The following circuit gives the circuit diagram of Telephone pickup Preamplifier. The central part of this circuit is the operational amplifier IC uA 741. This operational amplifier is used to pick up telephone signals. In this circuit, operational amplifier IC uA 741 acts as inverting amplifier. The gain of the amplifier is determined by the feedback resistor R4 (2M). This port resistor R4 can also be used for gain adjustments in this circuit. The gain to drive headphone Z1 is produced by the operational amplifier.
The tracking of telephone is done by high impendence magnetic core. If a high impedance magnetic core is placed near a telephone instrument or near one of the telephone wires, it picks up inductively both sides of the telephonic conversation without any electrical contact. The coil L1 used here can have 3000 turns of 0.4mm. The power supply for this circuit is 9 to 24 V DC supply.
- Use a good quality PCB or common board to assemble this particular circuit.
- The circuit can be used as a telephone bug too.
- Use 5000 turns for L1 if pick up with 3000 turns is not good.