Riot ST VCF by Guru Gara Synth

The Riot ST VCF is a stereo low-pass filter module from Guru Gara Synth. It’s based on the original Riot VCF, carrying forward its bold sonic character while expanding it into stereo. At 12HP, it’s not the most compact filter, but it combines strong sound design potential with a unique visual identity: its black, white, and exposed copper PCB art panel makes it stand out in any rack.

Sound & Core Character

The Riot ST VCF uses the same OTA core as the original Riot, so it retains the thick low end and untamed resonance that defined its predecessor. Sonically, it has character: powerful, raw, and musical in ways that encourage exploration. In stereo, this energy gains extra dimension, as movements in frequency become movements across space.

Stereo Routing

The module handles both stereo and mono sources. For mono, you simply patch into the left input, and the signal is internally routed to both channels, creating a stereo output. With stereo sources, each input feeds its own channel for true left/right processing.

The Stereo Space Fader

The defining feature is the Stereo Space control.

• Center position: Both channels share the same cutoff frequency, effectively acting like a mono filter in stereo.

• Moving left or right: One channel’s cutoff increases while the other decreases by the same amount, spreading the sound across the stereo field.

This can be done by hand or with CV. Patching CV into the Stereo Space input turns the fader into a polarity/scale control. In the middle, incoming CV has no effect. Slide the fader right, and the CV will push the right channel’s cutoff up while pulling the left down. Slide left, and the effect is mirrored: the left rises while the right falls. This makes modulation feel dynamic, spatial, and alive in a way other stereo filters don’t quite capture.

Performance & Control

The large Stereo Space fader is a key part of the design. Unlike most filters that limit hands-on play to cutoff and resonance knobs, this adds a third gesture—controlling stereo divergence. In performance, this feels immediate and expressive, giving you more than just another filter sweep.

Use Cases

• Stereo VCOs: Works well, but stereo divergence can feel unbalanced with some stereo VCOs.

• Two Mono Voices: Arguably the sweet spot. Each voice can move against the other without creating oddities in the stereo image.

• Mono to Stereo: Duplicate a mono source across both channels and use Stereo Space to create wide, evolving textures.

Ergonomics

At 12HP, the module takes more space than it strictly needs, though the larger layout gives the fader and controls breathing room. For performers who value tactile control, the trade-off makes sense. For cramped racks, it may be harder to justify.

Pros

• Characterful OTA-based sound

• Stereo Space fader adds unique stereo motion

• CV control over Frequency, Resonance, and Stereo Space

• Flexible mono and stereo routing

• Hands-on, performance-friendly layout

Cons

• Larger HP footprint than necessary

• Stereo divergence with some stereo VCOs

• Limited to low-pass mode, with fewer options than some competitors

Riot ST VCF In Sum

The Riot ST VCF is a straightforward but characterful stereo filter. It doesn’t overwhelm with modes or features; instead, it focuses on sound, playability, and a clever Stereo Space design. If you’re building a stereo signal path and want something rawer and more immediate than some of the more polished options out there, this module is worth a look. It may not be the most HP-efficient choice, but it delivers bold sound, satisfying modulation, and a performance-friendly interface that makes it stand out.