The core concept is elegant: a gate signal has a sharp transient attack followed by a sustain and release. By driving the QPAS’s radiate/resonance into self-oscillation, you can treat the filter itself as a pitched sound source, and then use the gate (often via an envelope) to excite it — like physically striking a tuned resonator.
The Core Technique: Filter Ping
The QPAS has a pair of bandpass filters in a stereo configuration with its unique Radiate parameter controlling the interaction between them. When you push Radiate high and increase Q (resonance), the filters approach and eventually enter self-oscillation — they ring at their cutoff frequencies like tuning forks.
A short trigger or gate sent to the audio input (or used to modulate the input level via a VCA) causes the filters to “ping” — they ring out at their tuned frequency and naturally decay. This is the same physics as hitting a bell or a drum: an impulse excites a resonant body, which then decays at its natural frequency.
Basic patch:
- Send a gate or trigger from your clock/sequencer into a fast envelope (e.g. Zadar or Maths)
- Run the envelope into a VCA, with white noise or the gate itself as the audio input
- Patch that VCA output into QPAS audio input
- Set QPAS resonance high (close to or at self-oscillation)
- The Radiate parameter controls the stereo character — low Radiate = both filters in phase (thicker, mono-ish), high Radiate = filters spread into stereo image
Drum Voices You Can Make
Kick drum: Tune QPAS low (60–100 Hz range), hit it with a short impulse, then modulate the cutoff frequency downward with a fast envelope (pitch drop is key for a convincing kick). The Zadar is great here — use one channel as the audio envelope, another for cutoff CV with a sharp initial peak that falls fast.
Tom / resonant thud: Slightly higher tuning, slower pitch envelope decay, moderate Q. The QPAS’s stereo bandpass character gives toms a rich spatial quality that mono filters don’t.
Hi-hat / snare-adjacent texture: Mix noise into the input, tune QPAS higher, use very short gate/envelope. Adjust Radiate to spread the stereo image — high Radiate creates a nice wash. Adding both cutoffs spread apart creates a more complex, metallic texture.
Clap / transient crack: Very short ping, fast envelope, mid-high frequency, high Q but not quite self-oscillating. The transient becomes the sound rather than the sustain.
Cowbell / metallic perc: This is where QPAS really shines. Because it has two filters, you can tune them to non-harmonically related frequencies (say, a minor 7th or minor 9th interval apart). The two pitches beating against each other create that characteristic metallic inharmonicity. Push Radiate to control how separated vs. blended they are.
Key Parameters to Modulate
The QPAS has dedicated CV inputs that make this really playable:
- ωA and ωB (the two cutoff frequencies) — Sweep these with a fast envelope for pitch drop/pitch bend. Patching both from the same envelope with slight offset between them creates movement.
- Radiate — Modulate with a slow LFO for evolving stereo width on repeating hits
- Q — Dynamic resonance control, can push into self-oscillation territory on peaks
- ΔFreq — Offsets the two cutoffs relative to each other; modulating this creates metallic shimmer