The following diagram is the circuit diagram of telephone amplifier, build based small amplifier IC LM386. This is a easy build telephone amplifier There is no extra electrical power supply required to power up the telephone amplifier circuit, as it draws power from the telephone line itself. The amplifier will supply fairly very good volume for the telephone conversation to be effectively heard in a living room.
A volume control for this telephone amplifier is included to adjust the volume as desired. The circuit is built around IC LM386. Diodes D6 and D7 are implemented to limit the input signal strength. Transformer X1 can be a transistor radio”s output transformer used in reverse. As original secondary (output) winding is connected in series with the phone lines, the speech signals passing through the lines cause alteration inside the magnetic flux in the core of transformer and thereby induce signal voltage across the primary winding. This audio signal is put to use as input for IC LM386. Diodes D2 via D5 connected in bridge configuration constitute a polarity guard to ensure that the amplifier is powered with correct polarity, irrespective of the line polarity, Zener diode D1 might have any breakdown voltage between 6 and 12 volts range.
There’s no require of a separate power switch as the circuit energises (via the generally open contacts of the cradle switch) when one lifts the handset. The circuit may possibly be wired on a general-purpose PCB or by etching a PCB for this circuit. The circuit can be easily tested by connecting a 6 volts supply to line terminals 1 and 2. A hissing sound will be heard from the loudspeaker. Now connect 6V AC from a transformer to terminals 1 and 2 and observe hum inside the loudspeaker. The volume of the hum may be changed through potentiometer VR1. Diodes D6 and D7 limit the input below ? 700 mV. This telephone amplifier is to be connected to the telephone lines in series with the telephone instrument, as shown inside the figure.
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