Here is the circuit diagram and PCB design for relatively low cost DIY bass guitar preamp pedal uses FET K117 or equivalent. There is a bit difficult to build this kind circuit project, it’s require a good knowledge in electronics. This preamp has several features for the bass guitar instrument such as attenuator, low cut, mid control and Hi boost. The circuit has some control: volume level, high frequency, high-mid frequency, low-mid frequency, bass frequency. Boost level and master level control also included in this circuit design.
Power supply circuit for bass guitar preamp also included in the PCB design. Simply connect the power supply section to the output of transformer with the secondary 15 + 15 V / 500 mA.