A simple adapter for recording a conversation from a telephone line. Scheme, description

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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING
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A simple adapter for recording a conversation from a telephone line. Encyclopedia of radio electronics and electrical engineering

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Encyclopedia of radio electronics and electrical engineering / Telephony

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Sometimes it is useful to record your telephone conversation on a tape recorder. It is impossible to apply a signal directly to the input of a tape recorder or through a capacitor with a TL. This is due to the fact that if the tube lies on the device, a constant voltage of 60 V operates in the line, which is quite enough to damage the input stages of the amplifier. The use of an isolation capacitor in the circuit will not save the tape recorder circuit - after all, at the time of the call, an alternating signal with an amplitude of up to 200 V can operate in the line, and the input of the tape recorder, for ease of use, must be constantly connected to the TL.

A simple adapter for recording a conversation from a telephone line. Scheme

Adapter diagram

The adapter diagram shown in the figure allows you to use any recording tape recorder that has an input for connecting an external microphone. The device does not require power and can be permanently connected to a break in one of the wires of the line going to the telephone set. (This does not degrade the quality of the connection and prevents damage to the input circuits of the tape recorder.)

The transformer provides DC isolation. The inclusion of diodes VD1 and VD2 as limiters prevents the occurrence of an alternating voltage in the secondary circuit of the transformer with a level dangerous for the input circuits of the tape recorder. And the desired signal level from the line is set by a tuned resistor R1 (its value may differ significantly from that indicated in the diagram).

For the manufacture of the T1 transformer, a magnetic circuit is used from any telephone transformer (they are all unified and easy to find). It has a design that excludes the magnetization of the magnetic circuit during the flow of direct current (when the tube is removed from the apparatus). We disassemble the transformer and remove the upper winding from the frame. In its place, 120 ... 140 turns are wound with PEL-2 wire with a diameter of 0,33 mm. The remaining windings are turned on as shown in the diagram, which allows you to increase the voltage in the secondary circuit.

With proper installation, the adapter does not need to be adjusted.

Publication: cxem.net

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