MiniMod Vintage Transistor Core VCO

AJH Synth 14HP
VCO OSCILLATOR

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Discrete all-transistor Moog Model D VCO emulation. Matched PNP expo converter and P-FET buffer, four simultaneous outputs including Sharktooth, CV-controllable PWM, hard sync, LFO mode. Normalled 1V/Oct from bus.

Patch Ideas · 7

★ Classic Model D voice
Saw to ladder filter · 8' octave · Envelope into filter cutoff · keyboard V/oct via bus · thick monosynth bass matching the original design intent
▸ Show diagram
Patch diagramPatch diagram with 5 modules and 5 connections. Modules: VCO, Filter, Envelope, VCA, Output. Signals: 3 audio, 2 cv.VCOFilterEnvelopeVCAOutputOctave: 8'SawaudioInaudioCutoffcvOutaudioOutcvInaudioCVcvOutaudioInaudioaudiocv
PWM string pad
Square out · slow triangle LFO into PWM-CV at ~70% · PWM Shape at noon centered · soft pad character with continuously shifting width
▸ Show diagram
Patch diagramPatch diagram with 4 modules and 3 connections. Modules: VCO, LFO, Filter, Output. Signals: 2 audio, 1 cv.VCOLFOFilterOutputOctave: 8'PWM CV: 70%PWM SHAPE: noonPWM-CVcvSquareaudioTricvInaudioOutaudioInaudio11. classic PWM sweepaudiocv
★ Dual-VCO hard sync lead
Master VCO free-running · slave VCO Sync Switch = S2 · master Saw to slave SYNC · slave frequency swept by envelope · screaming sync lead
▸ Show diagram
Patch diagramPatch diagram with 6 modules and 3 connections. Modules: VCO 1 (master), VCO 2 (slave), Envelope, VCO 1, VCO 2, Filter. Signals: 2 audio, 1 cv.VCO 1 (master)VCO 2 (slave)EnvelopeVCO 1VCO 2FilterOctave: 8'Octave: 8'Sync Switch: S2SYNC LEVEL: 3OutcvSawaudioSYNCaudioEXP-CVcvSawaudioInaudioaudiocv
LFO with four waveforms
Octave switch = LO · use Saw, Triangle, Square, Shark outs as four independent modulation sources from one oscillator · Shark gives a unique near-triangle curve unavailable on most LFOs
▸ Show diagram
Patch diagramPatch diagram with 5 modules and 4 connections. Modules: VCO, Filter, VCA, Delay, Mixer. Signals: 4 cv.VCOFilterVCADelayMixerOctave: LOTrianglecvSquarecvSharkcvSawcvCutoffcvCVcvTimecvPan CVcvcv
★ Sharktooth bass
Shark output into filter · tuned to 32' for sub · the Shark waveform sits between saw and triangle harmonically giving a warm, punchy low end without the buzz of pure saw
▸ Show diagram
Patch diagramPatch diagram with 5 modules and 4 connections. Modules: VCO, Filter, Envelope, VCA, Output. Signals: 3 audio, 1 cv.VCOFilterEnvelopeVCAOutputOctave: 32'SharkaudioInaudioOutaudioOutcvInaudioCVcvOutaudioInaudioaudiocv
Exp overmodulation FM
VCO B (modulator) into EXP-CV · EXP CV attenuator at 120% (past unity) · dirty exponential FM with aliased harmonic crashes well beyond standard FM depth
▸ Show diagram
Patch diagramPatch diagram with 3 modules and 2 connections. Modules: VCO A, VCO B, Filter. Signals: 1 audio, 1 cv.VCO AVCO BFilterEXP CV: 120% (max)Octave: 8'EXP-CVcvSawaudioTrianglecvInaudio11. overmodulated FMaudiocv
Linear vibrato on bus
Bus-normalled V/oct from MIDI interface · small triangle LFO into LIN-CV · LIN CV attenuator low · gentle ±semitone vibrato without detuning the main pitch reference
▸ Show diagram
Patch diagramPatch diagram with 3 modules and 2 connections. Modules: VCO, LFO, Filter. Signals: 1 audio, 1 cv.VCOLFOFilterOctave: 8'LIN CV: ~20%LIN-CVcvSawaudioTricvInaudio11. subtle vibratoaudiocv

Behaviors

Bus-normalled 1V/Oct no cable in 1V/Oct jack

Picks up the distribution-bus CV line if the case routes one (e.g., from a MIDI/CV converter). Patching any cable defeats the normal. Lets MiniMod voices track a master controller without visible cabling, matching the MiniMod system design.

Four simultaneous waveforms always

SAW, Triangle, Shark, and Square outputs all emit at once. Sharktooth is the distinctive Model D waveform, a softened saw with a triangular down-slope unique to this core. Useful as a built-in waveshaper menu via the patch cable.

LO switch = LFO mode Octave switch set to LO

Drops the core into LFO range. With -5V at the 1V/Oct input the VCO reaches approximately 0.02Hz (one cycle per 50 seconds). All four waveform outputs remain available, making it a full-waveform LFO.

Warm-up drift powering up from cold

Vintage analog core takes ~15 minutes to stabilize. Tuning before warm-up drifts noticeably. After stabilization pitch holds better than the original Model D due to modern SMT expo converter.

High-frequency tracking loss V/Oct input driving core above 10kHz

The vintage core design does not maintain 1V/Oct law above ~10kHz. Tracking flattens at the top. Matches the original Model D behavior — relevant if used as a modulation source at high rates rather than at pitch.

Hard sync threshold SYNC input with S1/S2 = S2

Sync reset is level-sensitive, set by Sync Threshold. Start fully CCW and turn up gradually. Classic 'laser harp' ripping sync sounds emerge when a swept slave VCO is synced to a fixed master. Falling ramps need inversion; saw outputs from other MiniMod VCOs are ideal.

PWM Shape as CV offset PWM CV patched while PWM Shape is adjusted

Under CV control the manual Shape knob becomes an offset. Lets the user center the PWM sweep — e.g., set Shape to 30% then modulate, so the duty cycle wobbles asymmetrically around that base width.

Controls

Global Octave Switch Rotary selector for preset octave ranges. LO setting drops frequency into LFO range for modulation use.
LO · 32' · 16' · 8' · 4' · 2'
Global TUNE Fine tune control. Approximately ±8 semitone range around the selected octave.
±8 notes · multi-turn trim behind panel for coarse set
Global EXP CV Attenuator for the Exp CV input. Ranges 0-120%, allowing overmodulation of the exponential core.
0–120%
Global LIN CV Attenuator for the Linear CV input. Modulates pitch subtly through the P-FET buffer stage.
0–100% · up to ±7 notes at full
Sync SYNC LEVEL Threshold/sensitivity for hard sync. Adjusts how much of the sync source reaches the core. Position 2–4 is typical with another MiniMod VCO as source.
0–10 · start CCW · very level-sensitive
Square output PWM CV Attenuator for PWM CV input. A triangle LFO at full gives classic sweeping PWM.
0–100% · input ±5V recommended
Square output PWM SHAPE Manual pulse-width control. Noon = 50% duty cycle. Remains active under PWM CV as offset.
~10%–90% duty · noon = square
Sync Sync On/Off Switch (S1/S2) Toggle to quickly enable/disable hard sync. S2 (down) = sync active; S1 (up) = bypassed. S1-triggered sync mode is no longer supported.
S1 = off · S2 = hard sync on

I/O

IN · 5

  • SYNC ±5V rising ramp GATE
    Hard sync input. Rising ramp (saw) of ±5V resets the core waveform. Falling ramps need inversion to trigger reliably.
  • 1 V/OCT ±7V accepted · tracking degrades above 10kHz CV
    Primary pitch input at exactly 1 volt per octave. Normalled to the Eurorack power bus CV line; patching a cable defeats the normalled signal.
    NORM → Eurorack distribution-bus 1V/Oct CV
  • EXP-CV ±5V CV
    Exponential modulation input. Behaves like 1V/Oct but attenuated via Exp CV knob. Accepts ±5V.
  • LIN-CV ±5V CV
    Linear modulation input. Through-zero style but not TZFM; produces subtle pitch modulation up to ±7 semitones.
  • PWM-CV ±5V CV
    PWM control voltage input for the square output duty cycle.

OUT · 4

  • SAW ±5V · 1kΩ out
    Rising ramp / sawtooth output.
  • Triangle ±5V · 1kΩ out
    Triangle wave output.
  • Shark ±5V · 1kΩ out
    Sharktooth waveform from the original Model D. Unique spectral character sitting between saw and triangle.
  • Square ±5V · 1kΩ out · 10–90% duty GATE
    Square / pulse wave output with duty cycle set by PWM Shape and PWM CV.