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Voltage controlled filter with discrete OTA core.


  • designed around discrete transistor OTA cores
  • switchable resonance character with optional ‘Solid bass’ behaviour.
  • switchable filter slope: 12 or 24dB/Oct
  • pleasantly saturating input stage and resonance with higher levels
  • linear and exponential FM options
  • envelope and modulation inputs with independent attenuvertors
  • soft-touch knobs and quality hardware (mostly Alps)

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Voltage controlled filter with discrete OTA core.

  • Eurorack format:
    • 8HP width
    • 3HE height
    • skiff friendly (32mm depth)
  • Power requirements:
    • 47mA @ +12V
    • 47mA @ -12V
    • (+5V not used)


Design overview

After discovering analog synths, modulars and the beautiful original and cloned analog filters of Moog, Korg, Roland, Sequential, and the likes I felt I wanted to try something a little different and see if I could come up with something that sounds unique and musical in its own right, rather than being a retake on proven circuits from the past. Furthermore, I was hoping to overcome dependency on (nearly) obsolete components, and benefit from the quality of modern components.




The core of the Nucleus contains a newly designed operational transconductance amplifier (OTA) circuit, built from discrete transistors. While there are technically superior alternatives, a discrete circuit excels in providing a warmer and more lively sound.

My wish was to make the sound of the Nucleus as versatile as possible. After studying many different filters to identify the key to their unique sound, I decided to include a filter slope switch (12 or 24 dB/oct), and a resonance character switch with three distinct modes (see box).

In each of these modes, the low-frequency resonance behaviour can be further tailored with the ‘Solid bass’ feature, cleaning and firming up the low-end, or leaving it to boldly resonate.

Resonance character:

  • Neutral – Classic resonance, reminiscent of vintage 4-pole designs. Inherently with a loss of low-end at high resonance.
  • Q-comp – Similar to neutral, but compensated for loss of low-end. Providing a firm and stable resonant sound, with slightly different saturation character.
  • K-style – The beefiest of all modes, with a thick mid range and fat low-end, and an aggressive resonant sound.

Concluding the design, I added a variety of modulation options for linear and exponential frequency modulation (FM), envelope modulation, and keyboard tracking (1V/Oct).

Additional information

Weight N/A
Dimensions N/A