Circuit diagram
While talking to a distant subscriber 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. e.
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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