According to marketing material issued in 1992 and 1993, the controls on the Climax Bass consist of “VOLUME, TREBLE, AND BASS” (aka PTB circuit) and 2 mini-toggles: “1) ACTIVE/PASSIVE” near the treble control and “2) PRE-AMP ON/OFF” near the bass control. For the L-1500 (Custom) included in the 1996 it only lists “ACTIVE/PASSIVE PREAMP ON/OFF” for the controls. Indeed, for both, the preamp on/off switch in the up position disengages the preamp. In the down position, the preamp kicks in with the sound increasing in volume and more oomph. Nothing out of the ordinary so far. With a name like “active/passive switch”, one would expect the preamp to be involved in one way or another too. However, even with its battery taken out, the switch near the treble control works just fine. This hints at it being a parallel/series switch instead, as confirmed by this Climax Bass/L-1500/L-1505 wiring block diagram available on the Basses by Leo website and included below for convenience. Given the measured DC-R values, this 1997 L-1500 Custom indeed has such a series/parallel switch but this 1993 Climax Bass has a single-coil/humbucker switch instead. The values for the different potentiometers and capacitors can also be gleaned from the diagram, with the usual 250kΩ Audio Taper pot for the treble cut in the PTB circuit for basses. The wiring harness of the L-1500/L-1505 is pictured in the Guitars by Leo Gallery. The passive/active wiring harness for both basses included below is the same as for the BBE-era L-series Bass v1.0 preamp using a LF441CN Low Power JFET Input op-amp instead of the LM4250CN as used before as well as v1.1 and later. Or does the use of a different op-amp merely have to do with these basses having only a single Magnetic Field Design pickup whereas all others have 2?