The J Retro 01 Bass Preamp is designed to fit directly into standard "Jazz" style basses without modification, including a PP3 battery. Its high quality circuit uses three stacked knobs and includes an active blend circuit plus a 3 band equaliser with bass, variable frequency mid, treble and bright function. A very gentle built in contour gives immediate colour without detracting from the basic sound of any bass. The active blend circuit prevents the loss associated with many passive basses where, when both pickups are up full, the signal from one pickup feeds into the other and vice versa. However, a switch is included to allow a passive equal mix of both pickups. A second switch - basic "get-you-out-of-trouble" passive mode is included in case the battery dies in crucial circumstances, but this mode allows lots of useful settings too. Easy to fit, no routing or modification on most basses. A standard PP3 battery fits under the plate in the standard cavity in most J-style basses.