AJH Synth MiniMod Dual Contour Eurorack Module
Two independant Contour Envelope Generators in a single 12hp wide Eurorack module. These are very accurate replicas of the Model D contour generators, with identical circuitry and component values running on the correct +9.3V and -10v supply voltages of the original; so they exactly replicate the original envelope response along with all of it eccentricities. Extra features include manual trigger buttons and envelope status LED’s
The contour generators played a larger part in the sound of the Model D than many people realise. There are two factors that make them unique, firstly the exponential slope when is used in conjunction with the (near) linear slope of the original VCA which is responsible for the famous Model D “punch”. Secondly, the re-trigger behaviour is quite unique, multiple retriggering increases the output level so that either the volume increases incrementally or the filter opens incrementally with each extra key press.
Several design compromises were made on the original Model D Contour generators to keep complexity, component count and costs lower. These compromises actually made a considerable contribution towards the signature sound of this synthesiser.
Here are the features of the MiniMod Dual Contour generator in more detail:
The discrete transistor circuitry used has a rather unusual response when retriggered. This is probably best explained by looking at the retriggering behaviour of a regular ADSR envelope generator and comparing it with the Model D contour generator.
It can be seen that if a key is repeatedly pressed on the Model D before the A/D cycle has completed then the decay is not fully reset and the level of the following attack peak is higher than the previous one. The effect of this is that if applied to VCA each retrigger will be louder than the previous one, or if applied to a low pass VCF the filter will “open” more with each retrigger or key press. Obviously after three or four rapid key presses the maximum level will be reached and subsequent key presses will be at this maximum level. This effect is variable and depends on the Attack and Decay pot settings.
The Gate input will activate envelope generation with a positive going pulse between 2V and 12V and it is protected against negative going voltages, so any Eurorack Modular signal sources or control voltages can safely be connected to it. A 16 way Eurorack power connector is used, and by default both envelope generators will react to trigger pulses on the distribution bus, but this is easily overridden by plugging a patch lead into the Gate input. We have included a manual trigger button for each contour generator so that the gate can be manually activated; this is independent from the Gate input pulse and can safely be used at the same time.
We have added independant Decay switches for each individual Contour Generator rather than the shared switch on the original synthesiser, these switches change the Contour response from AD (switch up) to ADSD mode (switch down).