Here is a simple 3 band graphic equalizer circuit made of a single op amp IC LF351 (IC1) and few passive components.The component values of this circuit are not very critical and can be replaced with nearest values with a little loss on the performance.This feature make it easy to be assembled from your junk box.
The opamp LF351 is wired to operate in three frequency ranges- high,medium and low.The circuit is designed such that the circuit produces +/-20 dB boost or attenuation for 50Hz,1KHz and 10Khz by varying POT’s R3, R6 and R9.The maximum amplification for any of these bands at maximum supply voltage is 20dB.The opamp LF 351 is wired as an inverting amplifier whose response to frequencies 50Hz,1Khz and 10KHz can be varied by adjusting POT’s R3,R6 and R9.
Circuit diagram with Parts list.
- The circuit can be operated from 6 to 24 V DC .What I recommend is 12V .
- Use a 12V battery or 12 V DC power supply for the circuit.
- All capacitors must be rated 15V.
- A 10uF/15V capacitor must be connected between +ve and -ve pins of LF351 if the power supply is far from the circuit.Not shown in circuit.