Input module of the mixing console. Scheme, description

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Input module of the mixing console. Encyclopedia of radio electronics and electrical engineering

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Encyclopedia of radio electronics and electrical engineering / Audio equipment

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The article proposes some options for the input module for an amateur mixing console. The description of this design, which aroused the interest of our readers, was published in the Radio magazine, 2003, No. 2, 3. The author designed several more modules that can be used as part of this console.

When establishing the input blocks for the modular console [1, 2], it turned out that different copies of the K548UN1A microcircuits have a rather large spread of parameters and require tuning. Usually, at the outputs of two amplifiers of the microcircuit, the constant voltage is noticeably different and the limitation of one half-wave of the signal occurs much earlier than the other. This reduces the overload margin, especially at low supply voltages. You have to deal with setting up each microphone amplifier by selecting resistors in the OOS circuit. In this case, the symmetry of the input is inevitably violated in the differential amplifier, i.e., we lose one of the main advantages of such a microphone amplifier. In addition, the current consumed by only one microcircuit reaches 15 mA, which is a lot for a multi-channel remote control if it is powered by batteries. In the modular design of the console, the blocks are easily replaced, which allows them to be improved as needed.

Another variant of the input universal amplifier with a transistor microphone amplifier (MU) and a linear amplifier (LU) based on an op-amp of wide application is proposed. It is usually not possible to use such op-amps in a microphone amplifier, since they cannot provide acceptable noise characteristics. It is necessary to install low-noise transistors at the input and even take out the transistor cascade to the microphone, transmitting an already amplified signal through the wires [3]. In the latter version, there are difficulties associated with the need to apply voltage to the transistor stage and at the same time maintain the symmetry of the input.

These problems are easy to solve if you remember how phantom power is usually applied to the input of a microphone amplifier. After all, the resistors through which the phantom microphone supply voltage is connected simultaneously to both inputs of the differential operational amplifier (through coupling capacitors) can play the role of the collector load of the transistors of another preliminary differential amplifier. This preamplifier can be placed on the same board and brought to the microphone, since it already has power (turned on instead of phantom), the input symmetry is preserved. The signal from the transistor collectors is fed through two wires of the microphone cable, and the braid serves as a common wire. It is enough to apply a small bias voltage from the collectors to the bases of the transistors, and you get a very good microphone amplifier. The operational amplifier can be used as a linear one. The entire module consumes no more than 10 mA. Two variants of the circuit of such an input amplifier are shown in Fig. 1.

(click to enlarge)

The only difference is the outputs. In the first of the options (Fig. 1, a) there is a common output level control and the signal is fed immediately to both output lines of the console, in the second (Fig. 1, b) - the "Panorama" control is installed at the output. The two knobs just don't fit on the front panel. Yes, and there is no need for this: for a stereo signal, there is a linear amplifier module, in which the signal level and timbre are adjusted simultaneously in both channels (on the TDA1524A chip or improved - LM1036) and the "Panorama" control is provided. Therefore, the printed circuit board (Fig. 2) is offered only for the first option.

(click to enlarge)

The linear amplifier (DA1.1) is assembled on a quad op-amp TL074 (TL084, KR1401UD4). The remaining op-amps are used in the tone control (DA1.2), overload indicator (DA1.3) and in the output stage (DA1.4). The gain is changed by about 10 times with a variable resistor R16. The calculation of a simple differential amplifier with gain control by a single resistor is quite simple [4]:

КУс = (R11+R12)/R8+2(R11xR12)/ /(R8xRp);

Rp = R16 + R15, R8 = R9, R11 - R12 = R13 = R14 = 10 kOhm.

The resistance Rp varies in the range of 1...48 kOhm. Accordingly, the gain is adjustable within 5,6 ... 0,6. Of course, you can choose a different range of adjustment. We draw attention to the fact that many domestic variable resistors can have a noticeable residual resistance between the terminals of the movable contact and the extreme terminals of the resistor at the corresponding extreme positions of the regulator. Of course, the control range is reduced in this case. It must be remembered that the supply voltage of the microcircuit is only 12 V and the voltage of the undistorted output signal slightly exceeds 2,5 V (3 V at Kr - 1%). To obtain a normalized output value of 250 mV, a signal with a voltage of 45 ... 450 mV can be applied to the input. For high voltage signals, you will have to use the output gain control R29.

The main gain comes from MU. You can use low-noise transistors in it (for example, KT3102E), choosing a pair with the same parameters, but it is easier to put KR159NT1V or KR159NT1E transistor assemblies. The initial gain of the MU is set by choosing the resistance of the resistor R7. If the highest sensitivity of the module corresponds to a signal with a level of 1 mV, then the total maximum gain (Kus lu = 5,6) should reach 250, and the MU should be about 50.

Measurements of the MU gain on transistors with h21E = 220 showed that at R7 = 560 Ohm Kusmu reaches 250, at 10 kOhm - 110, at 24 kOhm - 64, at 470 kOhm - 4,6. By the way, such a parameter change is sufficient for simple automatic level controls.

Input resistors R1, R2 determine the input impedance of the MU and allow, if necessary, to remove the connection of their common point with a common wire in order to apply phantom power to it. Capacitors C2, C3 help reduce unwanted high frequency noise. Switch S1 separates the microphone and line amplifiers, so nothing prevents the MU from being made in the form of an external board placed inside the dynamic microphone housing.

Laboratory measurements of the parameters of several input modules (they were connected to the path one by one in turn) showed that at the highest amplification, the level of integrated noise at the console output was -62...-65 dB relative to the normalized value. In this case, the harmonic coefficient Kg was less than 0,1%. An increase in the input signal level led to an increase in non-linear distortions. Thus, at Vin = 6...7 mV the Kg level reached 0,3%, and at Uin = 16 mV - 1%. Due to the low supply voltage, the overload capacity of the MU is small, but for dynamic microphones it is quite sufficient in most cases.

All holes on the front panel of the module and the places where the board is attached completely coincide with the module described earlier [2]. The input has an X1 JACK 6,3 connector. Either a microphone or a linear amplifier is connected to the input using switch S1.

The tone controls allow you to change the gain at frequencies of 50 Hz and 10 kHz by no less than ±12 dB. The sensitivity of the comparator, which registers the excess of the amplitude of the signal of any polarity of the set value ("Overload"), can be changed by selecting the resistor R24.

This module can be used as an independent single-channel remote control with line output. It is enough to place it in the case and supply power from the AC adapter. When the module is connected to a console that has a common stabilizer, the stabilizer DA2 and the protective diode VD5 become redundant (see Fig. 1,6). Instead, jumpers are soldered on the board.

If you use adjusting resistors SPZ-33-32, then they can be installed directly on the board. Then the corners for attaching the board to the front panel are not really needed. But you can’t do without them when using SDR-4 or imported variable resistors, which will have to be mounted on the front panel and connected to the board with wires.

It is not necessary to give a detailed description of the microphone module. It differs from the universal line only in the absence of the S1 switch (no line input) and the installation of the CANNON connector instead of the JACK connector, which is used in all professional microphones.

Literature

Kuznetsov E. Input amplifiers with balanced input. - Radio, 2002, No. 12, p. 16, 17.
Kuznetsov E. Amateur mixing console. - Radio, 2003, No. 2, p. 12-15; No. 3, p. 10-12.
Preliminary ULF. amateur schemes. Radio library. Issue 9. - M.: RadioSoft, 2001.
Gutnikov VS Integrated electronics in measuring devices. - L .: Energoatomizdat, 1988.

Author: E. Kuznetsov, Moscow

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