Aurora Curtain

What this section does

Aurora Curtain is the last filter in the chain. It sits after the distortion — after the hard clipper and after the fuzz — and its job is to decide how much of what those stages produced actually leaves the pedal. Clipping generates harmonics that were never in the instrument, and the higher ones are where a drive pedal gets its fizz and its harshness. A low-pass filter placed here removes them at the source, after they have been created and before they reach the amplifier.

This is the mirror image of Wall of Ice at the front of the pedal. That filter decides what the distortion is fed; this one decides what the distortion is allowed to say. Between them, the drive stages work on a signal that has been shaped at both ends.

Why filter after distortion

Clipping creates harmonics. A low-pass filter after the clippers changes how much of that high-frequency content reaches the next device. It makes the chosen distortion darker without turning the drive down.

Aurora Curtain combines two second-order sections on one control. Their corners increasingly separate as the filter is turned down, spreading the transition from passband to the four-pole stopband. This is the useful distinction of the design; it does not require a claim of perfect transient preservation.

The topology

The filter is a fourth-order Sallen-Key low-pass built as two cascaded two-pole sections, each with its own amplifier, sweeping together from a single four-gang potentiometer. All four capacitors are 330 pF Class 1 (C0G) ceramic; each section’s two resistive legs are a fixed 4.7 kΩ in series with one gang of a 200 kΩ reverse-log potentiometer.

Each section has gain 1 + 2.7 kΩ / 10 kΩ = 1.27. For the equal-component Sallen-Key model this gives Q ≈ 0.578, close to a second-order Bessel alignment. Cascading two such sections, especially with different corner frequencies, is not a conventional fourth-order Bessel alignment. A flat group delay or unchanged note attack is not specified for the combined circuit.

The two sections add 1.27 × 1.27, about +4.2 dB. On the bench-tuned board, input-trim resistor R17 is 43 kΩ (previously 33 kΩ), against 68 kΩ. That divider offsets the filter gain for approximately unity overall gain. The section Q, 43 kΩ gang-limiting resistors and OPA4191 are retained. Response plots are normalized to the passband.

The closed corner and presence shelf

The two second-order sections share a four-gang 200 kΩ control. The second section’s limiting resistors spread the transition as the filter closes; attenuation ultimately approaches a four-pole roll-off.

The November production bench brief places the closed corner around 3 kHz. That corner is matched to the drive’s U3A presence shelf, rather than using the older model’s approximately 2 kHz result as the current specification.

Across the knob

The 200 kΩ control has useful travel across its range. With Aurora Curtain open, the drive response includes about +4 dB presence above 3 kHz. Closed, the matched shelf and low-pass restore a broadly flat balance below the roll-off.

The supplied description places the open and closed curves together below 1 kHz; the closed response is flat within about 1 dB from 200 Hz to 3 kHz, then rolls off. A measured numerical closed-Curtain sweep is needed before plotting that curve against the supplied open response.

See the production drive sweep and shelf component values.

What it addresses, and what it does not

The open setting leaves the upper harmonics most available. The production drive sweep reaches about +5 dB relative to 200 Hz by 20 kHz, including the presence shelf and voicing. This amplitude result does not specify whole-pedal group delay or transient preservation.

What it does not do, at any setting, is remove distortion. It removes the high harmonics distortion produces. A fuzz set to collapse will still collapse; a hard-clipped signal will still be hard-clipped. What changes is whether the top two octaves of that clipping reach the amplifier. Nor does it affect the low end, which belongs to Wall of Ice at the other end of the pedal — the two controls do not overlap and do not fight.

Because Aurora Curtain is after the drive, turning it down does not reduce the gain that produced the clipping. It changes the spectral balance of the already distorted signal.

Specifications

Components

• OPA4191 quad amplifier, operating on the ±15 V rails. Its input range, stage gain and output swing all contribute to the filter’s clean headroom; the op-amp choice alone does not specify the complete filter’s distortion or noise.

• 330 pF C0G capacitors are used for their low voltage dependence and small temperature coefficient. Component tolerance and tracking between the pot gangs still affect the filter response.

• 200 kΩ reverse-log 4-gang — all four filter legs on one shaft; the C-taper distributes frequency evenly across the rotation.

• 1 % metal-film resistors — the section’s Q depends on resistor matching more than on absolute value.

Serviceability

The board takes +15 V and −15 V from the Ćuk-Linear Dual Reactor into its own local filtering. Faults are handled on the power board, where the parts designed to give way first are inexpensive and replaceable.

Notes

The current production closed-corner reference is approximately 3 kHz. The two-section circuit description is more precise than assuming a conventional fourth-order Bessel pole alignment or specifying flat group delay.