Telephone amplifier
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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Title: Telephone amplifier
electronic circuit
Source: www.electronics-lab.com
Published on: 2005-02-10
Reads: 770
Print version: 
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