Humble Audio — Quad Operator

• Manual PDF

Manual PDF

Humble Audio Quad Operator + Algo Expander Cheat Sheet

What it is

A 4-operator digital linear FM oscillator bank for Eurorack.

• 4 operators, each with:
• variable waveform: sine → triangle → square → saw
• independent output
• per-operator gain/VCA behavior
• modulation sends to any operator, including self
• Can run as:
• classic ratio-locked FM voice
• 4 independent oscillators in free mode
• quad/LFO modulation source
• Optional Algo expander:
• save/recall FM matrices
• crossfade between saved/live algorithms

Quick start

For classic harmonic FM

1. Set VCO/LFO to VCO
2. Put all 4 operators in LOCK
3. Set all Detune knobs to 12 o’clock
4. Set all Shape knobs fully CCW (sine)
5. Set all Mod 1–4 knobs fully CCW
6. Tune with Coarse and Fine
7. Use each operator’s Ratio to choose integer harmonic relationships
8. Turn up modulation sends gradually

For 4 independent oscillators

1. Put operator(s) in FREE
2. In free mode:
3. Ratio knob becomes coarse tuning
4. Ratio CV becomes 1V/oct for that operator
5. Patch from individual Op 1–4 outputs

For modulation/LFO use

1. Set VCO/LFO to LFO
2. Use Reset to restart phase
3. Keep operators in LOCK if you want phase-related complex modulation

Core concepts

LOCK vs FREE

LOCK - Operator frequency is an integer ratio of master pitch - Best for harmonic / classic FM sounds - Ratio knob selects approx x1/11 to x11 in integer steps - 1V/Oct, Coarse, and Fine affect all locked operators

FREE - Each operator becomes its own oscillator - Ratio knob = operator coarse tuning - Ratio CV = 1V/oct for that operator - Better for independent voices or inharmonic FM

Gain behavior

Each operator’s Gain CV controls: - its output level - how strongly it modulates other operators via its sends

So gain acts like a built-in VCA for both carrier level and modulation depth.

Mod matrix

Each row is a source: - Operator 1 - Operator 2 - Operator 3 - Operator 4 - AR FM external input

Each Mod 1–4 knob sends that source to the destination operator.

Example: - In Operator 2 row, Mod 1 = amount Operator 2 modulates Operator 1 - Self-modulation is allowed

Front panel controls

Master controls

• Coarse
• Master tuning
• Affects all operators in LOCK
• Exponential response over 8 octaves
• Fine
• Master fine tune
• Range: ±6 semitones
• VCO/LFO switch
• Sets frequency range for master tuning
• 1V/Oct input
• Pitch CV for all operators in LOCK
• LF FM input
• Low-frequency pitch modulation
• Best for vibrato, bends, envelopes
• Range corresponds to ±6 semitones
• AR FM input
• External audio-rate modulator input
• Reset input
• Resets phase of all operators

Per-operator controls

Each operator has:

• Lock/Free toggle
• LOCK = ratio-locked FM operator
• FREE = independent oscillator
• Ratio knob
• LOCK: integer ratio relative to master pitch
• FREE: operator coarse tuning
• Ratio CV jack
• LOCK: modulates ratio parameter
• FREE: becomes 1V/oct for that operator
• Detune knob
• Fine tuning
• Range: ±6 semitones
• Shape knob
• Crossfades sine → triangle → square → saw
• Shape CV jack
• CV over waveshape
• Gain CV jack
• Controls operator level and modulation intensity
• Normalled high, so operator is active without patching
• Mod 1–4 knobs
• Send this operator as FM to Operators 1–4
• Op output jack
• Individual output for that operator

External modulation section

For the dedicated AR FM input:

• AR FM jack
• External audio-rate modulator input
• AR FM Gain knob
• Adjusts external input level
• Set to avoid clipping
• Clip LED
• Red LED = external input clipping
• Gain AR FM CV jack
• VC over external modulation depth
• Normalled high
• Mod 1–4 knobs
• Send external AR FM source to Operators 1–4

Algo expander

Adds algorithm save/recall and crossfading of the modulation matrix.

What gets stored

Only modulation matrix values: - all Mod 1–4 knobs for: - Operator 1 - Operator 2 - Operator 3 - Operator 4 - AR FM input row

Slots

• A
• B
• C
• Live = current physical knob positions

Buttons / controls

• A / B / C / Live buttons
• Long press A/B/C until LED blinks twice = save to slot
• Short press = assign slot into crossfade selection
• Crossfade knob
• Morph between left/right selected algorithms
• Crossfade CV input
• CV control of algorithm morph

Important behavior

• If using only A/B/C slots, the physical Mod knob positions are ignored
• Double-tap Live if you want only current panel settings

Patch tips

Simple 2-op FM voice

• Op1 = carrier, monitor Op1 out
• Op2 = modulator
• Both in LOCK
• Op1 ratio = 1
• Op2 ratio = 2 or 3
• Op2 row: raise Mod 1
• Keep shapes near sine for cleaner FM

Feedback FM

• Use self-modulation by turning up an operator’s own destination knob
• Or patch an output externally and return it via AR FM

Layered outputs

• Since each operator has its own output, you can:
• mix multiple carriers
• process operators separately
• use one operator as audible carrier and others as CV/audio modulators

Voltage ranges and signal reference

Inputs

Outputs

Parameter ranges

CV scaling note

For Ratio and Shape parameters: - 5V of CV = full knob travel - Bipolar CV can sweep beyond current knob setting in either direction

Calibration

Use if pitch tracking seems off.

Patch:

• Op1 → Ratio1
• Op2 → Ratio2
• Op3 → Ratio3
• Op4 → Ratio4 and 1V/Oct via unbuffered split
• Important: use Op4 for the split

Trigger calibration:

1. Flip Operator 1 free/lock once
2. Flip Operator 2 free/lock once
3. Flip Operator 3 free/lock once
4. Flip Operator 4 free/lock once
5. Flip VCO/LFO switch 5 times quickly

Success indication: - Gain LEDs blink twice

Firmware / physical specs

• Firmware update via micro USB on back
• Manual mentions firmware version 1.0.4
• Quad Operator
• 30 hp
• 25 mm deep
• +12V: 140 mA
• -12V: 13 mA
• Algo expander
• 4 hp
• 25 mm deep
• +12V: 5 mA
• -12V: 0 mA

Best-use summary

• Use LOCK + sine + no detune for stable, harmonic FM
• Use FREE for separate oscillators or inharmonic textures
• Use Gain CV like a modulation VCA
• Use AR FM for external operators, feedback, or wild FM
• Use Reset in LFO mode for repeatable modulation shapes
• Use Algo to store and morph FM routings

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