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Telephone communication is sometimes such that the subscriber is almost inaudible, especially the person with the hearing impaired. The only solution in this situation is to build a simple amplifier 3H, which can be integrated into almost any domestic or foreign telephone. About this amp and the author tells article.

With all the advantages of telephone amplifiers, powered by an Autonomous source, their common drawback is the dependence of the state of source. In some cases this is due to the need to constantly have a spare galvanic cells and in a timely manner to replace depleted [1], others a break of power supply makes it impossible to use the amplifier [2].From structures of this kind differ devices that require a phone the line circuit through the apparatus. In this embodiment, the amplifier is always active, if a working line.

As experience shows, the gain of the received signal is implemented is very simple means. As an example, will tell how this was done common phone "Range-3" (TA-11320). This is a classic machine with rotary dialer, the sound projector MPC-1 and coal microphone MC-U-11, any many more.

The schema fragment of the aforementioned apparatus in a somewhat simplified form shown in Fig. 1, and in Fig. 2 is a diagram of the amplifier and its connection to circuits of the apparatus. Letters W, b, to the designated colors of the wires (green, white, red) cord, connecting the handset with the handset.

During a telephone conversation movable contact lever of the switch SA1 (its third contact, for simplicity, not shown) is located in the bottom under the scheme position. The constant line current flows through the coil 1 to 4 of the transformer T1 and the microphone WM at which the voltage drops 8 16...In, in depending on the design of the device. The variable component of the current conversation" the signal is transformed in the winding 3 to 4 and actuates the phone BF1 of the tube.

When introduced into the apparatus, the amplifier, the place BF1 is a variable resistor R1, having approximately the same resistance. With its engine, the incoming signal flows through the capacitor C1 to the base of transistor. With the collector load - resistor R3 is the amplified signal passes through a capacitor C2 to the phone BF1. Power to the amplifier is supplied with a clamping apparatus via the red and white wires, and the diode VD2. The Zener diode protects the amplifier from the high voltage call signal (sometimes it reaches 150...200!) when raised prematurely the handset.

Along with the simplicity of the device can be noted and ease of connection. The amplifier is able to work with any phone equipped with coal microphone and the so-called "protivoglistnym" transformer. To connect amplifier, you need to understand the three wires going from the handset to the handset: two are connected only with the telephone earpiece and microphone, one - General. Tracing the path of each of them (they are 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 clamp, to which summed up a common wire tube (usually white), the output from the diode VD2 - with clip wire from the microphone (red) wire from the phone disconnect from clip and fluster to the capacitor C2 of the amplifier, and the released clip connect with a variable resistor.

The device can be used fixed resistors MLT-0,125 or MLT-0,25, variable resistor - any small power from 0.15 watts. It can be parallel connected constant of resistor 270...330 Ohms and AC SP-0.4 470 Ohms. Oxide capacitors - C50-6 or other possible smaller dimensions. Specified in the diagram, the transistor will replace any in this series.

Instead of the Zener diode DG suitable DD, D or other voltage stabilization 10...12 V.

The amplifier is assembled on a printed circuit Board (Fig. 3) of one-sided glass material, which reinforce inside of the casing strip of insulating the ribbon.

The establishment of an amplifier is reduced to the selection of the resistor R2 such resistance, when which the collector current of the transistor will be about 7 mA. To avoid multiple resoldering can do: lift the handset from the handset, not yet having amplifier, measure the voltage across the terminals of the microphone. After connecting amp it will decrease by 2...3 V. With this in mind, turn the amplifier the corresponding voltage from an external source and install selection resistor the desired current - in the future it needs to stay the same and when the amplifier apparatus.

If due to a significant gain (when the slider of the variable resistor in upper circuit position) there will be acoustic feedback accompanied by whistling sounds, attach the body of the tube between the microphone and phone a thick pad of foam rubber.

Literature

  • Prokopcov Yu Amplifier to a telephone. Radio, 1996, No. 6, p. 36.
  • Bortnovsky G. Telephone amplifier with an inductive sensor. Radio, 1996, No. 10, p. 36, 37.
  • Author: Yu Prokopcov, Moscow