The Rob Hordijk Epoch Modular Benjolin Dual VCO Eurorack Module

benjolin eurorack production module

Published on May 7, 2013 Epoch Modular·1 video
Note this is the first Epoch Modular post on MATRIXSYNTH.

"A darling of the DIY synth community for several years now, Rob Hordijk's benjolin circuit is really like no other synth/noisemaker out there. The benjolin is the smaller sibling of Rob's now legendary Blippoo Box, the subject of his 2009 article in Leonardo Music Journal (Vol. 9). Both the Blippoo Box and the benjolin emerged from Rob's attempts to design a circuit that was, as he puts it, "bent by design". As such both modules function according to the basic principles of Chaos theory, where short to long sputtering patterns spontaneously alter themselves over time, at times gradually and at times quite suddenly, morphing into new pattern doublings and bifurcations. The result is two incredibly unique instruments that function in a sense autonomously or can "play themselves" if you like.

Both the Blippoo Box and the benjolin are based around similar "chaotic cores," which in the the case of the benjolin, is comprised of two vcos and a unique circuit designed by Rob, which he calls a rungler. The rungler is basically an 8 step shift register that takes its serial input from the squarewave of one oscillator and its clock input from the other. The digital outputs of the shift register are than put through a 3 bit digital to analogue converter to create stepped voltage patterns, which are then wired back into the oscillators. The effect of this arrangement is to create a complex interference pattern that gives the benjolin its unique, aleatoric character.

Along with this chaotic core, the benjolin also includes a 2-pole vcf with a unique topology that imparts an amount of all harmonic distortion to the filter outputs. The eurorack module I have designed has additional modifications not included in the original benjolin circuit, including HP and LP outs for the filter, as well as a rungler loop function that can be controlled either via control voltage, manually with an offset knob and also with a toggle switch that turns the offset on and off. These different controls may also be used in tandem, and all serve in someway to hold the rungler in a looping pattern, allowing one to cut pieces out of the chaos and create repetitive drones and beat-like patterns at whim.

As you can see from the video, there are 4 cvs in total (to the left side of the panel, top to bottom): one for oscillator A, one for oscillator B, one for the Filter cut off Frequency and of course the rungler loop function already mentioned. The CV inputs of the two oscillators they are normalized for cross modulation so when nothing is plugged in the triangle output of oscillator A goes into the input of B and vice versa. Similarly, the vcf cv input is normalized to the triangle output of triangle B. As you may be able to see from the attached photo, these three normalized cvs each have attenuator knobs as well.

Outputs include the Pulse and triangle wave outputs from oscillators A and B, the direct output from the rungler, an XOR (which is a logic output from the rungler) as well as a PWM output derived from the Triangle waves A and B. And of course there are also the three filter outputs mentioned above.

This video aims at showing the benjolin solely as "self playing" instrument, and as such no external control voltages are utilized. In addition, for the sake of simplicity, only the low pass output of the filter is used. I apologize for the terrible quality of the video (shot on my Iphone). I have just ordered a camcorder, so better quality video will be uploaded soon."


image via Richard Devine on Facebook

"Epoch Modular - Benjolin - dual VCO based on Chaos theory 'Chaotic Cores'."