AJH Synth MiniMod VCF Eurorack Module
The stuff of legends, the Moog transistor ladder filter changed the face of the musical landscape forever. The MiniMod VCF is a faithful replica of the 24db/octave low pass filter used in the early Model D synthesiser.
There is a three channel input mixer and a nice large filter cutoff control, along with “1/3” and “2/3” CV inputs to emulate the action of the original VCF keyboard switches, apply 1v/oct to either of these inputs and the VCF keyboard tracking will be the same as the original with Keyboard 1 (1/3) or 2 (2/3) engaged, or use the 1v/oct input to emulate both switches selected. There is a variable Exponential CV input which is about 30% more sensitive than the 1v/oct input, this allows some nice over modulation effects.
We have added Voltage Controlled Resonance (Emphasis) in addition to manual Emphasis control. This uses a vactrol as the control element. The fast vactrol used allows modulation speeds up into the audio range. The filter will of course self resonate and act as a sine wave oscillator too.
The Moog transistor ladder filter is a 24db/octave slope low pass design which was patented by Dr Bob Moog on 28th October 1969. It has gone on to become possibly the most famous filter ever, and since the expiry of the patent it has also became the most cloned and copied filter in the world too. But not all ladder filter replicas are born equal......
We initially thought that this would be an easy module to re-create, but it is only after considerable development work that we feel that we have now “nailed” the early ladder filter sound and captured its magic. We have taken great care to get as close to the original sound as possible, so we replicated the same circuitry and component values of the Early Model D VCF. In early incarnations of this filter all of the ladder transistors were hand matched pairs, whereas in the later filters only the top and bottom ladder pair were matched, presumably to save time and production cost as production volumes increased.
This change is cited as being responsible for the slightly better sound of the early Model D ladder filters, so in this application we use original specification TO-92 format transistors rather than SMD transistor packages so that we can hand select and match all of the ladder transistors, just like the early models.