This is the diagram od electro-harmonix graphic equalizer. You can specify the number of channels according to your needs. You just need to parallel the components: C1, C2, R1, an om-amp, Potensiometer and a 470 ohm resistor. The frequency to be boost decided by C1, C2 and R1. See the diagram for the C1, C2 and R1 combination versus the frequency.
This is the diagram of 5 band graphic equaliser circuit. The circuit is based on operational amplifier, the NE5532 or LM833 is the right choice for low cost op-amp chip with fairly good quality output. Each IC contains dual op-amps circuit, so you will need 4 ICs of NE5532 or LM833 to build this 5 band graphic equaliser.
The following diagram is the four band equalizer circuit. Equalizer is generally used to improve the fidelity of sound, to emphasize certain instruments, to remove undesired noises, or to create completely new and different timbres. The equalizer uses four pieces of variable resistor to cut and boost four range of audio frequency, that are:
The following diagram is the circuit diagram of 20 band stereo graphic equaliser which will controlling the audio signal in specific frequency range. This circuit must be connected before the amplifier circuit. For maximum performance, you may use high quality electronic component such as metalfilm resistor, MKM capacitor (nonpolar), tantalum capacitor (bipolar). You may replace the power supply module with regulated and stabilized power supply or just add IC LM7815 [...]