River Creative Technology Crux and Nucleus Eurorack Modules

Two new eurorack modules from new eurorack maker River Creative Technology.

Crux Dual voltage controlled amplifier with discrete OTA core:

"dual VCA around discrete transistor OTA core

switchable linear or exponential VCA response

switchable between Audio (AC) and CV (DC) use

starts to pleasantly saturate beyond levels of 10Vpp (clips at 20Vpp)

envelope (unipolar) and modulation (bipolar) CV inputs

smart normalization of unused envelope inputs to allow for full-range bipolar modulation

Design overview

After overlooking VCAs for many years, I began to appreciate the huge range of functions they can serve up. They’re really everywhere, in simple loudness contouring circuits, tremolo effects, tone controls, filters, overdrives, mixers, amplitude modulators, panners, cross-faders, and CV performance controllers. I also learned that VCAs put their subtle signature on the overall sound of an instrument.

The VCAs in the Crux are built around newly designed operational transconductance amplifier (OTA) circuits, built from discrete transistors. If you need a super-accurate VCA, this is not for you. However, if you appreciate the warm and lively nature of discrete electronics, the Crux will serve you well.

A phrase often used among modular musicians is “you can never have enough VCAs”. Probably followed by “I need more rack space”. That’s why the Crux is a compact module, simple and straight, a short signal path, and no frills. As there are no frills, there’s little fancy text here. It can do what a VCA does. It can do it twice. It can do it linearly or exponentially, and does it on audio or CV. And that’s it!"

Nucleus 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)

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.
nucleus_mod

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