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 basic advantage of this preamp is that it uses special JFET based circuitry (instead of an operational amplifier, like many preamp manufacturers do) which behaves very similar to the tube.

Operational amplifiers, when overdriven (which happens at the beginning of nearly every tone), get into the saturation. They need some time to recover (recovery time) and the circuit doesn't work as expected during this short time. The result is that we lose the signal - the beginning of the tone. The tube (and this preamp) doesn't behave like this and when it is overdriven, it softly distorts the sound by the second harmonics, which is an octave and it doesn't matter. And the first signal after the peak is immediately played (no recovery time).

This picture shows how it works.

BP-4 distortion

Another question is tone controls. Many preamps use the Baxandall tone control. It works like this:

Baxandall filter curves

But, tone controls for the bass are different. Treble for bass is not the same frequency as treble for guitar, cymbal, voice or saxophone. It is similar with the bass control. OK, 50Hz - 100Hz boost, but everything under 40Hz should be cut. No speaker (and bass box) will play it and there will be unpleasant interferences with the frequencies from the bass drum (kick). My tone controls are designed "musically", not "technically", and it works :-)

The following picture shows how:

BP-4 curves