Equalizer. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Tone, volume controls

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The equalizer, as you know, is designed to adjust the frequency response of sound reproducing equipment. It consists of several regulators, with the help of which it is possible to change the gain of the amplifying device in rather narrow frequency bands. This allows you to get a complex shape of the frequency response, which is impossible to implement with traditional tone controls. As a result, the listener has the opportunity to significantly change the nature of the reproduced sound picture and thus compensate for the frequency distortions introduced by the sources of sound programs, acoustic systems and listening rooms.

Equalizers are usually built on the basis of active band-pass filters on the op-amp, and the more filters, the more you can change the frequency response. However, a significant increase in their number greatly complicates the control of the equalizer, so the number of filters is usually limited to 8-10.

The following is a description of the eight band equalizer. The range of its operating frequencies is 20 ... 20 Hz; transmission coefficient - 000...3; the tuning frequencies of each of the eight filters are shown in the table; quality factor (ratio of tuning frequency to bandwidth) of the filter - 4; gain adjustment range - +_ 1,12 dB.

Schematic diagram of the equalizer is shown in fig. one.

Fig. 1

It consists of eight active filters connected in parallel on dual op-amps DA2-DA5. The op amp DA1 assembled input and output buffer amplifiers. A resistor R4 is connected in parallel with the filters.

Since all filters are inverting, and signals pass through the resistor R4 without inversion, the signals are subtracted in the output amplifier. Due to this, the frequency response is equalized at the edges of the filter passband and the required range of gain adjustment in each band is obtained.

The filter circuits are the same, and their tuning frequencies are determined by the capacitances of the capacitors C7-1-C7-8 and C8-1-C8-8, the values ​​\uXNUMXb\uXNUMXbof which are indicated in the table.

By moving the sliders of the resistors R7 - 1-R7-8, you can change the transfer coefficient of the corresponding filters, and, consequently, the frequency response in the band of these filters. In the extreme left position (according to the diagram) of the engine of these resistors, the transfer coefficient at the filter tuning frequency is maximum (+12,5 dB), and in the extreme right position it is minimum (-12,5 dB).

All parts of the equalizer, except for variable resistors, are placed on a printed circuit board made of foil textolite, a sketch of which is shown in fig. 2.

Fig. 2

In the equalizer, you can use fixed resistors VS and MLT, capacitors K50-6 (C5.C6) and KLS, KM, MBM (the rest), and capacitors with a small TKE should be selected for filters. Capacitors C7 and C8 are composed of two or three connected in parallel . The functional characteristics of variable resistors must be linear (group A), they can be either sliding, with linear movement, or axial.

When using sliding resistors (SPZ-23A), you can make a graphic equalizer. The position of the sliders of these resistors will clearly reflect the frequency response of the equalizer (Fig. 3).

Fig. 3

When using axial resistors SP, SPO, etc., the quality of the device will not deteriorate at all, but the visibility of the frequency response adjustment will decrease.

The equalizer does not require any special adjustment, it is only necessary to pre-select the capacitances of the filter capacitors with an accuracy of no worse than 5 ... 10%.

To power the equalizer, you need a bipolar stabilized power supply with a voltage of 12 ... 15 V and a current of up to 50 mA.

For a stereo complex, you will need to make two described equalizers and install dual variable resistors in them.

Author: I. Nechaev; Publication: cxem.net

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