MATRIXSYNTH: Zlob Introduces the VC F3DB Fixed Filter Bank Eurorack Module

Saturday, July 31, 2021

Zlob Introduces the VC F3DB Fixed Filter Bank Eurorack Module

What is a Fixed Filter Bank? | Chaos + Control with Zlob VC F3DBvideo upload by Sarah Belle Reid

"In this video, we explore the history of fixed filter banks in synthesizers (what they are, how they work), demonstrate some classic fixed filter bank concepts, and then dive into a series of patch examples exploring the Zlob Modular VC F3DB.

Zlob Modular's VC F3DB is a filter bank for Eurorack modular synthesizers. But it's much more than just that as well; while it can produce the clean, dry, nasal tones you might expect from a classic Moog or Buchla filter bank, it is designed to leverage feedback, clipping, and per-band envelope followers, allowing it to act as a chaotic sound source, spectral decoder, and intense distortion.

To access extended audio from this video (aaand there's a lot of it!), check out my Patreon page! You're free to use these sounds as materials for you own music, as well:

Video Timestamps:
00:00 Intro + Video Overview
01:40 What's A Fixed Filter Bank?
03:36 What's a Filter Bank Good For?
07:42 F3DB Module Overview
09:53 Patch #1: Clipping
11:37 Patch #2: Adding Modulation
16:00 Patch #3: Feedback
19:48 Patch #4: Feedback (But Less Crazy This Time)
22:00 Patch #5: Feedback with Modulation
24:43 Patch #6: Spectral Decoder"
Zlob VC F3DB First Look video upload by Brett Naucke

"A first look at this fantastic new 6-Band Voltage Controlled Filterbank by Zlob Modular. Patch examples including manual filtering, drum processing, voltage controlled filter animation, 'vocoding' and some exploitations using the filterbank as a chaotic audio source. This is a GREAT filterbank capable of some very unique overdrive and distortion functions that set it apart from all other filterbanks in eurorack modular."

via Zlob Modular

"VC F3DB stands for Voltage Controlled Fixed Filter Feedback Distortion Bank. The simplest description is it’s a 16hp six band voltage controlled fixed filter bank. But with the additions of so much gain, clipping, feedback, and self-oscillation it becomes a very flexible and 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 were specifically chosen to emphasize more ambiguous and dissonant intervals. Although the frequency bands are spread out over many octaves I 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 based on the Moog 914 filterbank, which it borrows the frequency bands from. But this circuit uses active filters with opamps inspired by YU Synth opposed to passive cells using inductors on the original. The 88hz is a low pass -24db/oct 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 is fairly standard which appears in many different iterations of classic guitar pedals. But it has a lot of gain and can even boost line level signals. What makes it a bit unique is the feedback on the clipping circuit which needs to be turned on by the GFB(gain feedback) switch and 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 or at the extremes it will self oscillate from audio rate to LFO sort of clicking range. All the clipping stages in the module are using silicon diodes.

Consult the Signal Flow Chart to the left 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.


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