Another telephone amplifier. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Telephony
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The magazine has already talked about amplifiers that make it possible to make the sound in the handset louder ("Radio", 1996, No. 6, p. 36). Another similar amplifier is described below, which is also used in industrial telephones.
The amplifier circuit is shown in fig. 1. It is powered directly from the circuits of the telephone set and does not have an output transformer, which made it possible to make the device small in size.
Variable resistor R1 adjust the volume. Instead, you can install a constant of the same denomination, and connect a toggle switch in parallel with it. When the contacts of the toggle switch are closed, the gain is maximum.
In the amplifier, any transistor of the MP series of p-n-p structure is applicable. Fixed resistors - MLT-0,125, variable - from a small-sized receiver. Capacitor C1 - K50-3, C2 - K50-6.
The details of the amplifier are mounted on a printed circuit board made of foil fiberglass (Fig. 2), which is placed directly in the handset. Capacitor C2 is installed on the board in the "lying" position.
Author: L.Poroshenko, Chistopol, Tatarstan
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