Mod Squad: Rob Hordijk Benjolin

Eurorack Format Modular Synth
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Above: The Benjolin’s panel design keeps cabling away from the knobs. It’s worth noting that each output has a different peak-to-peak voltage range: The filter outputs go from +2 to -2V, the triangle outs are ±4V, the rungler and XOR are ±5V, the pulses ±8V, and the PWM range is ±8.5V.

ROB HORDIJK made a name for himself with his mad programming skills on the Nord Modular G2 and his elegantly handcrafted hardware synths. Inspired by chaos theory, he designs instruments such as the Blippoo Box that let you explore unpredictability to a very high degree, using a minimum amount of circuitry.

One design that is popular among DIYers is the Benjolin, which Hordijk refers to as being “bent by design”—an allusion to the hardware hacking concept of circuit bending. In this instrument, Hordijk exploits “the interference patterns between two oscillators,” based on his imaginative use of cross-modulation. Sylvan Lee of Epoch Modular not only put Hordijk’s Benjolin behind a Eurorack panel, he added several features that greatly expand its usefulness.

Meet the Rungler The Benjolin begins with two analog oscillators that provide pulse and triangle signals at its output jacks. In addition, the triangle output of each oscillator is normaled to the modulation input of the opposing oscillator. External modulation inputs and attenuators are included.

The triangle waves are also fed into a comparator (oscillator A sets the frequency while oscillator B determines the pulse length) to create a pulse-width modulation signal, which has its own output. Furthermore, the PWM signal is summed with the output of the rungler (explained below) and sent into a resonant 2-pole filter that provides independent highpass, bandpass, and lowpass outputs. Oscillator B’s triangle wave is normaled to the modulation input controlling the cutoff frequency, with an external mod input and attenuator for both sources nearby.

What really defines the Benjolin, beyond all this cross-mod fun, is Hordijk’s famous rungler circuit. It starts with an 8-step shift register that accepts a serial pulse from oscillator A, which is treated as an XOR signal that is clocked by the pulse output of oscillator B: When oscillator A’s value is low and oscillator B’s signal is high, data in the shift register moves to the next stage.

The shift-register’s output goes through a 3-bit D/A converter, and the chaotic results are fed back into the modulation inputs of the oscillators and the filter; the Run A, Run B, and Run F knobs control the amount of rungler at each modulation input, respectively. Discrete outputs are provided for the rungler and XOR signals, which can be used elsewhere as audio sources, clocks, or CVs.

The rungler provides far more exciting and musical material than other entropy-generating modules because of the way Hordijk has integrated it into the Benjolin’s design. The slightest turn of a Run knob can send the module into an entirely new soundscape, with the rate of change happening glacially or at audio rates.

The Benjolin includes a very handy looping function that recirculates data in the shift register at a rate set by oscillator B. Looping also changes the waveshapes coming from the rungler and XOR outputs. A signal of 0.7V or greater at the loop CV input will begin the patterning. The associated toggle switch engages a bipolar-offset control that can be used on its own to create loops or to alter the bias of an incoming voltage. Lee suggests plugging a piezo into this jack, perhaps from an external percussion controller, to grab rungler patterns—very hip.

Source of Certainty It is important to note that, despite its chaotic capabilities, the Benjolin can also be used in more linear ways. For example, it’s great for creating drones, beats, and melodic patterns, as well as serving as a clock source, all with very predictable results.

Consequently, the Benjolin can supplement any modular system, thanks to its wide range of abilities. Yet, it’s very powerful on its own and capable of everything from glorpy burbling and slowly evolving textures to face ripping noise.