TWIN PINKS

Filter / VCV Rack

UD12

A nonlinear two-pole state-variable filter
based on the Polivoks circuit.

01 / Origin

From the original circuit
to Rack.

Once you’ve heard a Polivoks filter pushed hard, you’re unlikely to mistake it for anything else. Turn up the resonance and the sound can hang on one harmonic, almost motionless, before suddenly breaking apart.

We set out to bring that sound to Rack. The work took us into the Polivoks circuit, through transistor-level simulations and measurements of an original filter.

UD12 takes its name from the K140UD12, the Soviet operational amplifier used in the Polivoks filter.

UD12 product render
02 / Character

Input level is part of the sound.

UD12’s character depends on what you feed it. GAIN cuts or boosts the input by up to 6 dB, letting you hear different responses at the same cutoff and resonance settings. Higher input levels push the filter into saturation.

At high resonance, the filter self-oscillates with an asymmetric waveform. With no audio input, it can be played as an oscillator, tracking musically across the lower and middle registers.

We also spent an unreasonable amount of time making the potentiometers scratchy.

03 / Modulation

Two CV inputs. Wide modulation range.

The two cutoff CV inputs add together. CV2 provides 1 V/oct tracking. CV1 has a bipolar attenuverter with up to ×3 depth, allowing you to scale or invert incoming modulation.

A 5 V envelope through CV1 spans five octaves at unity depth; higher depth can sweep the full cutoff range, depending on where CUTOFF is set.

The experimental Fold mode reflects cutoff back down when modulation exceeds the upper limit, offering another flavour of audio-rate FM.

04 / Processing

Choose your processing mode.

UD12 uses oversampling for more accurate behaviour at high frequencies.

Eco, Normal and High let you choose the internal processing rate and CPU cost. In our tests in Rack, CPU usage did not increase at higher project sample rates.

Eco192 kHz
Normal384 kHz DEFAULT
High768 kHz
05 / Audio

Listen.

Preview layout — audio recordings to be added.

01 — Gain and resonance

Input level and resonance, heard through LP and BP.

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02 — Self-oscillation

No audio input. The filter played across the lower and middle registers.

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03 — Audio-rate FM

CV1 modulation, comparing Soft and Fold.

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04 — Hardware / UD12

The same source at matched input levels.

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