MiniMod Transistor Ladder Filter

AJH Synth 14HP
VCF FILTER DISTORTION

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Faithful 24dB/oct transistor ladder LPF replica of the early Model D filter. Matched discrete ladder transistors, three-input audio mixer, vactrol VC resonance, Model D 1/3 and 2/3 keyboard tracking CVs, internal overdrive jumper.

Patch Ideas · 7

★ Model D bass tracking
Sequencer V/oct → 1V/Oct filter input · VCO into IN 1 · Envelope into FREQ-CV with FREQ CV at 2 o'clock · EMPHASIS ~5 · classic Moog-style filter envelope with full keyboard tracking
▸ Show diagram
Patch diagramPatch diagram with 6 modules and 6 connections. Modules: VCF, VCO, Sequencer, Envelope, VCA, Output. Signals: 3 audio, 1 cv, 2 pitch.VCFVCOSequencerEnvelopeVCAOutputEMPHASIS: 5FREQ CV: 2 o'clock (pos 7)1V/Oct1v/octIN 1audioFREQ-CVcvOUTaudioV/Oct1v/octSawaudioPitch1v/octOutcvInaudioOutaudioInaudioaudiocvpitch
Two-thirds tracking lead
Pitch CV into 2/3 CV only · filter tracks at 1.5V/oct for that slightly-detuned 'half-follow' Model D character · EMPHASIS ~7 just below self-osc · bright but never fully opens up the top
▸ Show diagram
Patch diagramPatch diagram with 4 modules and 4 connections. Modules: VCF, VCO, Sequencer, Output. Signals: 2 audio, 1 cv, 1 pitch.VCFVCOSequencerOutputEMPHASIS: 7FREQ knob: noon2/3 CVcvIN 1audioOUTaudioV/Oct1v/octSawaudioPitch1v/octInaudio11. 1.5V/oct trackingaudiocvpitch
★ Three-oscillator fat unison
Three detuned VCOs into IN 1-3 · levels balanced ~70% each · J1 jumper removed for pre-filter overdrive · EMPHASIS ~4 keeps the fatness without squealing · classic thick Moog bass
▸ Show diagram
Patch diagramPatch diagram with 7 modules and 6 connections. Signals: 5 audio, 1 cv.VCFVCO 1VCO 2VCO 3EnvelopeVCAOutputJ1 jumper: REMOVED (overdrive)INPUT 1-3: ~70%EMPHASIS: 4IN 1audioIN 2audioIN 3audioFREQ-CVcvOUTaudioSawaudioSawaudioPulseaudioOutcvInaudioOutaudioInaudioaudiocv
★ Sine VCO from self-oscillation
No audio input · EMPHASIS at 10 · 1V/Oct from sequencer · filter sings as a sinewave VCO tracking ~3-4 octaves · FM the cutoff via FREQ-CV for timbral motion
▸ Show diagram
Patch diagramPatch diagram with 4 modules and 3 connections. Modules: VCF, Sequencer, LFO, Output. Signals: 1 audio, 1 cv, 1 pitch.VCFSequencerLFOOutputEMPHASIS: 10 (self-osc)INPUT 1-3: 01V/Oct1v/octFREQ-CVcvOUTaudioPitch1v/octOutcvInaudio11. timbral FMaudiocvpitch
Audio-rate resonance FM
VCO sine into EMP-CV · EMP CV knob ~5 · filter IN 1 fed by a second VCO · EMPHASIS 6 · vactrol modulates resonance at audio rate producing FM-like sidebands unique to this filter
▸ Show diagram
Patch diagramPatch diagram with 5 modules and 4 connections. Modules: VCF, VCO A, VCO B, VCA, Output. Signals: 3 audio, 1 cv.VCFVCO AVCO BVCAOutputEMPHASIS: 6EMP CV: 5IN 1audioEMP-CVcvOUTaudioSawaudioTrianglecvInaudioOutaudioInaudio11. audio-rate resonance modaudiocv
Dirty Model-D trick
J1 removed · EMPHASIS ~8 (just oscillating) · feed audio into IN 1 at maximum · VCA after filter attenuates the boosted output · saturated resonance growl like the vintage headphone-feedback trick
▸ Show diagram
Patch diagramPatch diagram with 5 modules and 5 connections. Modules: VCF, VCO, Envelope, VCA, Output. Signals: 3 audio, 2 cv.VCFVCOEnvelopeVCAOutputJ1 jumper: REMOVEDINPUT 1: 10 (max)EMPHASIS: 8IN 1audioFREQ-CVcvOUTaudioSawaudioOutcvInaudioCVcvOutaudioInaudio11. VCA turned down to compensateaudiocv
Overmodulation sweep
FREQ CV knob at maximum (130%) · wide envelope into FREQ-CV · filter cutoff is driven past its normal limits giving an exaggerated, dramatic sweep with pronounced pop on envelope peaks
▸ Show diagram
Patch diagramPatch diagram with 4 modules and 3 connections. Modules: VCF, VCO, Envelope, Output. Signals: 2 audio, 1 cv.VCFVCOEnvelopeOutputFREQ CV: max (130%)EMPHASIS: 6IN 1audioFREQ-CVcvOUTaudioSawaudioOutcvInaudio11. overmodulated sweepaudiocv

Behaviors

Three keyboard-tracking CV inputs applying V/oct to 1V/Oct, 1/3 CV, or 2/3 CV

Recreates the Model D filter-keyboard-follow switches. 1V/Oct = both switches on (full tracking), 1/3 CV = 3V/oct (partial), 2/3 CV = 1.5V/oct (two-thirds). Summing them produces classic Moog filter tracking behavior without a dedicated switch.

Vactrol voltage-controlled resonance CV into EMP-CV

Resonance is controlled by a fast vactrol, acting purely resistively so the filter core sound is unchanged. Supports audio-rate modulation for FM-like resonance timbres. Manual Emphasis becomes an offset when CV is present.

Overmodulation via Freq CV FREQ CV knob past 2 o'clock (position 7)

Freq CV range extends to 130% versus the 1V/Oct input. Pushing past 1V/oct point creates overshoot and exaggerated tracking useful for filter FM timbres and wide modulation sweeps.

Self-oscillation as sine VCO EMPHASIS set 8–10 with no audio input

Filter self-resonates into a clean sine wave. Tracks V/Oct reasonably over 3–4 octaves (CV scaling is uncorrected, drifts slightly with ambient temperature, matching original Model D behavior). Playable as an additional VCO.

Internal overdrive jumper J1 jumper removed on PCB

Boosts pre-filter signal gain by approximately 70%, overdriving the ladder front-end. Emulates the vintage Model D trick of routing the headphone output back into the external VCF input. Output level rises proportionally, so compensate downstream.

Emphasis still active with CV patching EMP-CV while Emphasis knob set

Manual Emphasis knob is summed as offset under CV control. Allows biasing the resonance up to near-oscillation then modulating the peak region without going fully into self-oscillation.

Controls

Global FREQ Large cutoff frequency knob. Acts as offset when CV is applied. CCW fully closes; CW fully opens.
Full audio range · offset when CV present
Global FREQ CV Attenuator for the Freq CV Input. Ranges 0-130%, enabling overmodulation past 1V/oct tracking. 1V/oct point occurs at 2 o'clock (position 7).
0–130% · 1V/oct = pos 7
Global EMP CV Attenuator for the Emphasis CV input feeding the vactrol resonance circuit. Manual Emphasis still acts as offset when CV is present.
0–100% · vactrol response allows audio-rate modulation
Global EMPHASIS Manual resonance (feedback) control. Self-oscillates between positions 8 and 10 producing a playable sine wave.
0–10 · self-oscillation ~8+ · 3–4 octave tracking range in self-osc
Audio mixer INPUT 1 / INPUT 2 / INPUT 3 Three independent level knobs forming an input mixer into the filter core. All AC-coupled.
0–100% each · summed to filter input
Internal PCB Gain Jumper (J1) Removing the onboard jumper boosts pre-filter gain by ~70% for Model D style overdrive. Output level rises proportionally.
Installed: unity · removed: +70% pre-gain overdrive

I/O

IN · 8

  • IN 1 ±5V audio AUDIO
    Audio input 1 into the three-channel mixer. AC-coupled.
  • IN 2 ±5V audio AUDIO
    Audio input 2 into the three-channel mixer. AC-coupled.
  • IN 3 ±5V audio AUDIO
    Audio input 3 into the three-channel mixer. AC-coupled.
  • FREQ-CV ±5V typical CV
    External cutoff CV. Scaled by FREQ CV attenuator. Overmodulation possible past 100%.
  • 1V/Oct 1V/oct
    Fixed 1V/octave input. Emulates Model D keyboard switches 1 AND 2 on. Tracking drifts with temperature (uncorrected, per original).
  • 1/3 CV 3V/oct CV
    Scaled keyboard tracking input at 3V per octave. Equivalent to Model D keyboard switch 1 on, switch 2 off.
  • 2/3 CV 1.5V/oct CV
    Scaled keyboard tracking input at 1.5V per octave. Equivalent to Model D keyboard switch 2 on, switch 1 off.
  • EMP-CV ±5V CV
    Resonance/emphasis CV input. Fed through vactrol for smooth response up into audio rate.

OUT · 1

  • OUT ±5V typical · THD 0.008% AUDIO
    Filtered audio output after the ladder core. Unity gain with single input at 100%; +70% higher with gain jumper removed.