure.