From the output of the source of musical programs, the signal goes to the resistive mixer R1-R3, which sums the signals of the left and right channels in stereo mode. At the same time, the mixer plays the role of a divider, which allows you to filter out extraneous noise and pickup signals, for example, those that occur during the operation of EPU engines, a tape recorder, or from the movement of the pickup needle along the plate.

As a rule, these noises are less than the minimum volume music signal. However, without a divider, due to the high sensitivity of the compressor, they can be amplified by it to a value that the screen device of the CDS can respond to. At the same time, the use of the divider R1-R3 made it possible to increase the sensitivity of the compressor to 200 mV, which corresponds to the voltage at the linear outputs of sound reproducing devices.

From the resistor R3, the audio frequency voltage is supplied through the capacitor C1 to the controlled divider R4VT1, in which the transistor is used as a voltage-controlled resistor. Next, the signal is fed to an amplifier made on transistors VT3 and VT4. Its gain is about 100. From the load of the amplifier (resistor R13), the signal is fed through the capacitor C10 to the SDU. At the same time, part of the amplified signal taken from the resistor R19 engine is fed to the cascade with the VT5 transistor. From the emitter of this transistor, the signal is fed to a detector made on diodes VD1, VD2. The constant voltage formed on the capacitor C5 is used to control the transistor VT1 (through the emitter follower on the transistor VT2). Any increase in the input signal level leads to an increase in the positive voltage on the capacitor C5 and a greater opening of the transistors VT1 and VT2. The resistance of the collector-emitter section of the transistor VT1 decreases, which means that the signal level on it also decreases. If the level of the compressor output signal is not enough for normal operation of the CDS, then to increase it to 5...7 V, an additional amplifier is connected between the CDS input and the compressor output, the circuit of which is shown in Fig. 3. The signal level at the SDU input can be changed by selecting the resistance of the resistor R3 in the amplifier and the tuning resistor R19 of the compressor. The amplifier is designed to work with SDU with an input impedance of at least 1 kOhm.

With a lower input impedance of the SDU, and also if there is a matching transformer at the input of the SDU, an amplifier should be used, the circuit of which is shown in Fig. 4. If a signal of 3 ... 0,5 V is sufficient for the operation of CDS with an input resistance of several hundred ohms to 0,6 kOhm, then it can be supplied from the emitter of the transistor VT5 of the compressor, while excluding the resistor R19, connecting the base transistor VT5 with the collector of the transistor VT4 and soldering the positive terminal of the capacitor C10 from the collector of the transistor VT4 to the emitter VT5. KT312V, G; any diodes of the series D315, D9 (VD10, VD1), D2, D223, KD226 (VD103). Capacitors C3, C3-K10-53, K1-53; C4-KD-4, the rest - K1-50. Fixed resistors - MLT-6 or MLT-0,25, trimmer - SPZ-0,125. Under these parts, a printed circuit board (Fig. 16) made of one-sided foil fiberglass is calculated. Capacitor C5 (it eliminates the excitation of the compressor at high frequencies) is installed on the side of the printed conductors. Resistors R4 - R1 are soldered directly to the terminals of the XS3 connector.

Diode VD3 and capacitor C9 are installed at the location of the elements of the main power supply unit of the SDU. If the SDU itself is powered by a constant voltage of +24 ... 30 V, it can be supplied to the compressor by excluding the VD3 diode and capacitor C9.

The adjustment of the compressor begins with checking the absence of self-excitation at high frequencies and, if it exists, increase the capacitance of capacitor C4. Then, by applying a 1 V audio frequency signal to the compressor input, the desired voltage (19 ... 0,7 V) is set at the compressor output with a tuning resistor R2,5.

Finally, set the sensitivity of the device. By connecting it to a signal source, resistors R1 and R2 are selected such that the previously set output signal level is maintained at the required minimum signal level at the XS1 connector.

Author: A. Anufriev; Publication: cxem.net

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