Parametric equalizer for modular console. Encyclopedia of radio electronics and electrical engineering

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For selective frequency correction when recording or sound amplifying speech, music and vocals, multiband frequency correctors or parametric correctors are used - with a tunable frequency and adjustable quality factor. About such a device, applicable in an amateur mixing console, is described in this article.

It is necessary to resort to frequency correction of the spectrum of sound signals both when recording musical programs, and when listening to them or sound amplifying them in halls and outdoor areas. your taste. We will not touch on the issue of frequency correction of musical signals when they are recorded in studios, since only experienced sound engineers master this art, and this is not a technical task. The modular console will most often be used on "live" speech transmissions, and the music will be played from phonograms already processed by sound engineers. The conditions under which musical ensembles work in schools or discos are far from studio and are unlikely to produce very high sound quality. Based on these considerations, it is useful to compare the use of various frequency correctors in a modular console.

The simplest tone controls for high and low frequencies installed in the input lines allow you to get an acceptable and approximately similar sound from all microphones and sound sources. But often this is not enough at all. For example, to improve speech intelligibility, a "speech-optimized" filter is often applied. The filter recommended for use in broadcasting broadcasting studios has a rise in the frequency response around 5 kHz by about 6 dB and its roll-off below 100 Hz and above 6 kHz. After processing with this filter, speech becomes more intelligible at higher noise levels, but sounds "drier", less natural.

Obviously, simple tone controls cannot provide the desired frequency response. There are other tasks that require the use of more complex frequency correction. For example, in a room where speakers are installed that can provide very high sound quality, due to the addition of direct and reflected sounds from the walls, standing waves are formed that completely change the picture of the sound of one of the speakers. People with good hearing hear it, but have no way to correct the situation. Or the trouble of sound amplification - the appearance of an acoustic "tie". You can lower the volume, but then what's the point in such amplification?

In these and many other cases, frequency correction is required in a very narrow band of the spectrum in order not to change the overall timbre of the sound. It is believed that the operation of a narrow-band notch filter, which "cuts out" only 7 octaves, is completely imperceptible to the ear, and the loss of 1/5 octave is also imperceptible on speech signals. Almost everywhere it is permissible to use a one-third octave filter. In professional equipment, multiband frequency equalizers are used. But a one-third-octave equalizer with 30 bands of adjustment is not so much complex as a very bulky structure and it is impossible to build it into a modular console.

Therefore, let's focus on the parametric equalizer, which differs from the graphic equalizer in that the center frequency in the correction band and the quality factor of each link can be adjusted over a wide range. It turns out that a parametric equalizer with two bands has about the same capabilities as a five-eight-band graphic one, and if you take four bands, then in most cases a third-octave graphic one can also be replaced. Typically, the adjustable frequency bands are chosen so that they overlap each other. It becomes possible to amplify, for example, low frequencies with a wide bandwidth with one link and “cut out” an amplified background with a frequency of 100 Hz with a narrow bandwidth of another link, or raise high frequencies and cut out high-frequency components at 8 kHz of “whistling” speech sounds (s, ts, h).

Each filter element in a two-band equalizer (its circuit in Fig. 1) is assembled on a quad op-amp of the TL084 type (TL074, 1401UD4). The frequency correction ranges are completely determined by the choice of the values ​​of the capacitors C5, C11 and C13, C14. With the indicated capacitance values, the tuning intervals of the center frequency are not less than 0,1 ... 2,5 kHz for the "lower" range, and not less than 0,2 ... 5 kHz for the "upper" range. These values ​​are chosen with the calculation of the overlap of the frequency range occupied by the speech signal, as well as the frequency range at which acoustic feedback usually occurs.

(click to enlarge)

When choosing a different correction range, you need to proportionally change the values ​​​​of the capacitors. With the capacitance of capacitors C5 and C11 equal to 1200 pF and 1,5 μF, respectively, the frequency range of the filter adjustment is 40 ... 1200 Hz, and with the capacitance of capacitors C13 and C14 equal to 39 pF and 0,039 μF, respectively, for another filter, - 1,2 ,15...XNUMX kHz.

The bandwidth of each filter can be changed from 0,25 to 3,6 octaves. The gain at the center frequency of the filter can be changed within ±16 dB. When correcting, you should not get involved in lifting, given the limited margin for overload due to the low voltage supply to the console. Toggle switch SA1 includes an equalizer; connector XS1 - additional line input of the equalizer for connecting an external signal source.

The nominal input and output voltage of the module is 0,25 V, the current consumption does not exceed 15 mA, the non-linear distortion Kr<0,07%.

A two-band parametric equalizer can be placed in the module. True, it must be borne in mind that it will not be able to correct the stereo signal, but, for example, to equalize the sound of two speakers in a room or eliminate the tie from one microphone, this is not required. The descriptions of the circuits of such equalizers have been given repeatedly [1-3].

The appearance of the module is shown in the photograph of the mixing console on the cover of "Radio", 2003, No. 2. In fig. 2 shows the PCB drawing of the equalizer. Capacitor C7 is mounted on the side of the printed conductors in the immediate vicinity of the pins of the DA1 chip. The SA1 toggle switch to turn on the equalizer and the additional line input XS1 (tulip connector) are installed on the top panel of the module.

On fig. 3 shows a drawing of the front panel of the unit. The article [4] describes the design of modules and their installation.

When operating the module, troubles are possible only because of the poor quality of variable resistors of the SPZ-33-23 type, the resistance of which in extreme positions is not always brought to "zero". Sometimes it cannot be set below 6 ... 9 kOhm (!), which, of course, reduces the upper limit of the adjustment range adjustment. Since this is quite common, you should check the minimum set resistance of these resistors before installation.

Instead of the op-amps indicated on the diagram, it is also permissible to use LM324 (the domestic analogue is K1401UD2, but with a different pinout); however, non-linear distortion and noise at the output of the module will increase. If necessary, you can do without the DA1.1 and DA2.2 op-amps, since they simply simplify switching and do not participate in the operation of the equalizer itself.

If the equalizer is used as an independent device, a jumper must be installed instead of the resistor R34; It is convenient to use an AC adapter as a power supply. In this case, to increase the overload capacity of the device, the supply voltage of the device can be significantly increased, since most op-amps can operate at a voltage of ±15 V.

When the module is integrated into the console, the DA3 stabilizer and the VD1 diode are not needed, since the protection against reverse polarity of the supply voltage and the voltage stabilizer are already in the MONITOR module. Instead of elements DA3 and VD1, jumpers are installed, and the capacitance of capacitors C20, C21 can be reduced to 0,1 - 0,33 uF. Resistor R34 is needed only when the equalizer is turned on before the MASTER output line, at the input of which there is an adder.

Literature

1. Shkritek P. Reference guide to sound circuitry. - M.: Mir, 1991.
2. Ataev D. I., Bolotnikov V. A. Functional units of high-quality sound reproduction amplifiers. - M.: Radio and communication, 1989.
3. Ataev D. I., Bolotnikov V. A. Practical schemes for high-quality sound reproduction. - M.: Radio and communication, 1986.
4. Kuznetsov E. Amateur mixing console. - Radio, 2003, No. 2, p. 12-15; No. 3, p. 10-12.

Author: E. Kuznetsov, Moscow

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