An analog distortion built for every instrument you own. Guitar, bass and synthesizer pass from 2 Hz to 58 kHz untouched, until you decide what to take away.
Three clipping voicesLED drive, hard clipper and fuzz, alone or stacked
A studio compressorVCA glue and a two-threshold gate after the drive
Filters at both endsSteep, sweepable, and out of the way when open
An effects pedal, not a guitar pedal
Most pedals quietly assume a six-string: coupling capacitors that roll off below 40 or 50 Hz, tone stacks that thin the low end, inputs that load a pickup. Put a bass through one and it turns to mud; put a synthesizer through one and half the sound never arrives. The Malice Madrigal was designed from the first schematic to treat every signal the same.
Guitar
Your pickups keep their voice. A 4.7 MΩ input with only 100 pF across it leaves the pickup’s resonant peak where the pickup put it, so nothing is dulled before the circuit starts. The drive is flat through the bass and lifts gently through the presence region, so the pick attack comes out the other side of heavy gain intact.
Pull Wall of Ice toward the middle for tight, percussive chugs. Open it all the way for doom.
Start here: Wall of Ice around 10 o’clock, Aurora Curtain around 2 o’clock, guitar tone knob wide open.
Bass
This is what the full-frequency design is for. A five-string’s low B sits at 31 Hz; the pedal’s lowest corner is below 2 Hz. The drive amplifies the whole signal evenly from 4.3 Hz upward, so the fundamental is distorted rather than thinned away before distortion.
Bass drive lives or dies on its top end. Aurora Curtain lets the grind sit behind the note instead of on top of it.
Start here: Wall of Ice open, Aurora Curtain between 10 and 12 o’clock.
Synthesizer
Synths and line-level gear see an open door, with clean headroom for line levels and most keyboards. Wall of Ice decides how much sub reaches the drive, which is where much of the character lives. Sweep Aurora Curtain like your synth’s own filter: its gentle, even slope is deliberately close in character to a classic analog filter.
Running a modular at ±10 V? A little attenuation in front keeps the first stage clean.
Start here: turn the synth’s output all the way down, plug in, then bring it up slowly.
Coupling below 2 HzEvery capacitor in the signal path is sized for it.
Settles in about 50 msDeep low-frequency reach without a long recovery.
Clockwise means moreOn every knob, without exception.
Try it on anythingVocals through a preamp, drum machines, whole mixes. Tell us what you find.
Five stations, in the order your signal meets them
Wall of Ice decides what the distortion is fed. Aurora Curtain decides what the distortion is allowed to say. Everything between them is yours to stack.
Wall of Ice
Input stage and high-pass filter. Lower-left knob, beside the footswitch.
A line drawn across the low end. Everything above it passes exactly as it came in; everything below falls away at 24 dB per octave. Fully clockwise it sits beneath hearing. Turn it back and the sub-bass goes, then the boom, then the body, until only a tight, cutting core remains. Because it sits before the drive, the distortion keeps that balance however hard you push it.
Start low, open up. Begin around 10 o’clock and turn clockwise while you play. Stop just before the bottom end starts to crowd the notes above it. More distortion usually wants a little less; the fuzz wants more.
4th-order Butterworth, two Sallen-Key stages, no bump at the corner
Corner ≈ 12 Hz to 490 Hz, 24 dB per octave below it
4.7 MΩ input with 100 pF, low-noise JFET first stage at 5 nV/√Hz
+6 dB of clean gain applied at the quietest point in the chain
Twin Assassins
Dual notch filter, blended against the dry signal.
Two notches set into the midrange, where boxiness and blur live. The blend decides how deep they cut, from out of the way to a carved, scooped voice. If a driven sound is blurry or boxy rather than harsh, this is the knob that fixes it.
Set Wall of Ice first. A controlled low end makes the notches far more precise. The two are meant to be used as a pair.
Two notches, centred near 400 Hz and 950 Hz
Wet/dry blend sets the depth
Works in the middle, between the two sweep filters, without overlapping either
The drive
Distortion knob, with hard-clipper and fuzz mode switches.
Three voices, used alone or stacked. Stacked, they clip in sequence: each takes a portion of the waveform the last one left, and the harmonics of the one before stay audible underneath.
LED drive
A high-gain precision amplifier clipping softly into LEDs, three per side, symmetric by construction. Clean and high-headroom at low settings, rounding gradually as you turn up, with odd-order, amplifier-like harmonics.
Hard clipper
An LED and a silicon diode to ground on each side. Firmer, more compressed, more aggressive: the classic clipped-mids grind, with the glassiness taken off the edge.
Fuzz
Two transistors that collapse rather than clip, with the gated, torn character only a saturating pair produces. It carries its own bass cut near 150 Hz, so it wants more low end coming in, not less.
The voice doesn’t change as the gain does. The Distortion control scales all three gain legs together, so the pedal sounds like itself at every setting rather than turning into a different circuit at the extremes.
Up to 42 dB of gain in the bass, rising to 52 dB at 10 kHz
Full-band from 4.3 Hz, where a classic drive starts thinning near 100 Hz
Soft clip ≈ ±4.8 V, matched LEDs from one reel, tracking in temperature
Hard clip ≈ ±2.4 V, about 6 dB below the drive’s own threshold
Fuzz on its own ground island with its own level control
Steward of the Seas
VCA compressor and gate, after the drive. Compressor mode switch.
Studio glue for distortion. Chords and single notes land at the same level, fast runs stop disappearing, sustain extends without added fizz. When you stop playing, a gate closes the noise down without chopping the tails of chords.
Set at the factory, finished for you. Thirteen trimmers on the board hold the preset, so the panel stays simple. More on the compressor.
SSI2162 dual VCA, the kind found in console channel strips
Up to 37 dB of gain reduction, with an adjustable knee
Fast and slow detectors working together
Gate floor ≈ −85 dB: when you stop, it is silent
Aurora Curtain
Staggered Bessel low-pass filter. Top row, centre knob.
Your guitar’s tone knob, done four times over and on purpose. Fully clockwise it is out of the way. Turned down, it takes back the fizz and glare that clipping creates. Its last two poles are held back deliberately, so the first octave above the corner rolls off gently: the sound goes dark without going shut, and notes keep their pick definition.
Set the drive first, then Aurora. Play the loudest thing you will play and turn it down until the sound stops being brittle and starts being solid. Leave the guitar’s tone open and the amp’s EQ neutral; this replaces them.
4th-order Sallen-Key, two Bessel sections, deliberately staggered
Corner ≈ 58 kHz to 1.8 kHz, spread evenly across the rotation
−0.3 dB at 20 kHz when fully open
330 pF C0G capacitors throughout, stable with voltage and temperature
See what each knob does to the response
This is the pedal’s filter path, computed from its component values. Turn a knob and the curve follows. The drive isn’t drawn here; this is what the filters let through to it and out of it.
Low corner—
High corner—
Notch depth—
Modelled from the design values in the technical references. Twin Assassins depth and width are approximate.
Listen
Hear it on the instruments it was built for. Each clip runs the pedal straight into a clean amp or interface, so what you hear is the pedal.
Starting points
The knobs are unmarked on purpose; the guides are your map. These get you to a good sound in the first minute.
Chug
Hard clipper on. Wall of Ice near 12 o’clock. Aurora Curtain around 2 o’clock.
Palm-muted riffs want definition more than depth, each hit landing as its own thump. The tightness is what makes fast, percussive playing read.
Sludge and doom
Fuzz or hard clipper on. Wall of Ice fully clockwise.
The pedal flattens 20–40 Hz fundamentals into square waves, which most drives simply cannot do. It is enormous, and hard on guitar speakers: bring the amp up gradually.
Mid-forward bite
Hard clipper on. Wall of Ice around 10 o’clock. Twin Assassins out of the way.
Shaving the bass before the drive leans toward the classic clipped-mids character: tighter, more cutting, more grind.
Bass grind
Wall of Ice open. Aurora Curtain between 10 and 12 o’clock.
The fundamental stays whole and the grind sits behind the note. Back Wall of Ice down only if the room starts to boom.
Synth sweep
Synth output down before you plug in. Wall of Ice rolled back, then opened to taste. Sweep Aurora Curtain while the patch plays.
You choose how much sub reaches the drive. Stop the sweep where the tone is right, the same way you would on the synth’s own filter.
Crushed drum machine
Drive up. Aurora Curtain turned down while the beat runs.
Cymbals and hats are where the harshness lives, and this is what removes them. Fast lo-fi, without a plug-in.
Steward of the Seas
A studio compressor and gate, inside a stompbox.
Most players put a compressor first, where it squashes the pick and feeds the drive a flat signal. In a studio the compressor goes after the thing that makes the sound. That is where this one lives: after the distortion, keeping every note at the level you meant and shutting the door when you stop.
A real VCA compressor
Nearly every compressor pedal is an optical cell or a single-transistor trick. This one is built the way rack units are: a voltage-controlled amplifier with a separate sidechain that measures the signal, shapes it, and tells the VCA what to do. That separation is what lets it be transparent at one setting and obvious at another.
Two detectors, not one
A fast detector catches the pick. A slow detector rides the phrase. The board follows whichever is larger at any instant, so single notes get caught without the gain pumping back up between them.
Parallel compression, built in
A blend mixes the untouched signal back in with the compressed one, in phase. The compressed path carries the sustain and body; the dry path carries the transients. It is the trick mix engineers use on drums and bass.
A gate with two thresholds
A working threshold closes the noise between phrases with smooth timing. Below it, a mute floor takes the pedal to about −85 dB. Hysteresis keeps a note sitting right at the threshold from chattering.
Set once, play forever. Thirteen trimmers set the factory preset: amount, depth, knee, four timings, blend, two thresholds, two gate timings and output level. Players get a pedal that sounds finished. Technicians get a full setup guide, a technical design guide with the response curves, and three test points to prove it.
The character of this pedal is fixed by its component values and by nothing else: not the adapter, not how long it has been on, not what the rest of the circuit is doing.
Real ±15 V rails
An on-board converter turns a standard 9 V supply into true bipolar rails with a real ground, filtered through capacitance multipliers. Every op amp gets room to swing, and switching residue sits more than 80 dB below the converter’s ripple.
Parts chosen for stability
Class 1 C0G ceramics set the filter corners, with no voltage dependence and no microphonics. Precision amplifiers run the signal path, and the clipping LEDs come from one reel so the two halves of the waveform match.
Serviceable by design
Each board takes power through a small entry resistor meant to be the first, cheapest part to open under a fault. Six-layer boards keep every signal trace over a solid ground plane.
Specifications
Sources
Guitar, bass, synthesizer, keyboards, drum machines, line-level gear. Unbalanced ¼″ in and out.
Frequency response
Coupling corners at or below 2 Hz. Flat within ±0.1 dB from 2 Hz to 20 kHz with the filters open; −3 dB near 58 kHz.
Settling
About 50 ms
Input
4.7 MΩ with 100 pF. JFET-input first stage, 5 nV/√Hz. +6 dB of clean preamp gain, matched at the output. Clean to at least ±6 V peak.
LED soft clipping (≈ ±4.8 V, symmetric), up to 42 dB in the bass and 52 dB at 10 kHz. Switchable hard clipper (≈ ±2.4 V). Two-transistor fuzz with its own level and a built-in 150 Hz cut.
Steward of the Seas
SSI2162 VCA compressor, 0 to ≈ 37 dB gain reduction, adjustable knee. Attack 2–26 ms fast, 33–360 ms slow. Release 27–50 ms fast, 190–360 ms slow. Phase-correct blend. Gate: two thresholds, 1–48 ms open, 47–520 ms close, ≈ −85 dB floor. Gain-reduction LED.
Aurora Curtain
4th-order staggered Bessel low-pass, ≈ 58 kHz to 1.8 kHz; 12 dB/octave through the first octave at the dark end, 24 dB/octave beyond
Controls
Five knobs, three mode switches (compressor, hard clipper, fuzz), footswitch. Clockwise means more.
Power
9 V DC pedal supply, about 120 mA. 12 V regulated is tolerated. Do not use 18 V. Reverse-polarity protected.
Internal rails
±15 V bipolar with a real ground, generated on board
Hand-built and set on the bench, with Steward of the Seas dialled to its factory preset
Full-frequency signal path for guitar, bass and synthesizer
Every guide online, for players and for technicians
Questions
Will it really work on bass?
Yes. That is what the full-frequency design is for. The pedal passes the fundamental of every note, including a five-string’s low B at 31 Hz, and the drive amplifies the whole signal from 4.3 Hz upward. Wall of Ice lets you tighten the low end when a room or rig starts to boom.
Can I use it with a synthesizer or drum machine?
Yes. Line levels, keyboards and most synthesizers fit within the input’s clean headroom. Turn the source’s output all the way down before you plug in, then bring it up while you listen. Modular gear running at ±10 V will drive the first stage into distortion; the input won’t be damaged, and a little attenuation in front fixes it.
What power supply does it need?
A standard 9 V DC pedal supply. It draws about 120 mA running, with a short surge at switch-on, so an isolated output with some current to spare is ideal. Regulated 12 V is tolerated. Do not use 18 V: the pedal’s protection is designed to sacrifice a part rather than let the converter be damaged, and it will need a repair.
Where does it go in my chain?
Anywhere a drive pedal goes. It already has its own compressor after the drive and complete tone shaping, so it is happy straight into a clean amp or an interface. Leave the guitar’s tone control open and the amp’s EQ flat or neutral, and let Aurora Curtain do the work.
Why are the knobs unmarked?
So the art can be the panel. Every knob follows one rule, clockwise means more, and the player’s guides are your map for each one.
Can the compressor be adjusted?
Steward of the Seas is set at the factory with thirteen on-board trimmers, so the panel stays simple. A technician can change the preset using the setup guide and three test points; the technical design guide has the response curves behind every setting.
Is the sub-bass safe for my speakers?
Fully open, the pedal will pass and distort energy far below what a guitar normally produces. Guitar speakers can’t always take it. Bring the amp up gradually, let the cabinet tell you where its limit is, and use Wall of Ice to keep the deepest content out when you need to.
What it does
Wall of Ice sets how much low end passes into the pedal. Turn it clockwise for more bass; turn it counter-clockwise to roll the low end off. Like every knob on the Malice Madrigal, clockwise gives you more.
It is not a bass tone knob in the usual sense. Instead of boosting or cutting a broad range, it draws a line: everything above the line passes exactly as it came in, and everything below the line is removed cleanly, at a steep rate. Fully clockwise the line sits at the very bottom of hearing and nothing is touched. As you turn counter-clockwise the line climbs — first the sub-bass goes, then the boom, then the body — until, fully counter-clockwise, only a tight, cutting core remains.
Why you would shape the low end
Low frequencies carry a lot of energy, and distortion works on everything it is fed at once. There is a point where the low end simply becomes too much: the bass grows large enough that it begins to cloak the notes above it, and the detail you want to hear — the pick, the chord, the top of a lead line — sits behind it and is harder for the ear to pick out. The sound is still good; it is just crowded. Wall of Ice is how you make room again, easing the low end back until the rest of the note steps forward.
The best place to shape the low end is before it reaches the distortion, not after. Once a signal has been distorted, whatever balance it had is set into the sound, and later filtering cannot fully reach back in. Wall of Ice sits at the front of the pedal for exactly this reason: it decides what the distortion works on in the first place. Set the low end where you want it here, and the drive keeps that balance no matter how hard you push it.
Clarity is not the only reason to reach for this knob. Sometimes less bass is the sound. The classic hard-clipper voice — the RAT character — lives in a midrange hump: it clips hardest in the mids and lets the low end stay comparatively clean, which is where its bite and definition come from. Shaving the bass down before the drive leans the pedal toward that character; a thinner, mid-forward signal clips differently than a full one, and the result is a tighter, more cutting grind. Open the knob back up and the same drive stage turns thick and heavy. Wall of Ice is not just a fix for too much low end; it is a way of choosing what kind of distortion you get.
A few places that idea takes you:
Chug. Palm-muted low-string riffs want definition more than depth — each hit should land as a distinct thump, not blend into the next. Pull Wall of Ice down toward the middle, run the hard clipper, and the chugs get their edges back; the tightness is what makes fast, percussive playing read.
Sludge and doom. The opposite move. Open Wall of Ice all the way, engage the fuzz or the hard clipper, and let the pedal work on the whole low end at once. Because the input passes everything down to 2 Hz and the drive stages have the headroom to clip it, the pedal will produce distorted low end that most drives simply cannot — fundamentals in the 20–40 Hz range flattened into square waves. It is enormous. A word of care: that much sub-frequency energy is hard on guitar speakers, so bring the amp up gradually and let the cabinet tell you where its limit is.
RAT bite. Shave the bass, run the hard clipper, keep the notch out of the way: a mid-forward, cutting grind with the classic clipped-mids character.
The one habit worth learning
It is tempting to start with the bass all the way up — more low end sounds bigger at first. Do the opposite. Start with Wall of Ice rolled most of the way down (counter-clockwise), then slowly open it up while you play.
Rolled down, the pedal is tight and focused and you can hear every note. As you bring the knob up, you add low end back a little at a time, and you will hear the point where the bass starts to crowd the rest of the sound. Stop just before that point — that is where the low end is as big as it can be while everything above it still comes through clearly. That spot is different for every instrument, every pickup, and every amp. Finding it by opening up to the edge of crowding, rather than backing down out of a crowded sound, is faster and lands you in a better place.
By instrument
Electric guitar. Start around the middle and open up until the low strings feel full but still clear. For heavy rhythm, leave it lower — the tightness is what lets fast passages stay defined under distortion.
Bass guitar. This is what the full-frequency design is for. Open the knob up; the pedal passes the fundamental of every note, including a five-string’s low B. Back it down only if a room or a rig is booming — you have the control to tighten up without losing the instrument.
Synthesizer. Synths can put out enormous low end, far more than a guitar. Roll Wall of Ice down first, then open it up to taste. You decide how much sub reaches the drive, which is where a lot of the character lives.
Microphone or vocals (through a preamp). Rolling the knob up past the middle removes rumble, stage thump, and handling noise before they reach the rest of the pedal, the way an engineer high-passes a vocal. Set it so the voice keeps its warmth but the low rumble is gone.
With the Fuzz engaged. The fuzz has its own built-in bass cut — it thins the low end automatically at around 150 Hz, between its two stages, which is what keeps a fuzz from turning to mush. Because of that, the fuzz actually likes more bass coming in: turn Wall of Ice up further than you would for the other modes, and let the fuzz’s own voicing do the tightening. If the fuzz sounds thin, give it more bass, not less.
A note on hot sources
The pedal is happy with instrument and line levels. Synthesizers, keyboards, and line outputs are usually stronger than a guitar, so before plugging one in, turn that device’s volume or output all the way down. Plug in, then bring its level up slowly while you listen. Stop when it sounds full and driven but not harsh or spitting. This protects your ears and your speakers and lets you find the sweet spot instead of overshooting it.
Working with the other controls
With the Notch. Set Wall of Ice first to get the low end where you want it, then use the Notch to carve the mids. They work as a pair; a controlled low end makes the Notch far more effective.
With Distortion. The more you drive the pedal, the more the bass character shifts (see above), and heavy settings can want Wall of Ice a little lower to keep the definition up front. If a high-gain sound is starting to feel crowded on the bottom, open a little room here before touching anything else.
With the three mode switches. Compressor, hard clipper, and fuzz each change how the low end behaves. The fuzz in particular wants more bass than the others because of its built-in cut. Reach for Wall of Ice whenever you flip a mode.
Try it on everything
This pedal was built to be used on more than guitar. Put a bass, a synth, a drum machine, or a whole mix through it and Wall of Ice will treat them all the same — a clean, steep low-end control that never colors what it keeps. If you find a use that surprises you, we would like to hear about it.
What it does
Aurora Curtain sets how bright the pedal is. Clockwise opens it up; counter-clockwise darkens it. Fully clockwise the filter is out of the way entirely — the pedal passes everything, right through the top of hearing and beyond. Turn counter-clockwise and it progressively removes the top, from a gentle trim of the air down to a deep, dark, rounded tone. Like every knob on the Malice Madrigal, clockwise gives you more.
It sits at the end of the chain, after the distortion. That matters: what it is removing is not part of your instrument — it is the high harmonics the clipping created. Fizz, glare, that thin sizzle riding on top of a driven sound: those are made inside the pedal, and this is where they are taken back out.
Think of it as your tone knob, only better
Your guitar already has one of these. The tone knob on an electric guitar is a low-pass filter — one capacitor, one potentiometer — and it has survived seventy years unchanged because it is gentle: it darkens the sound without making it sound closed or smeared. The problem is that it is not very strong. Rolled far enough to take the fizz off a driven amp, it has already taken the life out of the instrument.
Aurora Curtain is the same idea built four times over, with all four stages designed to work together, so it can go much further without the sound going flat or losing its attack. Notes keep their pick definition even when the tone is dark, which is the thing simple tone controls cannot do.
The practical consequence: when this pedal is in your chain, you do not need your other tone controls. Leave the guitar’s tone knob wide open and set the amplifier’s treble and presence flat or neutral, then use Aurora Curtain for everything. Stacking three tone controls that were never designed to work together is what makes a rig sound smeared; this replaces all of them with one.
What to expect as you turn it
Position
What you hear
Fully clockwise
Out of the circuit. Nothing removed. Use this when your amp is already dark, or when you want the drive as raw as the pedal makes it.
4–5 o’clock
Air only. Takes the top sparkle off without changing the body. Good for taming a bright amp or bright speakers.
2–3 o’clock
The fizz zone. This is where most of the harshness from hard clipping and fuzz lives. For many rigs this is the setting.
12 o’clock
Clearly darker. The sound gets rounder and thicker while the notes stay defined.
9–10 o’clock
The main tone-shaping region. Warm, dark, vocal — a covered, smoky drive sound.
Fully counter-clockwise
Dark. Heavy, round, with the top gone — but still articulate, which is the part most filters get wrong.
The knob is designed so the useful range is spread evenly across the rotation rather than bunched at one end. The first fifth of the travel from fully clockwise is deliberately subtle — that is the filter getting out of the way — and then it starts working.
How to set it
Set the drive first, then Aurora Curtain. Get the Distortion knob and the mode switches where you want the gain and the character, and accept that it may sound harsh; then bring Aurora Curtain down until the harshness goes. That order matters, because the amount of fizz to remove depends entirely on how hard you are driving the pedal.
A reliable method: turn it fully clockwise, play the loudest, most aggressive thing you are going to play, and slowly turn counter-clockwise. There is a point where the sound stops sounding brittle and starts sounding solid. Stop there. If you keep going you will start losing pick attack and clarity — not immediately, and not unpleasantly, but you will know when you have gone past what you wanted.
By instrument
Electric guitar. Start around 2–3 o’clock with the drive up. Humbuckers into a dark amp often want it further clockwise than you would expect; single coils into a bright rig usually want it lower. Remember to open the guitar’s own tone knob all the way first.
Bass guitar. Bass distortion lives or dies on the top end — too much and it sounds like a broken speaker. Roll Aurora Curtain down until the grind sits behind the note instead of on top of it, usually somewhere between 10 and 12 o’clock, and let Wall of Ice handle the bottom.
Synthesizer. This is where the filter really shows. Synths can produce far more high-frequency content than a guitar, and distorting them produces more still. Use Aurora Curtain as you would a synth’s own filter: sweep it while a patch plays and stop where the tone is right. Its gentle, even slope is deliberately close in character to a classic analogue synth filter.
Microphone or vocals. A little goes a long way. Vocals reveal a dark filter faster than instruments do — start at 4–5 o’clock and move down only until the sibilance and the edge of the distortion are under control.
Drum machines and full mixes. Turning it down while a beat runs through the drive is a fast way to get a lo-fi, crushed sound; cymbals and hats are where the harshness lives, and this is what removes them.
Working with the other controls
With Distortion. More drive makes more high harmonics, so heavier settings want Aurora Curtain further down. If you turn the drive up and the sound gets brittle, this is the knob that fixes it — not the drive knob.
With the three modes. The hard clipper and the fuzz each generate their own upper harmonics, and the fuzz in particular can get sharp. Expect to move Aurora Curtain whenever you flip a mode.
With Wall of Ice. They work on opposite ends and do not overlap. Wall of Ice decides what the distortion is fed; Aurora Curtain decides how much of what it produced comes out. Set the low end first, then the top.
With Twin Assassins. The notch works in the middle, between them. If a sound is harsh, try Aurora Curtain; if it is blurry or boxy, that is a job for Twin Assassins. They fix different problems and it is worth knowing which one you have.
One thing it cannot do
Aurora Curtain will not make the pedal clean. It sits after the distortion, so by the time the signal reaches it the clipping has already happened; turning the knob down makes that same distortion darker, not gentler. If you want less distortion, that is the Distortion knob or the mode switches. What this control does is decide how much of the result reaches your amplifier — which, for most of the harshness players complain about, is the more useful thing.
Try it on everything
Every source has a top end, and distorting anything makes more of it. Bass, synths, drum machines, a whole mix — Aurora Curtain treats them all the same, and on sources that are not guitars it often does more work than any other control on the pedal. If you find a use that surprises you, we would like to hear about it.
1. What the board does
The compressor evens out the level of what comes out of the distortion stages — quieter notes come up, louder ones come down — so that chords and single notes sit at a similar volume and the pedal feels consistent under the pick. It is meant as a polish, not a sustainer. After it, a gate shuts the signal off between phrases so the noise that distortion brings up is not heard when you stop playing.
Thirteen trimmers on the board set all of this. They are meant to be set once, on the bench, to a factory preset; the player then has the pedal’s main knobs. This guide explains what each trimmer does, how to tell what it is doing, and in what order to set them.
1.1 How to read the board
Compression is controlled by a voltage the board makes from the signal. You can see it two ways: the gain-reduction LED (brighter = more compression; at full brightness the board is pulling the signal down by about 30 dB), and test point TP3, which reads 0 V with no compression and about 1 V at 30 dB. The gate has its own voltage at TP2: near 0 V when the gate is open, about +2.8 V when it is closed. If a setting doesn’t seem to do anything, these two points tell you whether the board is doing what you asked.
2. The map
Component side. Amber numbers are the trimmers, in the order described below; blue marks the jumpers, test points and connectors.
3. Starting point
Set everything here first, then adjust in the order given in §5. Rotation is the fraction of the trimmer’s travel, clockwise from fully anticlockwise.
#
Control
Start at
Why
13
Out_LVL1
35 %
unity gain; adjust last
1
ERR1
40 %
moderate sidechain gain — enough to hear compression, not enough to over-compress
2
C_LVL1
70 %
most of the envelope reaches the VCA
3
Knee1
50 %
between hard and soft; refine by ear
4
FST_ATT1
20 %
≈ 7 ms — catches pick attacks without clicking
5
FST_REL1
100 %
longest fast release
6
SLW_ATT1
50 %
≈ 180 ms — rides the phrase, not the note
7
SLW_REL1
100 %
longest slow release
8
BLEND1
75 %
mostly compressed, a little dry for attack
9
G_LVL1
25 %
gate opens on normal playing, closes on noise
10
RV_THR1
10 %
floor just above the noise
11
G_FAST_ATT1
10 %
≈ 6 ms — opens before the note is heard
12
G_SLOW_REL1
40 %
≈ 235 ms — holds through short gaps
A
JP1
B (feedback)
see §6 — the gentler, studio-style curve
B
JP2
A (compressed)
gate keyed from the compressed signal
4. The controls, one by one
1. ERR1 — compression amount
What it does. Sets how strongly the board reacts to level. Internally it is the gain of the amplifier that measures the signal before it is turned into a control voltage: at fully anticlockwise there is no measurement and no compression at all; at fully clockwise the measurement is at its most sensitive. This is the closest thing on the board to a ratio control.
How to set it. Play a steady chord and turn ERR1 up from zero while watching the gain-reduction LED. It stays dark, then begins to glow, then brightens quickly. The useful region is from where it first glows to where it is fairly bright on your loudest playing; past that the board pulls hard on everything.
What you’ll hear. Low settings: the pedal sounds untouched, with a little evenness added. Mid settings: notes and chords land at a similar level, sustain feels longer. High settings: pumping, and in feed-forward mode (JP1 = A) a ‘ducking’ effect where hitting harder actually makes the output quieter.
Range. 0 to about +17 dB of sidechain gain. With C_LVL1 at 70 %, compression starts to be audible around 25–30 % and is strong by 60 %.
Interacts with C_LVL1. ERR1 decides at what level compression begins; C_LVL1 decides how deep it goes once it has begun. If you want compression to start earlier but stay gentle, raise ERR1 and lower C_LVL1.
2. C_LVL1 — compression depth
What it does. Scales the control voltage before it reaches the VCA. At zero, the detector can work as hard as it likes and the VCA never moves; at maximum, 82 % of the detected envelope is applied and the board can reach about 37 dB of gain reduction.
How to set it. With ERR1 where you left it, play and turn C_LVL1 from zero up. The LED and TP3 rise with it. Set it so your hardest playing produces the amount of gain reduction you want — for a polish, the LED noticeably brightening on accents but never blazing.
What you’ll hear. Raising it deepens everything proportionally — the curve keeps the same shape and gets steeper. It does not change where compression starts.
Range. 0 to 82 % of the envelope; maximum gain reduction 0 to ≈ 37 dB.
3. Knee1 — knee hardness
What it does. The board limits how large the measured error can get. Knee1 sets whether that limit is a hard stop (anticlockwise) or a gradual one (clockwise). It shapes the top of the compression curve — how the board behaves on the loudest transients — not the onset.
How to set it. Hit hard, single notes with ERR1 and C_LVL1 at working settings. Anticlockwise, the loudest hits all land at exactly the same level with a slightly abrupt feel; clockwise, the loudest hits still get louder, just less.
What you’ll hear. Hard: more consistent, more ‘limited’. Soft: more dynamic on accents, more natural.
Range. 0 (hard) to 5 k (soft). Subtle unless you are driving the board hard.
4. FST_ATT1 — fast attack
What it does. How quickly the fast detector responds when a note arrives. The fast detector is what catches the pick attack.
How to set it. Play staccato single notes. Fully anticlockwise (2 ms) the attack is clamped almost instantly — pick clicks may be softened, and on bass notes you may hear a faint distortion as the gain changes within one cycle. Turn clockwise until the pick retains its edge; around 15–30 % is usually the balance.
What you’ll hear. Fast: smooth, slightly flattened attack. Slow: the first few milliseconds of each note come through untouched, then the compressor grabs — more ‘snap’.
Range. 2 ms to 26 ms.
5. FST_REL1 — fast release
What it does. How quickly the fast detector lets go after a note. Short releases follow each note individually; longer ones smooth between notes.
How to set it. Play repeated eighth notes. Anticlockwise, the gain recovers between every note and you may hear the level ‘breathe’. Clockwise, it holds. On this board the range is deliberately short, so the effect is modest; leave it near maximum unless you want an obvious pumping effect.
What you’ll hear. Short: lively, each note pumps up slightly. Long: smoother.
Range. 27 ms to 50 ms (the fixed circuit around it limits the top of the range).
6. SLW_ATT1 — slow attack
What it does. How quickly the slow detector responds. The slow detector rides the overall phrase and stops the fast one from pumping back up between notes.
How to set it. Play a sustained chord and let it ring. With the control anticlockwise (33 ms), the slow path joins the fast one almost immediately. Clockwise (360 ms), the compression settles in over a third of a second. Set it so a strummed chord sounds even across its first second.
What you’ll hear. Fast: the compressor arrives all at once. Slow: a gentle settling in.
Range. 33 ms to 360 ms.
7. SLW_REL1 — slow release
What it does. How long the slow detector holds after you stop playing.
How to set it. Play a phrase and stop. With the control clockwise the compression holds for about a third of a second after the last note before the gain recovers, which keeps noise from swelling up in short gaps. Anticlockwise it recovers in under 200 ms.
What you’ll hear. Short: more dynamic, a hint of swell after notes. Long: more ‘glued’, quieter gaps.
Range. 190 ms to 360 ms.
Two detectors, one result. The board always uses whichever of the fast and slow detectors is larger at the moment. Fast attack + slow release is the standard polish setting: the fast path catches the hit, the slow path keeps it caught.
8. BLEND1 — wet / dry
What it does. Mixes the uncompressed signal back in with the compressed one before the gate. Fully anticlockwise is dry (no compression heard at all); fully clockwise is entirely compressed.
How to set it. Set the compressor the way you want it, then back BLEND1 off until a little of the natural attack and dynamics returns. Around 70–85 % keeps the polish while letting the pick through.
What you’ll hear. This is ‘parallel compression’: the compressed signal supports the sustain and body, the dry signal supplies the transients. It makes heavy settings of ERR1 and C_LVL1 usable without sounding squashed.
Range. 0 % (dry) to 100 % (wet). The two signals are in phase, so any mix is clean.
9. G_LVL1 — gate threshold
What it does. The level below which the gate begins to close. This is the main gate control.
How to set it. Stop playing and let the pedal sit on its noise. Turn G_LVL1 anticlockwise until the gate opens on the noise (you hear it), then clockwise until it just closes — then a little further for margin. Play lightly: the gate should open on your quietest intentional note. If it doesn’t, come back anticlockwise a touch.
What you’ll hear. Too low: the gate never closes and noise is heard between phrases. Too high: soft notes and the tails of chords get cut off.
Range. About 0.31 to 0.78 V peak at the detector. In the normal position of JP2 that is the compressed signal, so the threshold tracks the compressor settings rather than the drive knob.
The gate has built-in hysteresis — it opens at a slightly higher level than it closes — so a note that is right at the threshold does not make it chatter.
10. RV_THR1 — floor (mute) threshold
What it does. A second, lower threshold. Below it the gate goes to its deepest setting (about −85 dB — effectively silent) rather than merely closing. Think of it as the mute level under the gate’s working range.
How to set it. Set it below G_LVL1. Start fully anticlockwise, then raise it only enough that the pedal is silent when nothing is playing. It should never be above the gate threshold.
What you’ll hear. Below the floor: silence. Between the floor and the gate threshold: the gate’s normal, smoothly timed closing.
Range. About 0.31 to 0.63 V peak at the detector. Its action is smoothed over ≈ 20 ms so it does not click.
11. G_FAST_ATT1 — gate open time
What it does. How quickly the gate opens once the signal crosses the threshold.
How to set it. Play sharp, percussive notes from silence. Fully anticlockwise (1 ms) the gate opens before you can hear it move. Turn clockwise only if the opening sounds clicky on bass notes; past about 20 % (≈ 10 ms) the start of each note begins to be audibly rounded off.
What you’ll hear. Fast: transparent. Slow: a soft ‘fade-in’ on each note from silence.
Range. 1 ms to 48 ms.
12. G_SLOW_REL1 — gate close time
What it does. How long the gate takes to close after the signal drops below the threshold.
How to set it. Play a phrase with a short rest in it. Anticlockwise (47 ms) the gate snaps shut in the rest — tight, but tails are cut. Clockwise (520 ms) it fades out over half a second. Set it so the natural decay of a chord is not chopped but the gap before the next phrase is quiet.
What you’ll hear. Short: tight, choppy. Long: natural decay, noise lingers a moment.
Range. 47 ms to 520 ms.
13. Out_LVL1 — output level
What it does. The gain of the final stage, after the gate.
How to set it. Set it last. With the pedal in bypass and then engaged, match the two volumes by ear or with a meter at the output. Unity is around 35 % rotation.
What you’ll hear. Below 35 %: quieter than the input. Above: up to +12.6 dB of make-up gain, which compression usually needs.
Range. −33 dB to +12.6 dB.
5. Setting it up, in order
Jumpers first (§6). For a studio-style polish put JP1 on B. Put JP2 on A.
All trimmers to the §3 starting point. Out_LVL1 to 35 %.
Gate wide open: G_LVL1 and RV_THR1 fully anticlockwise, so the compressor can be heard on its own.
Compressor onset: ERR1 up from zero until the LED glows on normal playing.
Compressor depth: C_LVL1 until accents bring the LED up clearly but it never pins.
Attack: FST_ATT1 for the pick, SLW_ATT1 for the phrase. Then releases, both near maximum to start.
Knee: by ear on hard hits. Blend: back off from fully wet until the attack returns.
Gate: raise G_LVL1 until the noise closes it; check your quietest note opens it. Then RV_THR1 just high enough for silence at rest, below G_LVL1.
Gate timing: G_FAST_ATT1 as fast as is click-free; G_SLOW_REL1 so chord tails aren’t chopped.
Output level: match bypass and engaged.
Play for ten minutes and revisit ERR1 and G_LVL1 — they are the two that drift in your ear.
6. The two solder jumpers
JP1 — where the compressor listens. Position A (pads 1–2) takes the signal before the compressor: feed-forward. Position B (pads 2–3) takes the signal after it: feedback. In A the board can over-react — above a certain level, hitting harder makes the output quieter — which is dramatic and can be useful as an effect, but is not what a glue compressor does. In B the compressor regulates its own output, the curve is a gentle soft knee, and the controls behave the way players expect. The board as shipped is bridged to A; for the polish role, move it to B.
JP2 — what the gate listens to. Position A keys the gate from the compressed signal, so the threshold follows the level the compressor produces regardless of how much drive is dialed in upstream. Position B keys it from the clean guitar signal supplied by the high-pass board (amplified ×11 on this board), which makes the gate ignore the distortion stages entirely and respond only to the strings. A is the normal setting; B is for a tighter, pick-controlled gate on high-gain sounds.
7. If something’s wrong
Symptom
Check
Likely cause
No compression, LED never lights
TP3 stays at 0 V while playing
ERR1 or C_LVL1 fully anticlockwise; JP1 not bridged
LED on with no signal
TP3 above 0 V at rest
Noise upstream is above the onset; raise the gate first, then lower ERR1
Pumping / breathing
LED pulsing with the rhythm
Releases too short, or ERR1 too high; try JP1 = B
Hitting harder gets quieter
—
Feed-forward over-compression (JP1 = A with high ERR1); lower ERR1 or move to B
Gate chatters on sustained notes
TP2 flicking between 0 and +2.8 V
G_LVL1 set right at the sustain level; lower it slightly or lengthen G_SLOW_REL1
Gate never closes
TP2 stays near 0 V at rest
G_LVL1 too low, or noise upstream above threshold
Gate never opens
TP2 stays at +2.8 V while playing
G_LVL1 or RV_THR1 too high; JP2 on B with nothing on J10
Notes start with a click
—
G_FAST_ATT1 fully anticlockwise on bass-heavy signal; turn up a little
Chord tails chopped
—
G_SLOW_REL1 too short, or G_LVL1 too high
Output distorted at high settings
—
Out_LVL1 too high for the stages after it; check it is not above unity when not needed
8. Reference
#
Control
Value
Function
Range
Clockwise
1
ERR1
50 k
Compression amount
0 to +16.6 dB sidechain gain
more
2
C_LVL1
100 k
Compression depth
0 to ≈ 37 dB max gain reduction
deeper
3
Knee1
5 k
Knee hardness
hard → soft
softer
4
FST_ATT1
50 k
Fast attack
2 – 26 ms
slower
5
FST_REL1
470 k
Fast release
27 – 50 ms
longer
6
SLW_ATT1
100 k
Slow attack
33 – 360 ms
slower
7
SLW_REL1
500 k
Slow release
190 – 360 ms
longer
8
BLEND1
10 k
Wet / dry
dry → wet
wetter
9
G_LVL1
100 k
Gate threshold
0.31 – 0.78 V pk at detector
higher
10
RV_THR1
100 k
Floor threshold
0.31 – 0.63 V pk at detector
higher
11
G_FAST_ATT1
100 k
Gate open
1 – 48 ms
slower
12
G_SLOW_REL1
100 k
Gate close
47 – 520 ms
slower
13
Out_LVL1
20 k
Output level
−33 to +12.6 dB
louder
Test points: TP3 (ENV_C) 0 V = no compression, ≈ 1 V = 30 dB. TP2 (ENV_G) ≈ 0 V open, ≈ +2.8 V closed. TP1 (Loop_ERR) is the raw error signal; it should swing no further than about ±1.8 V on peaks.
Ranges are calculated from the component values; your board will land within a few percent of them.
What this section does
Everything the pedal does starts here. The input stage receives whatever you plug in — guitar, bass, synthesizer, a vocal through a preamp — protects it, and hands it to the rest of the circuit at the right level. Right behind it sits a steep, adjustable high-pass filter that decides how much low end reaches the distortion stages.
That order matters. A high-pass filter placed after distortion can only trim the result; placed before it, the filter changes what the distortion is fed. Cutting mud before the drive keeps the clipping stages working on the part of the signal that has definition, which is why the pedal stays articulate at settings where most drive circuits turn to blur. The filter is not a tone control in the usual sense: below its corner it removes, above its corner it leaves the signal exactly as it was.
Why this pedal is full-frequency
Most effects pedals are designed for a six-string guitar and quietly assume it: coupling capacitors that roll off below 40 or 50 Hz, tone stacks that thin the low end, inputs that load a pickup. Put a bass through one and it turns to mud; put a synthesizer through one and half the sound never arrives. The Malice Madrigal was designed from the first schematic to be an effects pedal rather than a guitar pedal — to treat every signal the same, whatever produced it. The low end is not removed until the player decides to remove it, and the top is never dulled by the input. That intent shows up as three rules on every board, and this section is where they are most visible.
Full frequency, by design. Every coupling capacitor in the signal path is sized for a corner at or below 2 Hz, and every stage’s passband is flat to beyond 20 kHz. A five-string bass’s low B at 31 Hz, a synthesizer’s sub-octave, a baritone guitar — all arrive intact. Where low end is removed, it is because the player chose to remove it with the high-pass knob.
Gain where it is quietest. The only fixed gain ahead of the drive stages is applied by the lowest-noise amplifier in the pedal, so the signal reaches every later stage well above that stage’s noise floor.
Clockwise means more. Every knob on the pedal adds as it turns clockwise. On this section, clockwise admits more low end.
The input stage
The input is verified against the most demanding source it will meet: a passive guitar pickup, which is a fussier source than most gear admits. An input that treats a pickup correctly treats everything else correctly too. A pickup is a coil, and its tone lives in a resonant peak somewhere between 2 and 5 kHz. Anything the pedal connects across that coil — resistance, capacitance — moves the peak and dulls the guitar before any circuit has had a chance to do anything. Many pedals load pickups audibly; this one is built not to.
4.7 MΩ input impedance — high enough to leave a pickup’s resonance where the pickup put it, high enough for piezo and acoustic pickups, and irrelevant to a low-impedance source like a synthesizer or a preamp, which simply sees an open door.
Only 100 pF of capacitance across the input — less than a short cable adds — as a shunt to ground behind a small series resistor, which together keep radio-frequency interference out of the amplifier. The signal then passes through a 22 nF C0G coupling capacitor; it is in series, so it adds no loading to the pickup, and its Class 1 ceramic dielectric has none of the voltage dependence or microphonics of ordinary ceramics.
A low-noise JFET-input amplifier as the very first stage. Its noise is the lowest of any amplifier in the pedal, and because every later stage multiplies whatever this one contributes, that is where low noise counts most.
+6 dB of clean gain applied here, at the quietest point in the chain, so the signal arrives at the drive stages well above their noise floor.
Protection diodes that never conduct in normal use but catch static discharge and cable-plugging spikes before they reach the amplifier.
The coupling capacitor and its 4.7 MΩ bias resistor set a corner at 1.5 Hz — the lowest note on a five-string bass is 31 Hz, and the input does not know it is there.
This is the most sensitive node in the pedal and it is treated that way on the board: the input network sits within a few millimetres of the amplifier pin, on a single layer, with nothing routed near it. The protection diodes guard the hardware against static and cable-plugging spikes; they do not make the input a line-level input. The amplifier handles roughly ±6 V cleanly, which covers instrument level, line level, keyboards and most synthesizers with margin. Only the hottest sources — modular synthesizers running at ±10 V, for instance — exceed it; the input will not be damaged, but the first amplifier will distort, and a small external attenuation is the fix.
The high-pass filter
The filter is a fourth-order Butterworth: four poles, a slope of 24 dB per octave below its corner, and a passband with no ripple and no resonant bump at the corner. “Maximally flat” is the textbook description, and it is the reason the filter can be set aggressively without adding a hump of its own. What is above the corner passes untouched; what is below it falls away at 24 dB per octave, which is to say an octave below the corner is already nearly silent.
The corner frequency is set by the front-panel knob and sweeps from roughly 12 Hz to 490 Hz. Fully clockwise the filter is effectively out of the circuit — a bass guitar’s fundamentals pass at full strength. Turning counter-clockwise raises the corner: first boom and mud go, then body, and at the far counter-clockwise end what remains is a tight, cutting core that stays clear under heavy distortion. Like every knob on the pedal, clockwise means more.
Because the filter is placed ahead of the notch and the drive stages, its setting changes how those stages behave, not just what you hear at the output. Two practical consequences:
Low-end cleanup before distortion tightens the drive and reduces intermodulation — the low notes stop smearing the high ones.
The notch filter that follows becomes more precise once the low end is controlled; the two are meant to be used together.
Specifications
Input impedance
4.7 MΩ in parallel with 100 pF
Input type
Instrument or line level, unbalanced, 1/4″; clean to ±6 V peak
Preamp gain
+6 dB (2×), fixed
Filter type
4th-order Butterworth high-pass, two Sallen-Key stages
Series resistance, RF filter, clamp diodes to supply rails
Internal supply
+15 V regulated (14.8 V typical), 7.5 V reference (7.4 V typical)
Serviceability
Each board receives its supply and its reference voltage through small isolation resistors — 4.7 Ω on the supply, 22 Ω on the reference — feeding local filter capacitors. Their first job is electrical: they keep noise on the shared rails out of the board and keep the board’s own currents out of its neighbours. Their second job is practical: under a fault that draws heavy current, the small entry resistor is the part designed to fail first, before heat can spread to the rest of the board. They are not rated fuses, but in practice a single inexpensive part opening is what a fault looks like, and it is replaceable.
Notes
The +6 dB of preamp gain is intentional and is accounted for in the rest of the pedal’s gain structure; bypass and engaged levels are matched at the output stage. Suggested settings and the knob’s interaction with the notch and drive stages are covered in the user’s guide.
This pedal was built to be tried on things pedals are not usually tried on. Bass, synthesizers, drum machines, vocals, a whole mix — the input and the filter will treat them all the same, and the results are often the interesting part. If you find something that works, or something that doesn’t, tell us. The design will keep improving, and that feedback is how.
What this section does
This is the heart of the pedal: the stage that turns a clean signal into a distorted one, and the two additional clipping circuits that sit alongside it. Everything before it exists to decide what this stage is fed; everything after it exists to decide what leaves. Three distinct voices live here, each switchable from the panel — a high-gain amplifier clipping into LEDs, a hard clipper to ground, and a two-transistor fuzz — and they can be used alone or stacked.
The drive amplifier is a descendant of the classic LED-clipped distortion circuit, rebuilt around this pedal’s full-frequency philosophy. The original design deliberately thinned the low end inside the feedback loop, because on nine volts a full-range signal would have overwhelmed it. This one does not need to: with a 15 V rail and a precision reference underneath it, the whole signal can be amplified and the decision about what to keep is left to the filters and to the player.
The drive amplifier
A single precision amplifier is configured as a non-inverting stage with the Distortion control as its feedback resistor — a 50 kΩ audio-taper potentiometer. From the amplifier’s inverting input, three resistor-and-capacitor legs run to ground in parallel. Each leg sets how much gain the stage has in a particular part of the spectrum, and where that leg’s contribution begins:
Leg
Values
Takes effect above
Ultimate gain
Full-band
390 Ω + 94 µF
4.3 Hz
129× (42 dB)
Upper-mid
270 Ω + 220 nF
2.7 kHz
186× (45 dB)
Presence
390 Ω + 47 nF
8.7 kHz
129× (42 dB)
The full-band leg is the important departure. Its capacitor is large enough — two 47 µF in parallel — that its corner sits at 4.3 Hz, below anything an instrument produces. The stage therefore amplifies the entire signal equally from the bottom of hearing upward, where the classic circuit would have started rolling the bass away around 100 Hz. A five-string bass’s low B is distorted, not thinned before distortion.
The two remaining legs add gain on top of that, progressively, as frequency rises: about 3 dB more through the upper midrange and another 3 dB above it. The result is a drive characteristic that is flat through the bass and lower mids and tilts gently upward through the presence region — roughly 42 dB at the bottom and 52 dB at 10 kHz with the control wide open. That tilt is deliberate. Clipping compresses; a signal that arrives with slightly more energy where the pick attack lives comes out the other side with its articulation intact.
Because all three legs share the same feedback resistor, turning the Distortion control scales all of them together. The voicing does not change as the gain changes, which is why the pedal sounds like itself at every setting rather than becoming a different circuit at the extremes.
Symmetry
The drive amplifier clips into light-emitting diodes placed across its feedback path — three in each direction, in series, with a 100 Ω resistor in the return path. Two things about that arrangement matter.
It is symmetric by construction. The positive and negative halves of the waveform see the same number of identical devices, so both halves begin to round at the same voltage and in the same way. Asymmetric clipping produces even-order harmonics and a characteristic lopsided tone; this circuit produces odd-order harmonics from a symmetric transfer curve, which is the cleaner, more amplifier-like result. Because all six devices come from the same reel and sit within millimetres of each other on the board, they also track each other in temperature — the symmetry holds as the pedal warms up.
LEDs clip softly, and three of them clip late. A light-emitting diode turns on around 1.6 V rather than the 0.6 V of a silicon diode, so the stage amplifies cleanly far longer before any clipping begins. Three in series set the threshold near 4.8 V — high enough that at low Distortion settings the circuit is a clean, high-headroom preamp, and the transition into distortion happens gradually as the signal grows. The 100 Ω series resistor softens the knee further: as the diodes conduct, the current through them develops a small voltage across it, so the clipping level rises slightly with signal level instead of flattening abruptly.
This is also why the drive stage has so much usable range. With the clipping threshold at 4.8 V on a rail that swings to 7 V, the amplifier is never slammed against its supply; the diodes always act first, and they act gently.
The hard clipper
The second voice is a hard clipper: a pair of diode branches from the signal to ground, engaged by a panel switch. Where the drive amplifier’s LEDs round the waveform inside the feedback loop, this circuit flattens it from outside — a firmer, more compressed, more aggressive result.
Each branch is a light-emitting diode in series with a small-signal silicon diode, setting a threshold near 2.4 V. The combination is deliberate. The LED provides most of the threshold and its abrupt turn-on; the silicon diode adds a gentle exponential knee over the couple of hundred millivolts before it fully conducts, which rounds the corner of the clipped waveform and takes the glassiness out of what would otherwise be a hard edge. Engaged, it sits about 6 dB below the drive amplifier’s own clipping threshold, so the two stages clip in sequence rather than fighting: the LEDs round the peaks first, then the hard clipper flattens what remains.
The fuzz
The third voice is a two-transistor fuzz on its own section of the board, with its own ground island and its own supply filtering, fed through a level control that sets how hard it is driven. It is the most extreme of the three — a circuit that collapses rather than clips, with the gated, torn character that only a saturating transistor pair produces.
It carries a built-in bass cut between its two stages, with a corner near 150 Hz. That is characteristic of the topology and it is left in place deliberately: a fuzz fed the full low end turns to mush, and thinning the signal between stages is what keeps it articulate. The practical consequence for the player is counterintuitive and worth knowing — the fuzz wants more low end coming in than the other two voices do, because its own circuit is going to remove some.
Because the fuzz’s clipping point is set by transistor saturation rather than by diodes, it is the one voice whose threshold is set by the player: its input level control decides how far into collapse it goes. Set so that it is driven to roughly a third of the hard clipper’s amplitude, the three voices clip in a deliberate sequence — each taking a portion of the waveform the previous one left, and each leaving the harmonics of the one before it audible underneath.
How the bias is maintained
Every stage on this board is referenced to the pedal’s 7.5 V precision reference rather than to a local resistor divider. The drive amplifier’s non-inverting input is held there through a 100 kΩ resistor; the output buffer’s two halves are each held there through their own 100 kΩ; the fuzz’s bias network is derived from the same reference. Because that reference is servo-regulated with an output impedance in the milliohms, every stage sits at the same voltage, and none of them can move the others.
That matters more in a distortion circuit than anywhere else, for a specific reason. Clipping is defined relative to the bias point: the signal swings above and below it, and the diodes conduct when the difference exceeds their threshold. If the bias drifts — with load, with temperature, with supply sag — the two halves of the waveform stop clipping at the same level and the symmetry described above is lost. A bias that does not move is what keeps a symmetric clipper symmetric under real playing conditions.
The board also takes its supply and reference through its own isolation resistors into local filtering, and the fuzz section has a further filtered sub-ground of its own. Each stage is decoupled locally at the pin. The intent throughout is that no stage’s current can appear as another stage’s reference noise.
How the corner frequencies are maintained
Two kinds of corner frequency exist on this board, and both are held deliberately.
The coupling corners. Every capacitor that passes signal from one stage to the next is sized against the resistance it works into so that its corner lands at or below 2 Hz. Nothing in the audio band is rolled off between stages; the low end that arrives at the board is the low end the drive amplifier sees, and the low end the drive amplifier produces is what reaches the output. Where a corner exists at all, it is the one in the fuzz, which is there on purpose.
The voicing corners. The three feedback legs’ corners — 4.3 Hz, 2.7 kHz and 8.7 kHz — are the drive stage’s voice, and they are set by capacitors chosen for stability rather than by whatever was cheapest. Their values do not shift with signal level, with temperature, or with the supply voltage, so the stage sounds the same cold as it does after an hour. And because the legs are referenced to the board’s own filtered ground rather than to the chassis, their corners are not modulated by currents from elsewhere in the pedal.
The consequence, taken together with the bias arrangement, is a drive circuit whose character is fixed by its component values and by nothing else — not by the adapter, not by how long it has been on, not by what the rest of the pedal is doing.
Output
After the drive stages the signal passes through a precision dual amplifier configured as buffers, with the notch filter’s wet/dry blend between them, and leaves through a 100 Ω series resistor. The buffer isolates the clipping stages from whatever comes next so that cable capacitance and the following board’s input impedance cannot load them, and the series resistor keeps the output stable into a long cable.
Specifications
Drive stage
Non-inverting precision amplifier, gain set by a 50 kΩ audio-taper control
Maximum gain
≈ 42 dB at low frequencies, rising to ≈ 52 dB at 10 kHz
The board receives its supply and reference through small isolation resistors into local filter capacitors, with the fuzz section further isolated through its own resistor and ground island. Under a fault the small entry resistor is the part designed to fail first, and it is replaceable.
Notes
Figures 1 and 2 are computed from the circuit’s component values; measured sweeps will be added as bench data becomes available. Note that a gain curve measured at high Distortion settings will differ from the computed one, because the clipping diodes are conducting and the stage is no longer linear — the computed curves describe the amplifier’s voicing, which is what sets the character of the distortion, not its behaviour once it is clipping.
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.
The same idea a guitar already uses — done properly
Every electric guitar has a tone knob, and it is a low-pass filter: one capacitor and one high-value potentiometer, usually 250 kΩ or 500 kΩ, rolling the treble off the pickup. It is the simplest filter there is — a single pole, 6 dB per octave — and there is a reason it has survived seventy years unchanged. A single pole is gentle. It darkens the sound without erasing the leading edge of a note, because it barely disturbs the timing relationship between the frequencies passing through it. Guitarists trust their tone knob precisely because it never makes the instrument sound closed or smeared.
The trouble is that a single pole is also weak. Rolled far enough to remove the fizz from a driven amplifier, it has already taken most of the presence with it, because at 6 dB per octave everything above the corner comes down together at the same lazy rate. So players end up stacking filters: the guitar’s tone control, then the amplifier’s treble control, sometimes a third somewhere in a pedal. Each of those is designed on its own, in a different circuit, with a different corner and a different behaviour, and nothing coordinates them. Stacked, they interact in ways nobody designed — the corners land where they land, the phase relationships between them are arbitrary, and the result smears transients in exactly the way a single tone control does not.
Aurora Curtain is the same idea done four times, on purpose, with all four poles placed where they were meant to go. Four poles on one shaft, aligned to a Bessel response so the group delay stays flat and the attack of a note survives the filtering, with the last two deliberately held back so the first octave above the corner rolls off gently and only the region above it gets the steep treatment. It is a tone control with the authority of a fourth-order filter and the manners of a single-pole one.
The practical implication is worth stating: when this pedal is in the chain, the other filters in the signal path do not need to do anything. Leave the guitar’s tone control wide open and the amplifier’s tone stack flat or neutral, and let Aurora Curtain do the work. That is not a limitation imposed by the pedal — it is the point of it. One control, designed as a whole, in place of three that were never designed to work together.
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 is given a gain of 1 + 2.7 kΩ / 10 kΩ = 1.27, which is not a level choice but a shape choice: in an equal-component Sallen-Key section the gain sets the Q, and 1.27 produces Q = 0.578 — a second-order Bessel section to three decimal places. A Bessel alignment is the one that preserves the shape of a waveform passing through it: its group delay is flat, so every frequency in a transient arrives at the same time and the attack of a note is not smeared. The trade is a gentler knee than a Butterworth or Chebyshev, which for a musical filter is a feature rather than a cost.
The gain the two sections add (1.27 × 1.27 = +4.2 dB) is paid back by a fixed divider at the board’s input, so the section is level-neutral: a 33 kΩ / 68 kΩ divider attenuates by 3.4 dB and the board comes out within a dB of unity.
The stop resistors, and why this filter is unusual
A conventional four-pole filter puts all four of its poles at the same frequency and sweeps them together. Do that with a Bessel alignment tuned dark and the result is a curtain in the literal sense — everything above the corner disappears at 24 dB per octave and the instrument loses not just its fizz but its presence and its air at the same rate.
Aurora Curtain does something different. Across the second section’s two gangs sit a pair of 43 kΩ resistors. They do nothing at the open end — in parallel with a pot approaching zero ohms they are invisible — but as the knob turns toward dark they progressively limit how far that section’s resistance can rise. The first section is free to sweep the full 200 kΩ; the second is held to about 43 kΩ. The two sections therefore separate as the control is turned down:
Knob position
Sweeping section
Limited section
Board −3 dB
Fully clockwise (open)
102 kHz
102 kHz
58 kHz
Halfway
13.9 kHz
21.6 kHz
9.3 kHz
Fully counter-clockwise (dark)
2.4 kHz
12.0 kHz
1.8 kHz
At the dark end the filter is therefore not a 24 dB/octave slope at 2.4 kHz. It is a 12 dB/octave slope beginning at 2.4 kHz, which steepens to the full 24 dB/octave once the signal passes 12 kHz and the second section joins in. The audible consequence is a filter that darkens without closing: the first octave above the corner rolls off at half the rate a conventional four-pole would, so the instrument keeps its body and its definition, and the steep part of the slope is reserved for the region where only clipping artefacts live.
Across the knob
The potentiometer is a 200 kΩ reverse-log (C-taper) four-gang, chosen so that frequency moves evenly with rotation rather than bunching at one end. With the taper’s 15 % midpoint, the control distributes as follows:
Rotation toward dark
Board −3 dB
Response at 20 kHz
What it is doing
0 % (fully open)
58 kHz
−0.3 dB
Out of the way
10 %
37 kHz
−0.8 dB
Trimming air
25 %
21 kHz
−2.7 dB
Removing fizz
50 %
9.3 kHz
−12.8 dB
Audible shaping
75 %
4.2 kHz
−30.2 dB
The main tone-control region
100 % (fully dark)
1.8 kHz
−47.8 dB
Dark, without collapsing
What it addresses, and what it does not
Fully open, both sections sit at 102 kHz and the board is flat to within 0.3 dB at 20 kHz — roughly a third of a decibel at the top of hearing, with the group delay of a Bessel alignment. For practical purposes the filter is out of the circuit: it is not tone-shaping, not dulling, and not something the player has to work around. That is deliberate. A filter that cannot get out of the way is a filter you end up fighting.
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.
One further consequence of the placement is worth stating plainly: because Aurora Curtain is after the drive, turning it down does not make the pedal cleaner. The distortion has already happened. It makes the same distortion darker, which is a different and more useful thing.
Specifications
Filter type
4th-order Sallen-Key low-pass, two cascaded Bessel sections, deliberately staggered
Section alignment
Q = 0.578 per section (2nd-order Bessel), gain 1.27 each
Ultimate slope
24 dB/octave; 12 dB/octave through the first octave above the dark-end corner
Unity within 1 dB (filter gain offset by input divider)
Amplifier
OPA4191 precision quad, rail-to-rail, JFET-grade input current
Internal supply
+15 V regulated, 7.5 V precision reference
Components
OPA4191 — precision quad op-amp: rail-to-rail input and output, 5 µV offset, 20 pA bias current, 140 µA per channel. The rail-to-rail input matters here: in clean mode the signal can approach ±7 V and a conventional amplifier would run out of common-mode range before the filter did.
330 pF C0G — Class 1 ceramic: capacitance independent of voltage and temperature, no piezoelectric effect, distortion below film. The filter’s corner frequency is only as stable as its capacitors.
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 its supply through a 4.7 Ω isolation resistor and its reference through 22 Ω, each into local filtering, with the reference capacitors returning through a quiet-ground island. Under a fault the small entry resistor is the part designed to fail first, and it is replaceable.
Notes
Figures 1–3 are computed from the circuit’s component values and the potentiometer’s taper; measured sweeps will be added as bench data becomes available. The staggered arrangement is the part of this design worth knowing about: a conventional fourth-order low-pass is a well-understood circuit, and it is not what is in here.
