Below we present an Audio Graphic Equalizer circuit, based on Philips Semiconductors Application Note 142 (published on October 1984). The circuit itself has great performance and uses a top performance operational amplifier; the NE5532.
The graphic-equalizer consists of an input buffer (IC1 -a), a variable-boost/ cut active filter (lC2-a), and an output summing amplifier (IC1-b). The IC1-a circuit is designed for unity gain and is used mainly for impedance-matching between the input source and the equalizer filters. The filter is a variable-bandpass or notching device, depending on the setting of the control potentiometer R2.
Any number of equalizer filter-stages can be used within the range of about 20 Hz to 20 kHz. However, the more stages you have, the easier it is to boost or cut a particular frequency without affecting the response at adjacent frequencies. All the filter stages use the same R-C feedback-network configuration, to provide a maximum of about 15-dB of boost or cut at Fo, the center frequency. The only differences in each stage are in the values of C1 and C2, which set the values of Fo. Table 1 lists the values for R1, R3, C1 and C2 for many center frequencies in the audio spectrum. Note that C1 is ten times as large as C2 and that the values for R1 and R3 are both related to the value of R2 by about a factor of 10. The center frequencies have been adjusted so that C1 and C2 are standard, off-the-shelf, values. We recommend using linear slide potentiometers for R2.
The value of R6 depends on the number of filter stages used. It insures that the gain across the equalizer is unity when all controls (R2's) are in the FLAT or 0 dB position.
The value of R6 is 100K divided by N, where N is the number of stages used. Note that only one audio channel is shown in the circuit schematic. In order to build a Stereo version of the above Audio Graphic Equalizer you'll need two of those circuits.
Table 1. Component Values