Equalizers - for correction apparatus amplitude-frequency characteristics (frequency response) reproducing complexes. With their help, you can adjust the tonal character of phonograms, to correct their shortcomings, as well as to achieve optimum sound speakers for a particular room.
The paper proposes a diagram and description of seven-band graphic equalizer with active filters.
Each of the filters individually represents a "unified" module, all the modules are identical in circuit and its components, which greatly simplifies fabrication of the device.
The input sensitivity of the circuit - 250 mV, the depth adjustment of each frequency band of +/- 15 dB. Active filters with operational amplifiers (F1, F2... F7) are included in the feedback op-amp OP2 circuit and tuned on the frequency: 40 Hz, 100 Hz, 270 Hz, 700 Hz, 2 kHz, 5 kHz, 12.5 kHz.
The operational amplifier OP1 serves as a normalizing (matching) of the input stage.
tuning frequency (bandwidth) of each of the filter elements it is determined RC circuits. To work correctly, the equalizer as a whole, "smoothness" adjust the frequency response line, it is very important to choose the denomination frequency control elements as accurately as possible.
As seen from the filter circuit (circuit in the blue square) resistors R2 and R8 have the same resistance (1.5 ohm) in each module.
Changing only nominal values of capacitors C1 and C2. Capacity of these capacitors should be matched with an accuracy of 1... 3%. For their selection can be applied high-precision measuring containers and use the method of parallel or serial connection of capacitors.
As is known, capacitance in parallel connection of capacitors are summed, and sequential calculated by the formula: C = C1 * C2 / (C1 + C2).
For convenience, selection table, which shows the possible options (combinations) including filter capacitors F1... F7 to obtain the necessary capacities values.
In the embodiments shown therein the parallel connection of capacitors (with the sign "+") and consistent (the sign "-"). For example, for F1, capacitance filter capacitor C2 will be equal to: 22h10 / 22 + 10 = 220/32 = 6.875 uF (according to the above given formula).
All capacitors in the circuit it is desirable to apply high quality, with good stability parameters (in capacitance and temperature). In the case of sequential electrolytic inclusion (oxide) capacitors, they must include the counter (positive to positive or negative to negative).
Operational amplifier (op amp) may apply any suitable in the parameters to be used in high-end audio equipment. Here, the focus should be low-noise and high slew rate value of the input voltage. Power to be fed from the op-amp stabilized, a bipolar power supply. The voltage and current of the power supply depends on the particular type Applied on chips.
All resistors in the circuit type MLT-0.125 (0.25). Variable resistors for frequency bands of slide knobs is better to take, with a linear characteristic, for good visibility of frequency response form by mutual arrangement of their "engines" on the front panel.
The original scheme, and a more detailed description can be found in the magazine "Radio", 1991, №12, p.63