Epoch Modular Benjolin Chaos Eurorack Module

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Analogue chaos in Eurorack format

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Manufacturer: Epoch Modular

Category: Keyboards & Synths



The benjolin is a multifunction synthesizer designed by Rob Hordijk. The

module consists of four separate function blocks: two VCOs, a state variable

filter and an additional circuit, invented by Hordijk himself, called a rungler. This

particular arrangement emerged from Hordijk’s efforts to design a synthesizer

that was, as he puts it, "bent by design". As such, the module functions

according to principles of chaos theory, where short to long sputtering patterns

spontaneously transform themselves, at times, gradually, at others, quite

suddenly, morphing into new pattern doublings and bifurcations.

The chaotic character of the benjolin is anchored in Hordijk’s rungler circuit. In

essence, the rungler is an 8­step shift register that takes its data input from one

oscillator (OSC A) and its clock input from the other (OSC B). The output of the

shift register is then fed into a primitive, digital­to­analogue converter, creating a

stepped voltage pattern of variable length and depth, what Hordijk terms a

“stepped havoc wave”. This rungler wave is then wired back into the oscillators,

via the attenuator knobs (Run A and Run B), creating the complex interference

system that lends the benjolin its unique, aleatoric character.

The inclusion of a voltage­controlled filter expands these possibilities further into

the audio realm. Its design includes Hordijk’s signature, Z­plane modulation

circuitry, which produces an all­harmonic distortion. This gives the filter the kind

of warm, organic character, reminiscent of classic tube amplifiers. This is most

palpable when the resonance of the filter is turned to the maximum, with both

oscillators running in the audio range.

The filter’s input is a mix of a PWM wave (derived from the triangle waves of

each oscillator) and the rungler signal itself. And as with each of the oscillators,

a rungler attenuator (RUN F) allows modulation of the filter as well.

To round things out, CV inputs for both the oscillators and filter are normalized,

each via their respective attenuators. This allows for cross modulation between

the oscillators and a “sweeping” effect for the filter, hitched to Triangle B.


Oscillator Manual Controls: Two knobs at the top center of the panel (OSC A

and OSC B) are the manual controls for each oscillator. They determine the

central rate/frequency/pitch of each respectively. Both oscillators cover around

18 octaves, spanning From LFO into VCO range.

Filter Cutoff Manual Control: Third knob down at center (FREQ), dictates

central cutoff frequency of filter. As is the case with SVFs in general, maximum

cutoff for High Pass and Low Pass are located at either end of the knob’s

rotation. Both have 12db curves, while the band pass is 6db. Like the

oscillators, the cutoff frequency range is around 18 octaves.

Filter Resonance Control: Fourth knob down, to the right (RES). This knob

controls the amount of resonance or damping of the filter. It is designed to have

a slight anti­logarithmic curve. In addition, you will notice a significant increase

in the loudness when resonance approaches the max setting. This is due to the

interplay between the filters resonance and all­harmonic distortion.

External Modulation Controls: Three knobs to the left of the panel (MOD A,

MOD B and MOD F) are basic attenuators for their corresponding CV input.

Without CV inputs, they control the level of internal modulation via their

normalized inputs. Again these are: Triangle A to MOD B, Triangle B to MOD A

and Triangle B to MOD F.

Rungler Modulation Controls: Three knobs to the right of the panel (RUN A,

RUN B and RUN F) are standard attenuators that interface the Rungler with the

three other function blocs in the circuit.

Rungler Loop Controls: Both the offset knob (LOOP) and the toggle Switch, to

its right, serve to augment the looping function of the rungler. When the rungler

enters loop mode, the data contents of the shift register are recirculated, thus it

no longer accepts new data from Oscillator A. This effectively, removes the

benjolin from the chaotic domain, cutting pieces out of the rungler pattern, and

using them to feed repetitive drones, beat­like patterns and so on.

Additional Info

Additional Info


Modulation CVs: Three top most CV inputs, corresponding to MOD A, MOD B

and MOD F attenuators. Each of these approximates a 1V/O curve, however

the range for accurate tracking is limited to around two to three octaves. Fine

for a sequencer, or simple quantizer, but not ideal for tracking a keyboard.

Rungler Loop CV: Input allows for triggering rungler loop mode externally via

CV. Nominal threshold, at +.7V


Pulse A and B: Average ­/+ 8V or 16V P/P.

Triangle A and B: Average ­/+ 4.5V or 9V P/P.

XOR: Logic output from rungler. Averages ­/+ 5.5V or 11V P/P.

Rungler: Averages ­/+ 5.5V or 11V P/P.

PWM: Averages ­/+ 8V or 16V P/P.

Filter Outs: Filter outputs are around line level (approx. ­/+ 1.5V or 3V P/P)

Module Width: 18hp

Current draw: ~45 mA@+12V

~45 mA@­12V