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Telephone
Amplifier
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| While
talking to a distant sub- scriber on telephone, quite
often we feel frustrated when the voice of the distant
subscriber is so faint that it is barely intelligible.
To overcome the problem, circuit of an inexpensive amplifier
is presented here. It can be assembled and tested easily.
There is no extra power source needed to power up the
circuit, as it draws power from the telephone line itself.
The amplifier will provide fairly good volume for the
telephone conversation to be properly heard in a living
room. A volume control is included to adjust the volume
as desired. The circuit is built around IC LM386. Diodes
D6 and D7 are used to limit the input signal strength.
Transformer X1 is a transistor radio’s output transformer
used in reverse. As original secondary (output) winding
is connected in series with the telephone lines, the speech
signals passing through the lines cause change in the
magnetic flux in the core of transformer and thereby induce
signal voltage across the primary winding. This audio
signal is used as input for IC LM386. Diodes D2 through
D5 connected in bridge configuration constitute a polarity
guard so that the amplifier is powered with correct polarity,
irrespective of the line polarity, Zener diode D1 may
have any breakdown voltage between 6 and 12 volts range.
There is no need of a separate power switch as the circuit
energises (via the normally open contacts of the cradle
switch) when one lifts the handset. The circuit may 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 in the loudspeaker. The volume of the hum can be changed
through potentiometer VR1. Diodes D6 and D7 limit the
input below ± 700 mV. The circuit is to be connected to
the telephone lines in series with the telephone instrument,
as shown in the figure. |
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