Skip to content

About Swell

Overview

Most filters shape your sound. Swell transforms it.

At its core, Swell is a 14-HP voltage-controlled 4-pole low pass filter (LPF). But where it diverges from every other filter in your rack is what sits in the signal path: a Soviet military-spec subminiature vacuum tube, originally designed for MiG fighter avionics and early orbital satellites. This isn't nostalgia or gimmickry—it's a deliberate marriage of vintage valve technology and modern synthesis architecture, engineered to give you a sonic palette that no solid-state circuit can replicate.

The valve doesn't just "colour" your sound. It actively reshapes it, generating harmonics, compressing dynamics, and folding waveforms in ways that evolve with every voltage change, every envelope, every modulation source you patch. And because Swell gives you unprecedented control over how the valve operates—switching between triode and pentode modes, shifting bias from clean to heavily saturated, and placing the valve before or after the filter—you're not locked into a single "tube sound."

You have access to a continuum: from transparent filtering with subtle warmth, through classic tube overdrive, all the way to extreme asymmetric wave-folding that turns simple waveforms into complex, evolving textures.

The low-noise 4-pole filter (24 dB/Oct) provides 1V/Oct tracking, temperature compensation, a unique character under resonance and the ability to compensate for output loss gain as well as different timbre with and without compensation.

The addition of audio frequency modulation with a dedicated attenuverter control enables the filter to enter into the uncharted territories by applying different levels of feedback into the filter frequency control.

This is a filter that responds. That breathes. That feels alive under your hands.

Swell front panel Swell — 14HP Eurorack voltage-controlled filter with integrated vacuum tube stage

What Makes Swell Different?

Swell will stop you in your tracks the first time you face it. Six toggle switches. Four independent gain stages — input level, drive, boost, and output gain — each placing saturation at a different point in the signal chain. Three switches governing how the valve itself operates: mode, bias, and placement before or after the filter. That is a lot of surface area for a filter.

It is deliberate. Every one of those controls exists because valve behaviour is fundamentally multidimensional. You are not just setting "how much" — you are deciding where non-linearity is applied to the signal, what character it takes, and how the filter then responds to it. The gain architecture determines whether saturation is gentle colouration at the grid or hard compression deep inside the valve. The valve controls determine whether you get even-order warmth, odd-order bite, soft compression, or asymmetric wavefolding. Pre or Post changes whether the filter shapes a complex saturated signal, or whether resonance peaks drive the valve dynamically as you sweep.

The result is not one "tube sound" — it is a palette. From transparent 4-pole filtering with barely a whisper of warmth, through classic overdrive, into extreme wavefolding territory that turns a simple waveform into a complex, evolving texture. And because each control shifts the valve's operating point, every modulation source, every envelope, every patch changes the character in real time. The sound possibilities are genuinely open-ended.

Once the logic behind each control clicks, the complexity becomes the point. You will not reach the edge of what this filter can do.

Dual Harmonic Modes: Triode and Pentode

Flip a switch and change the valve's internal configuration. Triode mode generates predominantly even-order harmonics—warm, musical intervals that stack in octaves and fifths, adding body and presence without obscuring pitch. Pentode mode emphasizes odd-order harmonics, delivering the aggressive bite and edge of classic guitar tube amplifiers. It can also enter into wavefolding territory when pushed hard. One valve, two distinct timbral characters.

Variable Bias: From Clean to Chaos

The Bias switch doesn't just add "more drive"—it fundamentally shifts the valve's operating point, and its effect is completely different depending on which Mode you are in.

In Triode mode, the valve produces a clean sound with minimal compression. COLD bias adds subtle warmth at signal extremes; HOT bias moves the operating point away from the cutoff non-linearity, giving the most linear response of all four combinations — open and transparent across the full signal swing.

In Pentode mode, the character escalates. COLD bias already compresses both hemicycles asymmetrically via the pentode kink region. HOT bias pushes further — the quiescent point enters the grid current region (A2 operation) at lower drive levels, folding the positive peak back via voltage drop across the grid resistor, while the negative hemicycle compresses hard toward cutoff. This is where the asymmetric wavefolding and "wild" character live. Some valves will exhibit this characteristic more than others — that is the nature of these vintage components.

Although each module is carefully calibrated by hand in our London labs, each valve exhibits subtle variation in this region due to manufacturing tolerances from decades ago, giving every Swell its own character. Like vintage analog synthesizers, no two modules sound identical—your Swell is unique.

Pre/Post Valve Placement

The Valve switch determines whether the valve saturates your signal before filtering (Pre mode) or after filtering (Post mode). This isn't a subtle difference—it's a fundamental change in behaviour:

  • Pre mode: The valve generates rich harmonic content, then the filter shapes it. Resonance bites into a complex spectrum, producing thick, evolving sweeps.
  • Post mode: Clean signal is filtered first, then the filtered output drives the valve. Resonance peaks can push the valve into saturation dynamically, creating timbral shifts that track the filter cutoff.

It's like having two entirely different filters, selectable on the fly.

Extensive Gain Staging: Control Where Saturation Happens

Swell doesn't just give you "a drive knob." It gives you independent control over gain at four points in the signal path: a Gain knob and Drive knob before the valve, a Boost switch reconfiguring the valve stage itself, and a Gain switch after the valve. This architecture means you decide where distortion occurs—early saturation at the grid, heavy compression within the valve, or final-stage clipping at the output. Different gain distributions produce radically different timbres.

FM with Feedback: Self-Modulation and Chaos

The fm input is normalized to the module's output, creating a deliberate feedback loop — no patch cable required. The "fm" attenuverter controls both the amount and polarity of the feedback, and polarity makes a fundamental difference to the character.

Negative settings feed back with inverted polarity — peaks suppress the cutoff, troughs open it, progressively enriching the waveform with upper harmonics without becoming unstable. Positive settings create a regenerative loop that intensifies resonant peaks at moderate levels, then destabilises into beating, pulsating behaviour at higher settings. See Filter Overview — FM Modulation for full detail.

Soft-Knee Saturation: No Harsh Clipping

Unlike solid-state clippers that hit a hard ceiling, the valve's transfer curve saturates gradually. Even when pushed into heavy distortion, Swell maintains musical coherence. There's no brittle, metallic edge—just smooth, evolving harmonic complexity that responds proportionally to input level, like an acoustic instrument reacting to playing intensity.

Saturation: Visualisation Feedback

The circular window above the valve carries a LED indicator: white when the output stage is saturating (brightness tracks level), and red in the specific HOT + Pentode condition where the grid driver saturates without the output stage simultaneously clipping — the module's most extreme operating zone. See Indicator LED for the full reference.

Low power consumption

Many of the existing Eurorack modules which use valves, use the wrong ones for the job consuming hundreds of mA to power hungry filaments. Swell, on the other hand will only draw 50mA on the +12V rail and 55mA on the −12V rail, making it a perfect fit for Eurorack compact cases.

A Valve That Lasts

Swell valves were built to live a lifetime. In the unlikely event the valve fails, Bartola will replace it for you. See Warranty terms for details.

Key Features

  • Voltage-controlled low pass filter, 4-pole (24 dB/Oct) design with variable CV-controlled resonance and temperature compensation.
  • Soviet military subminiature vacuum tube (1950s-era, designed for MiG avionics and satellites)
  • Dual operating modes: Triode (even harmonics) and Pentode (odd harmonics)
  • Variable bias: COLD (warm in Triode; compressed in Pentode) and HOT (most linear in Triode; asymmetric wavefolding and A2 operation in Pentode)
  • Pre/Post valve switching: Place valve before or after the filter for two distinct characters
  • Four-stage gain architecture: Gain knob, Drive knob, Boost switch, Gain switch
  • FM modulation with feedback: fm input normalized to output for self-modulation and chaos
  • Q compensation: Maintains consistent levels at high Q settings whilst providing different timbre options.
  • 1V/octave tracking: Precise cutoff frequency control across multiple octaves
  • Visual feedback: saturation indication depending on saturation and clipping activity.
  • Reverse polarity protection: Keyed power connector and MOSFET protection circuit
  • Power requirements: ±12V power, 50mA (+12V) / 55mA (-12V)
  • Output level: Eurorack-compatible, 10Vpp output, 1K impedance
  • Size: 14HP
  • Module depth: 30mm (including power connector)
  • Nominal Noise Floor: <-48dBV (Unweighted, 1Hz - 24kHz bandwidth)
  • Signal-to-Noise Ratio (SNR): 59dB (Relative to 10Vpp, unity gain)
  • Maximum Bandwidth (Unpatched): 16 kHz (-3dB) with Frequency knob fully open
  • Maximum Bandwidth (CV Driven): 24 kHz (-3dB) with +10V applied to 1V/Oct input

In Practice

Swell has a split personality — like Dr Jekyll and Mr Hyde. Drive her subtly and you get warmth, compression, and lovely timbres — the result of the asymmetric non-linearities introduced by the valve and filter circuit. Drive her hard and you get wild, chaotic responses: sometimes unpredictable, sometimes extreme. A full palette of timbres in between.

The instinct will be to push the gain immediately. Resist it. Start low and work outward — the range is far larger than it first appears.

Turn the Gain and Drive knobs to zero. Set Mode to Triode, Bias to COLD, Boost OFF, Gain switch to Low. Patch a simple sawtooth wave into the input. Sweep the filter cutoff.

You hear clean filtering with a hint of warmth—the valve's soft-knee compression adding subtle body without obvious distortion.

Now flip Bias to HOT. The filter opens up — HOT bias moves the valve away from its cutoff non-linearity, giving a cleaner, more transparent sweep. The same sawtooth sounds more defined, with sharper resonance peaks and less colouration. This is the most linear the valve gets.

Switch Mode to Pentode. Everything changes. Odd-order harmonics bite through the mix immediately — and now HOT bias has a completely different effect. The quiescent point enters grid current territory, clamping positive peaks and folding the waveform asymmetrically. Resonance sweeps snarl and spit. Push Boost ON and Gain switch to High to recover full output level and push deeper into saturation.

Flip Valve to Post mode. Suddenly the filter behaves differently. Clean signal is filtered first, then saturated. Resonance peaks drive the valve dynamically—the timbre shifts as you sweep, tracking the filter cutoff. Patch the fm output back into itself. Turn the fm knob past noon. Feedback takes over. The filter begins to howl, oscillate, fold back on itself in chaotic, unpredictable ways.

This is Swell. One module. Infinite trajectories.

Swell doesn't replace your other filters. It expands what filtering can be—turning a utilitarian function into an expressive, evolving instrument that responds to your touch, your patches, and your imagination.