Amplifier to the Spectrum. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Telephony

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Telephone communication is sometimes such that the subscriber is almost inaudible, especially to a person with hearing loss. The only way out in such a situation is to build a simple 3H amplifier, which can be built into almost any domestic or foreign telephone set. The author of the article talks about such an amplifier.

With all the advantages of telephone amplifiers powered by an independent source, their common drawback is the dependence of operation on the state of the source. In some cases, this is due to the need to constantly have spare galvanic cells and replace depleted ones in a timely manner [1], in others, a power supply interruption makes it impossible to use the amplifier [2]. In this version, the amplifier is always active if the line is good.

As experience shows, the amplification of the received signal is implemented by very simple means. As an example, I will tell you how this was done for the widespread Spektr-3 telephone set (TA-11320). This is a classic device with a disk dialer, a PDK-1 sound emitter and an MK16-U-11 carbon microphone, there are many more.

A fragment of the scheme of the named device in a somewhat simplified form is shown in Fig. 1, and in fig. 2 shows a diagram of the amplifier and its connection to the apparatus circuits. The letters h, b, k indicate the colors of the wires (green, white, red) of the cord connecting the device to the handset.

During a telephone conversation, the movable contact of the SA1 lever switch (its third contact is not shown for simplicity) is in the lower position according to the diagram. In this case, the direct current of the line flows through the winding 1 - 4 of the T1 transformer and the BM1 microphone, on which the voltage drops 8 ... 16 V, depending on the design of the device. The variable current component of the "conversational" signal is transformed into a winding 3 - 4 and drives the phone BF1 of the handset.

When an amplifier is introduced into the device, the place of BF1 is occupied by a variable resistor R1, which has approximately the same resistance. From its engine, the incoming signal enters through the capacitor C1 to the base of the transistor. From the collector load - resistor R3 - the amplified signal enters through the capacitor C2 to the phone BF1. Power is supplied to the amplifier from the terminals of the device through the red and white wires, as well as the VD2 diode. The zener diode protects the amplifier from increased ringing voltage (sometimes it reaches 150 ... 200 V!) When the handset is prematurely picked up.

Along with the simplicity of the device, one can also note the simplicity of its connection. The amplifier is able to work with any telephone equipped with a carbon microphone and a so-called "anti-local" transformer. To connect the amplifier, it is enough to understand the three wires that go from the handset to the device: two are connected only to the telephone capsule and microphone, one is common. Having traced the path of each of them (they are also multi-colored), it is easy to determine the clamps to which they are connected. The output from the common wire of the amplifier is connected to the terminal to which the common wire of the tube is connected (it is white), the output from the VD2 diode is connected to the wire clip from the microphone (red), the wire from the phone is disconnected from the clip and soldered to the capacitor C2 of the amplifier, and the freed clamp is connected to a variable resistor.

The device can use fixed resistors MLT-0,125 or MLT-0,25, a variable resistor - any small-sized power from 0,15 W. It can be made up of a constant resistor with a resistance of 270 ... 330 Ohms connected in parallel and a variable SP-0,4 at 470 Ohms. Oxide capacitors - K50-6 or other possibly smaller dimensions. The transistor indicated in the diagram can be replaced by any of this series.

Instead of the Zener diode D814G, D814D, D813 or another one with a stabilization voltage of 10 ... 12 V is suitable.

The amplifier is assembled on a printed circuit board (Fig. 3) from a one-sided foil material, which is reinforced inside the apparatus case with a strip of insulating tape.

Establishing an amplifier is reduced to selecting a resistor R2 of such a resistance at which the collector current of the transistor will be about 7 mA. To avoid repeated soldering, you can do this: after removing the handset from a device that does not yet have an amplifier, measure the voltage at the microphone leads. After connecting the amplifier, it will decrease by 2 ... 3 V. With this in mind, apply the appropriate voltage to the amplifier from an external source and set the desired current by selecting a resistor - in the future it should remain the same when the amplifier is working with the device.

If, due to significant amplification (when the variable resistor slider is in the upper position according to the diagram), acoustic feedback occurs, accompanied by whistling sounds, put a thick foam rubber gasket into the tube body between the microphone and the phone.

Literature

1. Prokoptsev Yu. Amplifier for a telephone set. - Radio, 1996, No. 6, p. 36.
2. Bortnovsky G. Telephone amplifier with an inductive sensor. - Radio, 1996, No. 10, p. 36, 37.

Author: Yu.Prokoptsev, Moscow

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