Swell as an Oscillator¶
Overview¶
At maximum resonance, Swell's filter enters self-oscillation — producing a sustained tone at the cutoff frequency without any input signal. This is a fully usable oscillator voice: pitch-stable and 1V/Oct trackable across 7 octaves.
The valve's role depends on the Valve switch position. In Post mode, the filter's self-oscillation output passes through the valve stage before reaching OUT — Bias, Mode, and Boost directly shape the oscillation waveform. In Pre mode with no input signal, the valve receives nothing and is idle; the oscillation output is unaffected by the valve stage. Pre mode gives a clean filter sine; Post mode gives valve-coloured character.
The FM input adds vibrato, frequency modulation, and feedback chaos in either position. Used this way, Swell is not just a filter — it is a standalone sound source.
Reaching Self-Oscillation¶
The Res. knob controls resonance. Self-oscillation begins when the resonance feedback exceeds the filter's loss — the threshold varies with module settings. Comp ON lowers the threshold slightly by maintaining output level at high Q.
| Control | Setting | Notes |
|---|---|---|
| Gain (knob) | 0 (fully CCW) | No input signal needed |
| Drive | 0 | — |
| Res. | MAX or near | Adjust to taste once oscillating |
| fm | Centre | Avoid unintended feedback affecting pitch |
| Comp | ON | Lowers threshold; maintains level through resonance |
| Valve | Pre | Recommended starting point — see Pre/Post below |
No input patching required. The filter self-oscillates from internal feedback; external audio in is not needed and can interfere with the oscillation stability if present.
1V/Oct Pitch Tracking¶
Swell tracks 1V/Oct in self-oscillation across 7 octaves (C1–C7) when correctly calibrated. The factory calibration procedure uses self-oscillation as the reference condition.
Conditions that degrade tracking:
- fm knob off-centre (no cable patched): The FM normalisation feeds the output back to the cutoff. Moving the fm pot off-centre shifts the base pitch and introduces a PWM effect on the waveform, but 1V/Oct interval tracking is maintained. For a stable, unmodified pitch reference keep fm at centre, or patch a cable into the fm jack to break the normalization entirely.
- Post mode: The valve sits within the FM feedback path in Post mode. Any saturation in the valve stage is fed back through the FM normalization loop, which can perturb pitch. Pre mode is the more stable choice for precise tracking.
- High Drive or Gain: Pushing the valve into heavy saturation in Pre mode can affect the filter's input operating point. Use minimal Gain and Drive when pitch accuracy is required.
Oscillation Character by Bias × Mode¶
Post mode oscillation¶
In Post mode, the four Bias × Mode combinations each produce a distinct oscillation character. The filter produces an approximately sinusoidal output at the cutoff frequency; the valve then shapes that waveform.
The Q Compensation mode plays a key role in both four modes creating further possibilities. When Q compensation is removed, the filter is less overdriven therefore producing a cleaner waveform. When Q compensation is added, a sharper and more aggressive waveform is produced overall thanks to the filter overdrive:
| Q Comp | Triode | Pentode | |
|---|---|---|---|
| COLD | ON | Square wave — heavily saturated sine wave | Square wave — heavily saturated sine wave |
| COLD | OFF | Highest gain level; slightly saturated 10Vpp sine wave. Adding extra gain shifts toward a square wave. | Clean sine — lower gain levels yield a clean 10Vpp sine wave. Adding Drive or Gain will saturate the waveform. |
| HOT | ON | Square wave — heavily saturated sine wave | Aggressive — asymmetric wavefolding on positive peaks, compressed negative. At maximum Drive (Drive=max, Boost=OFF, Gain switch=High) the waveform becomes a kinked asymmetric shape rich in harmonics; the LED turns red — intentional, not a fault. |
| HOT | OFF | Clean sine — 10Vpp output with minimal valve colouration. Adding Drive and Gain will saturate the waveform. | Less aggressive — asymmetric wavefolding on positive peaks (kinked folded hemicycle), compressed negative. At maximum Drive (Drive=max, Boost=ON, Gain switch=High) the waveform becomes a kinked asymmetric shape rich in harmonics, more rounded than with Q Comp ON. |
[Waveform diagrams to be added for each combination.]
Pre mode oscillation¶
In Pre mode, the oscillation output bypasses the valve so the gain and mode controls do not alter the oscillation. The only control that it does is the Q compensation. Without it, a 4Vpp clean sinewave is produced, whereas with Q comp, a slightly saturated 10Vpp sinewave is generated.
Pitch Control¶
Cutoff (FR knob)¶
Sets the base oscillation frequency. Range covers the full audio spectrum and beyond. The unit is calibrated to produce C1 when the FR knob is at minimum (fully CCW) with no signal on the 1V/Oct or fm inputs.
1V/Oct CV input¶
Tracks pitch across 7 octaves. Connect a keyboard, sequencer, or quantiser CV output for melodic use. Calibration verified across C1–C7.
FM input (vibrato, modulation, and fine tuning)¶
The fm attenuverter controls frequency modulation depth and polarity:
- Slow LFO → fm jack: Vibrato. Depth set by fm knob.
- Envelope → fm jack: Pitch envelope with built-in depth control. The fm attenuverter sets the envelope intensity directly — no external attenuator needed. Can be used instead of the 1V/Oct input for pitch modulation, or combined with it: 1V/Oct carries the keyboard/sequencer pitch while the fm input handles the envelope sweep independently. The two inputs sum at the filter cutoff.
- Audio rate → fm jack: FM synthesis tones. Valve stage adds harmonic complexity to the FM sidebands.
- fm knob with no cable (feedback): Self-modulation — see Audio FM Feedback below.
- DC offset CV → fm jack: Fine pitch adjustment. A precision DC offset patched into the fm jack, trimmed with the attenuverter, shifts the oscillation frequency independently of the FR knob and 1V/Oct input. Use this to tune Swell precisely against other oscillators or to add a fixed interval offset on top of 1V/Oct tracking.
Audio FM Feedback in Self-Oscillation¶
With no cable in the fm jack, the module's output normalizes back to the fm input, creating a feedback loop around the filter cutoff. In self-oscillation this loop is active at the oscillation frequency itself:
- Negative fm (CCW from centre): Inverted feedback suppresses the cutoff at signal peaks. This lowers the base pitch and introduces a PWM effect — the positive hemicycle widens relative to the negative. Waveform grows denser and more complex without becoming unstable. 1V/Oct interval tracking is maintained.
- Positive fm (CW from centre): Regenerative feedback raises the base pitch and narrows the positive hemicycle (PWM effect in the opposite direction). 1V/Oct interval tracking is maintained.
In self-oscillation, Swell remains stable across the full range of the fm attenuverter in both directions — no chaotic or runaway behaviour at the extremes.
Pre vs Post Valve Position¶
| Pre | Post | |
|---|---|---|
| Signal path | Valve → Filter (oscillates) → Out | Filter (oscillates) → Valve → Out |
| Valve role | Idle (no signal through valve with Gain=0) | Shapes the oscillation waveform directly |
| Output level | Comp OFF: 4Vpp clean sine. Comp ON: 10Vpp soft-clipped. | Waveform shaped by valve; Comp ON adds filter saturation (fuzzier); Comp OFF gives rounder, warmer valve shaping. See Post Oscillation mode section for details. |
| 1V/Oct stability | Better — valve outside signal and FM paths | More sensitive to valve saturation state |
| Character | Pure filter sine tone | Valve-coloured — from sine to square wave and waveshaping across Bias × Mode combinations |
Recommendation: Use Pre for stable, melodic oscillator use — the output is a clean sine at 4Vpp (Comp OFF) or a soft-clipped 10Vpp (Comp ON). Use Post when you want the valve to actively shape the oscillation timbre.
Patching Ideas¶
[Expand this section with tested patches.]
- Simple melodic voice: 1V/Oct from sequencer → 1V/Oct jack. Res. at self-oscillation threshold. VCA on OUT. Mode: Triode, Bias: HOT for clean tone.
- Percussive ping: Short envelope → fm jack (positive polarity, moderate depth). Res. near self-oscillation. Fast decay pulls pitch down on each trigger. [Need to test and refine]
- FM bass: Audio-rate oscillator → fm jack. Low fm depth for subtle FM character; increase for harmonic complexity. HOT + Pentode adds asymmetric distortion to sidebands. [Need to test and refine]
- Feedback drone: No cable in fm jack. Positive fm setting toward instability threshold. Bias: HOT, Mode: Pentode. Bias and Drive interact with the feedback to produce evolving, unpredictable tones. [Need to test and refine]