Here’s a simple 2 band Equalizer circuit for your bass guitar. Can be powered by 9V battery.
About the circuit
The circuit it made up with 2 opamps from TL072 IC. You can use OPA2272 for better performance. The volume control is not included on the circuit but it is connected after the pickup. VR1 is a trimmer resistor meaning it is only for gain trimming which is adjusted once and forget it. you can replace it with a fixed resistor once you determine the best value.
Simulation Result
Frequency response graph of bass and treble at 0%-100% , 10% increment.
