Telephone monitor

It is sometimes necessary to make a telephone conversation audible to more than one person at either end or to make a tape recording of it (as in many businesses). Often, this is done with the aid of a microphone fitted with a suction cup. This does not work very well, though, hence the present circuit.

Use an old, but still serviceable, a telephone set connected across the telephone line in parallel with the receiver to be monitored. Connect a length of screened audio cable across the receiver inset and to an amplifier or tape recorder, as the case may be. As soon as a conversation has to be recorded or amplified, take the receiver from the hook and adjust the input level of the amplifier or recorder as required. Do not set the amplifier gain 100 high, because this may give rise to howling.
The hook contact of the additional telephone set prevents the bell voltage (50 V or more) reaching the audio equipment.1 Furthermore, the audio equipment is electrically isolated from the telephone line by the 1:1 telephone transformer. T.

In most west European countries it is now allowed for telephone sets to be connected in parallel, in spite of the resulting paucity of the signal quality (primarily caused by the reduced echo attenuation).
Telephone monitor Schematic diagram

 

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