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AJH Synth

MiniMod VCO

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    Beschrijving MiniMod VCO

    The discrete transistor core of this Voltage Controlled Oscillator is an exact emulation of the original and now very rare R A Moog  Model D Oscillator from 45 years ago, and the resulting waveforms and sounds are exactly as the original, as big, fat and sweet as ever!

    It features front panel  switchable octave selection and four independent waveform outputs including the famous “Sharktooth” wave, Sawtooth wave, Square wave and Triangle wave.

    The Square wave output now has fully adjustable Pulse Width Modulation, with manual and CV control rather than the more restricting selection of two static fixed pulse widths or square wave of the original design.

    We have added extra features too, including exponential and linear modulation inputs and oscillator sync which help to make this a very versatile Eurorack VCO in its own right.


     When discussing the Model D oscillators, enthusiasts often refer to the “early” or “late” model VCO cards, however  contrary to popular belief there were not two but three different types of oscillator cards fitted to the Model D during it's lifetime, comprising: 

    1 –The very early and now very rare all discrete transistor RA Moog model.

    2 - The “Early” CA3046 transistor array design with matched dual fet amplifier stage.

    3-  the “Late” UA726 and op-amp based VCO, which was the most common model.

    When we decided to recreate the Model D in Eurorack format we tested, evaluated and built prototypes of all three VCO designs before settling upon the very early design, it just sounded slightly “sweeter” to our ears.

    The very early oscillator cards were a fully discrete transistor design.  Of the 17,000+ Model D’s produced less than 300 featured this oscillator. Early Model D’s fitted with this VCO card are the most highly prized of all Model D’s, and currently sport asking prices of up to US $15,000. Many Model D enthusiasts regard this as the sweetest sounding and fattest of the three VCO, however it was quickly discontinued and replaced with the 3046 design because the temperature stability was terrible and customers complained that it would not stay in tune! This was caused by a design problem in the CV summing amplifier circuitry, it was not fully temperature compensated and the pitch would drift due to heat from the power supply or changes in ambient conditions.

    To address the original temperature stability issue we have redesigned the Control Voltage summing amplifier front end of the VCO and upgraded it by using precision op-amps and voltage references which were unavailable back in the day when this oscillator was designed. This successfully addresses the temperature stability problem and the good news is that the modifications which we have made are to the voltage control element of the VCO only, we did not need to make any changes to the audio and wave shaping parts of the circuitry, so the resulting sounds and waveforms remain unchanged. The audio circuitry is still identical to the original and retains the PNP current source and PFET transistor voltage follower along with the fast switch dual transistor Schmidt trigger core, so that it retains all of the sonic characteristics of the original design. We have used identical resistor, capacitor values and transistor specifications and it remains true to the original in all ways except for temperature stability, which is now vastly improved.


    More information you can find here: AJH MiniMod VCO

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