Telephone Amplifier Diagram

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.

Above schematic diagram is a small 325mW amplifier circuit based on LM386 with a voltage gain of 200 that can be used as a simple bench amplifier, signal tracer or used to amplify the output from personal radios, MP3 players, MP4 Players or iPods.

How this circuit works:

The gain of the LM386 can be controlled by the capacitor across pins 1 and 8. With the 10u cap shown above, voltage gain is 200, omitting this capacitor and the gain of the amplifier is 20.

The IC works from 4 to 12Volts DC, 12Volt being the maximum recommended value. Supply from USB port might be able to supply this bench amplifier circuit. The internal input impedance of the amplifier is 50K, this is shunted with a 22k log potentiometer so input impedance in this circuit will be lower at about 15k. The input is DC coupled so care must be taken not to amplify any DC from the preceeding circuit, otherwise the loudspeaker may be damaged. A coupling capacitor may included in series with the 22k control to prevent this from happening.