Filter-Based Preamp for Bass Guitars
A modern design paying tribute to the original Alembic filter
Having owned a couple of Alembic basses, I've always felt there was something special about their electronics. Rather than using a passive tone control or a standard active 2-or 3-band EQ, they have this 12dB-per-octave low pass filter with a widely adjustable cut-off frequency and a resonant peak just before the cut. It can go from the darkest dub or reggae tones to the brightest slap and funk and anything in between with just one knob that has musically useful tones in every position. Synth players use resonant filters a lot, but most bassists are not familiar with them, since they only seem to come preinstalled on a few really expensive boutique basses.
I eventually sold my Alembics, but not before I took an original filter apart and reverse-engineered it.
At first I built an exact clone, which worked great, but then I saw some room for improvements. I optimized the op amp biasing scheme, more than tripling the battery life (it only draws 0.3mA!). I also made some changes to make sure it would work with virtually any bass, or in a typical stomp box (the Alembic original uses an unusual grounding scheme that’s incompatible with the way most basses and pedals are wired). This led to the first Distiller revision:
The second revision adds Alembic-compatible 2-and 3-pin headers (Molex KK 254 series). This makes it super easy to integrate with Alembic pickups/basses without any soldering. Oversized soldering pads are still present on the back of the board, for those (like myself) who prefer to solder everything.
Another REV2 addition is a built-in output summing resistor. This allows you to combine two Distillers by simply connecting their outputs in parallel (via a pickup selector or blend pot).
The third revision (purple board) adds support for two different potentiometer sizes on the same board. This was necessary because I could no longer source the original, grey potentiomenter, so I found a smaller one that's more readily available. When equipped with the small green pot, I call this "Distiller SE" (Special Edition). The larger, grey pot can still be fitted if it becomes available again.
Distiller REV3 (SE)
Distillers flying in formation. I think they're migrating southward to escape the Canadian cold.
Input impedance 1 MΩ
Output impedance 2.7kΩ
Supply voltage 9V or 18V
Current draw 0.3mA
Battery life (alkaline) 1200 hours
Noise floor -99dBu
Dynamic range 108dB
Cutoff freq. range 250Hz - 8kHz
Resonant peak 0 / 5 / 9dB
Gain trim range +3 to +12dB
Board size 21x34mm