BP-4 bass preamp

Technical specifications:
• Powered by 9V battery
• Current consumption 0,95 mA
• Battery lifetime cca 1000 hours
• Gain +12dB
• Tone controls:
• Bass: +14dB/-4dB @55Hz
• Treble: +14dB/-12dB @6,5kHz

BP-4 bass preamp

The BP-4 bass preamp (the active electronics) was designed in 2001 (revision_A, now exists revision_L). The goal was to make an output impedance lower (more energy), the frequency controls adjust "musically", not technically.

In 2001, Stuart Spector and PJ Rubal visited my workshop and I demonstrated the circuit built on a breadboard. They were interested and Stuart wanted a sample...

Since 2003, these circuits are used in Euro Series and Legend Series Spector basses under the TonePump, respective TonePump Jr. brand names.


The following picture shows how it works.

BP-4 curves