A Eurorack VCO module based on the AS3340 chip from Electric Druid — a drop-in clone of the classic Curtis CEM3340. Analog 1V/oct tracking with simultaneous saw, triangle, sine, and pulse outputs.

• 1V/oct tracking with two summed V/oct CV inputs (use both for transpose/unison)
• Course tune with jumper-selectable range (narrow / wide via JP1) plus Fine tune pot
• Linear FM input with attenuator pot (Lin FM, ±5V CV)
• PWM input with amount attenuator (±5V CV), feeds the chip's pulse-width modulator
• Hard / Soft Sync input with panel switch for selection (AC-coupled, 1 nF)
• Four simultaneous outputs (10 Vpp): Saw, Triangle, Sine (diode-shaped from triangle), Pulse
• Three calibration trims:
  • Scale trim (RV1) — V/oct gain
  • High-Frequency Tracking trim (RV7) — accuracy at high notes
  • Sine amplitude trim (RV6) — tri-to-sine shape
• Polystyrene timing capacitor (C8, 1 nF) for low temperature drift
• Eurorack ±12V power via either 16-pin IDC or 3-pin JST connector

   V/oct 1 ─┐
   V/oct 2 ─┼──[summing node, 10K + 10K + 47K (course) + 1M5 (fine)]─► AS3340 Freq Control
   Course  ─┤                                                        ├─► Saw    ─► [×2.09 buffer] ─► Saw Out
   Fine ────┘                                                        ├─► Tri    ─► [×2.09 buffer] ─► Tri Out
                                                                     │                          └► [diode shaper + RV6 amp trim] ─► Sine Out
   Lin FM ─► [×RV2 amount] ─► [bias network: 1M5/+12V, 470R/GND] ──► AS3340 Lin FM In           
                                                                     │
   PWM CV ─► [×RV3 amount + 100K/200K/100K conditioner] ───────────► AS3340 PWM In ◄─── [10K hysteresis from Pulse Out]
                                                                     │
   Sync in ─► [1 nF AC-couple] ─► SW1 ─► Hard Sync / Soft Sync ────► AS3340 Sync
                                                                     │
                                                                     └─► Pulse  ─► [×2.09 buffer] ─► Pulse Out

   Calibration trims:
     RV1 (Scale)  → AS3340 Scale1/Scale2 pins (V/oct gain)
     RV7 (HFT)    → AS3340 HFT pin (high-frequency correction)
     RV6 (Sine Amp) → tri-to-sine output buffer (sine amplitude)

   Eurorack ±12V ─► U3 (78M05) / U4 (79M05) ─► ±5V rails (op-amp + chip VEE)
   Timing cap: C8 = 1 nF polystyrene (critical: do not substitute ceramic)

• Eurorack ±12V via J10 (3-pin JST) or J11 (16-pin IDC) — populate one
• Local U3 (78M05) makes +5V from +12V
• Local U4 (79M05) makes −5V from −12V; this becomes the AS3340's VEE pin voltage
• R39 / R40 (10K) are optional minimum-load resistors
• D3 / D4 — reverse-polarity protection diodes

AS3340 VEE protection. The chip has an internal 7.4V Zener; the absolute max for VEE-to-GND is −6V. Because VEE here is driven from −5V (not −12V directly), no current-limiting resistor (REE) is needed.

If you ever modify the design to feed VEE directly from a more-negative rail, the datasheet formula is REE = (|VEE| − 7.4) / 0.008:

The board has three trim pots that interact. Adjust in this order. You'll need: a multimeter, a frequency counter or tuner (a phone tuner app works), and a stable temperature environment (let the module warm up ~10 minutes first — the chip drifts thermally during warm-up).

Step 1 — Scale trim (RV1). Sets the V/oct gain. This is the most important trim for musical accuracy.

1. With no CV patched and Course/Fine at 12 o'clock, note the output frequency.
2. Patch a known reference voltage into V/oct 1 (e.g., 0V from a calibrated source or shorted to GND). Tune Course/Fine until the saw output matches a reference note — say C2 (65.41 Hz).
3. Apply +1V to V/oct 1. The output should rise exactly one octave (C3 = 130.81 Hz). If it's high, lower RV1 slightly; if low, raise it.
4. Step the input voltage up to +4V or +5V and verify the output is still tracking 1V/oct. Iterate RV1 + the tune knob until accurate across 5+ octaves.

Step 2 — High-Frequency Tracking trim (RV7). Compensates for internal comparator delay at very high notes.

1. With the V/oct calibrated (Step 1), apply +6V or +7V to push the output to ~8 kHz+.
2. Compare actual output frequency to expected (e.g., from 65.41 Hz baseline, +7 octaves = 8372 Hz).
3. Adjust RV7 to correct the deviation. RV7 mostly affects high frequencies — it should not noticeably shift low-frequency tuning.
4. Re-verify Step 1 calibration. If low-frequency tracking shifted, iterate.

Step 3 — Sine Amplitude trim (RV6). Sets the tri-to-sine shape and amplitude.

1. Patch a scope on the Sine Out jack with a steady tone (e.g., A4 = 440 Hz).
2. Adjust RV6 until the waveform looks like a clean sine — not too peaked (under-trimmed) and not flat-topped (over-trimmed).
3. By ear: clean smooth fundamental, no harsh harmonics.

Local archived copies live in references/ so this repo stays useful if the upstream links die.

• AS3340 datasheet — local copy · upstream (alfatriode.lv)
• Kassutronics 3340 VCO docs (v2.4) — local copy · upstream (GitHub) — primary design inspiration
• Electric Druid AS3340 product page
• Electric Druid CEM3340 VCO designs
• YuSynth VCO
• Tri-to-sine shaping techniques (Tim Stinchcombe)

What's ready for builders today, and what's still on the TODO list:

Production assets (what you need to actually fabricate and assemble a final unit)

• Schematic — Rev 0.1.2 (PDF)
• PCB layout — in progress — single working layout in kicad/, not yet separated for fab. The current rev is labeled "Prototype."
• Gerber files for fabrication — none yet
• BOM — Rev 0.1.0 PDF (critical: use polystyrene for C8 timing cap, not ceramic)
• Final front panel (SVG/PDF for fab) — none yet
• License — none yet

Prototype assets (for breadboard / perfboard / 3D-printed-panel builds before final PCB)

• 3D-printed prototype panel STL — 3340_VCO.stl
• Falstad simulations of sub-circuits — falstad/

Documentation

• Photos of the assembled module — none yet (drop in photos/)
• Demo video — none yet
• Build / assembly instructions — none yet
• Calibration / tuning notes — see "Calibration" section above

Want to help fill a gap (build photos, gerbers, an assembly guide)? Open an issue or PR.