ajh-synth/minimod-transistor-ladder-filter
MiniMod Transistor Ladder Filter
Faithful discrete transistor replica of the Moog Model D 24 dB/oct four-pole lowpass VCF, with three-channel input mixer, vactrol-controlled voltage-controlled resonance (Emphasis), 1/3 and 2/3 CV inputs emulating original keyboard switches, variable exponential CV input, and optional overdrive jumper.339
Visualization filter_response
Capabilities 2
- Self-Oscillationself-oscillation
- Filter self-resonates as a sine wave oscillator when Emphasis Level is fully clockwise; pitch tracks 1V/oct via the 1V/Oct input.339
- Vcfvcf
- Four-pole 24 dB/oct lowpass ladder filter, faithful replica of the Moog Model D VCF with matched transistors and vactrol-controlled resonance.339
Jacks 7
| Name | Signal | Voltage | Description |
|---|---|---|---|
| Inputs (7) | |||
| 1 V/Oct Input | cv | — | Changes filter cutoff at 1V/oct; when filter is fully self-oscillating, controls pitch of the resulting sine wave at 1V/oct. Emulates having both Keyboard Control switches 1 and 2 On in the original Model D.339 |
| 1/3 CV Input | cv | — | Changes filter cutoff at 333 mV/oct (1/3 of 1V/oct). Emulates Keyboard Control switch 1 On and switch 2 Off on the original Model D.339 |
| 2/3 CV Input | cv | — | Changes filter cutoff at 666 mV/oct (2/3 of 1V/oct). Emulates Keyboard Control switch 2 On and switch 1 Off on the original Model D.339 |
| Audio Input 1 | audio | -5V to 5V | Part of a 3-input audio mixer at the filter's front end; mixed with Inputs 2 and 3 at the level set by Audio 1 Level control. AC coupled; DC signals will not cause harm but are not useful.339 |
| Audio Input 3 | audio | -5V to 5V | Part of the 3-input audio mixer; mixed with Inputs 1 and 2 at the level set by Audio 3 Level control. AC coupled.339 |
| Emp. CV Input | cv | — | External CV input for voltage control of filter Emphasis (resonance). Amount passed to filter core is set by Emp. CV Level control (0–100%).339 |
| Freq CV Input | cv | — | External CV input for voltage control of filter cutoff frequency; amount passed to filter core is set by the Freq CV Level control. Approximately 30% more sensitive than 1V/oct at maximum level.339 |
Parameters 5
| Name | Type | Range | Behavior |
|---|---|---|---|
| Cutoff Freq. | knob | — | Manually varies the filter cutoff frequency. At minimum (fully CCW) all frequencies are cut off; at maximum all pass. When using CV control, acts as an offset control.339 |
| Emp. CV Level | knob | 0% – 100% | Varies the amount of CV from Emp. CV Input sent to the filter core (0–100%). The manual Emphasis Level control remains active as an offset when external CV is in use.339 |
| Emphasis Level | knob | — | Manually regulates the amount of internal feedback (resonance) applied to the filter core. Acts as an offset when Emp. CV is in use. When fully clockwise the filter self-resonates as a sine wave oscillator.339 |
| Freq. CV Level | knob | 0% – 130% | Sets the amount of CV from Freq CV Input sent to the filter cutoff circuitry, 0–130%. At position 7 (2 o'clock) the response is 1V/oct from the Freq CV Input.339 |
| Gain Jumper | jumper | — | When the jumper is removed, the gain to the filter front end is doubled, substantially overdriving the filter core for added harmonic dirt. Reconnecting the jumper returns gain to normal levels.339 |
Firmware history 0 versions
No firmware history recorded.
References 2
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[339]
AJH Synth MiniMod VCF User Manual (Internet Archive)manual● verified 2026-05-14
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[340]
AJH Synth MiniMod VCF product page (2025-01-04 archive snapshot)product_page● verified 2026-05-14