The VC F3DB is a Voltage Controlled Fixed Filter Feedback Distortion Bank. The simplest description of the module is that it’s an analog six band vc fixed filter bank. But with the additions of so much gain, clipping, feedback, and self-oscillation it becomes a very flexible and a unique gestural sound sculpting tool capable of; graphic equalization over 6 octaves, multi-band distortion, spectral processing, crude vocoding, multi-frequency envelope following, self-modulation, and even as an instrument on its own. With no input it can be used as a pseudo chaotic “harmonic” feedback oscillator!
The filter frequencies have been specifically specifically chosen to emphasize more ambiguous and dissonant intervals. Although the frequency bands are spread out over many octaves, Zlob have purposely tried to leave out “3rds” besides a major 10th at the top frequencies to even out all the dissonance below. But the frequencies aren’t exactly equal tempered, so it’s relative. The bands roughly equate to F2, B3, F#5, C6, F7, A8.
The design is inspired by the 914 filterbank, which it borrows the frequency bands from. But this circuit uses active filters with opamps inspired by YU Synth, as opposed to passive cells using inductors on the original model. The 88hz band is a low pass -24db/oct filter and 7k is a high pass -24db/oct, the rest are -12db/oct band pass filters all with a Q around 4.
The hard and soft clipping circuit on the input has a lot of gain and can even boost line level signals. What makes it unique is the feedback on the clipping circuit which is turned on with the GFB(gain feedback) switch. The amount can be controlled by the Gain FB pot which can add some fuzz/bite to the signal depending on which clipping switch is engaged. At the extremes it will self oscillate from audio rate to LFO ‘clicking’ range. All the clipping stages in the module are using silicon diodes.
Consult the Signal Flow Chart below for a rough visual breakdown of the controls,in/outs,normalizations, and optional settings for the module.
Each band can also be muted by the top toggle switches or clipped(at the VCA) by the top slide switch which makes this design a bit different than other filter banks. As well as the overall feedback section(bottom middle) takes the sum/all bands out through a VCA(controlled by the feedback pot and FB CV) back to the input gain/clip circuit. and you can also tap into this using different sends/bands to the FB In jack to break the normalization. With this extra feedback “resonance” you can get more whistley and howly filter sounds kinda like the Serge res eq, which can also go into self oscillation.
All cv ins are expecting +5v to open the VCAs(they wont respond to negative voltage), once you get closer to +8v or so it can clip the vca regardless of the gain settings or clip switches. Output amplitude in high gain settings will exceed 10vpp and can get up to 20vpp. The envelope followers tend to stay around 0-5v out, but with clip switch active they can go up to 10v.
Since this is a high gain module there can be bleed between the bands in certain configurations and situations. Also in high gain modes with higher frequencies the 2.8k and 7k bands can bleed a little with the VCAs closed.
This is a highly involved DIY all through hole project. It is a long build with 5 different pcbs to solder and assemble.
F3DB – Build Guides & BOM (scroll to bottom of page)
THIS IS NOT A BEGINNER OR INTERMEDIATE PROJECT. This module is a labor intensive build, since it is all through hole, soldered by hand, and has a lot more quality control and calibration/testing than other Zlob modules.
You must be very proficient at soldering and not prone to mistakes since the boards are tightly packed and traces are small, there is not much room for error. And of course you have to really understand how to troubleshoot on your own using the eagle files. Since it’s such a complicated module with so many variables – Zlob can only offer so much support through email.
- Width: 16HP
- Reverse Power Protected
- Current Consumption: +12v: 75mA, -12v: 75mA
- Depth: 45mm, 1.77″