Question

How can I configure the 'Quad Operator' FM Oscillator to produce squelching melodic acid tones?

Modules In Scope

• Humble Audio Quad Operator
• Humble Audio Quad Operator Algo Extension

Answer

I've read the full Quad Operator manual (and its Algo expander section). Here's a patch recipe built from the module's actual controls.

Squelchy Acid Tones on the Humble Audio Quad Operator

The trick to acid sounds on an FM oscillator: there's no filter, so the "squelch" comes from sweeping the modulation index — an envelope into a Gain CV input stands in for the filter-cutoff envelope of a 303, and operator feedback stands in for resonance.

1. Start from the manual's "Neutral Position"

Per the Achieving Harmonic Frequency Modulation Results section:

• VCO/LFO switch → VCO
• All four operators → Lock state (integer ratios to master tuning — essential for harmonic, melodic results)
• All Detune knobs → 12 o'clock
• All Shape knobs → full counterclockwise (sine)
• All Mod sends → full counterclockwise (no modulation yet)

2. Build a two-operator acid voice

Use Op 1 as the carrier and Op 2 as the modulator:

• Ratio 1 → ×1 (carrier at the fundamental)
• Ratio 2 → ×1 or ×2. A 1:1 ratio gives a saw-like spectrum (very 303); 2:1 gives a hollower, square-ish tone
• On Op 2's row, turn up the "Mod 1" knob — this is the send from Op 2 into Op 1's frequency. Set it around 10–1 o'clock as your baseline "cutoff" level
• Take your audio from the Op 1 output only

Optionally nudge Shape 1 slightly toward triangle/saw for extra bite, but keep Op 2 a sine — modulating with overtone-rich shapes gets noisy/aliased fast (the manual warns about this).

3. The squelch: envelope → Gain 2 CV

This is the key move. From the manual: "Gain CV — affects both the output level of the operator signal as well as how intensely it modulates other operators via its modulation sends. Normals to gain of 1."

• Patch a snappy decay envelope (fast attack, short-medium decay) into Gain 2. Note Gain CV expects 0–5 V; once patched, it replaces the normalled full gain
• Now every gate closes Op 2's virtual VCA: each note starts bright (high FM index) and sweeps dark as the envelope falls — exactly the wah/squelch of a filter envelope
• Decay time = your "cutoff sweep" speed; Mod 1 send level = your "cutoff/env amount"
• For accents, use a sequencer accent output to raise the envelope level (or mix a second envelope into Gain 2) on accented steps — louder envelope = brighter, harder squelch

Put a regular VCA + amp envelope on the Op 1 output after the module, as usual.

4. "Resonance": operator feedback

• On Op 2's row, also turn up "Mod 2" — Op 2 modulating itself. Small amounts add a growly, resonant edge that reads like filter resonance; too much tips into noise (easy to ride live)
• Because Gain 2 scales all of Op 2's sends, your envelope sweeps the feedback and the carrier modulation together — the squelch and the "resonance" breathe as one, which is very acid
• For more extreme resonance-style effects, the manual suggests the AR FM input: take an operator output, process it externally (e.g., through a wavefolder or filter), patch it back into AR FM, and use the AR FM row's Mod sends. Watch the red clipping LED and trim the Gain AR FM knob

5. Melodic sequencing, slides, and consistency

• Pitch sequence → the 1V/Oct input (controls all locked operators together, so the whole FM stack tracks in tune)
• Slides (the other half of the acid formula) → use your sequencer's glide/portamento on the 1V/Oct CV; alternatively patch a slewed CV into LF FM (±6 semitones) for bend effects
• Patch your gate/trigger into Reset to restart operator phase on each note — this keeps the FM transient identical note-to-note for a tight, consistent pluck

6. Bonus: Algo expander as a macro "filter knob"

Save a dark algorithm (low Mod sends) into slot A and a screaming one (high sends + feedback) into slot B, then use the Crossfade knob or its CV input to morph between them — a one-knob acid "cutoff macro" you can sweep or sequence over the pattern, or crossfade against the Live knob positions while you tweak.

Quick summary: Op 2 (sine, ratio 1–2×, in lock) FMs Op 1 via the Mod 1 send; a fast decay envelope into Gain 2 sweeps the FM index for the squelch; a touch of Mod 2 self-feedback fakes resonance; sequence via 1V/Oct with glide, trigger Reset per note, and take audio from Op 1.