AJH Synth MiniMod VCO Eurorack Module
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 two different types of oscillator sync which help to make this a very versatile Eurorack VCO in its own right.
The MiniMod VCO is fully integrated into Doepfer standard Eurocard format and uses the full 16 way power header, and is configured so that it receives the 1v/Oct input from the distribution bus by default. This can be overridden by connecting a patch lead to the 1v/oct input, this cancels the normalling from the distribution bus. If the other end of this patch lead is not connected to a control voltage then the oscillator will free run, in the same way as Model D when VCO 3 keyboard control is disabled.
The original Model D was powered by +/-10v supply rails, so we have used exactly the same voltages for the MiniMod oscillator core. Eurorack power supply rails deliver +/-12v to modules, so we have included two onboard low ripple voltage regulators so that the VCO core operates at a very stable +/-10v. This design approach has the added advantage that it provides some isolation to each VCO from noise on the supply rails and prevents the VCO’s “locking” together when they are at or very near to the same frequency, which can sometimes be a problem with modular synth VCO’s – it would stop the slow beating together of the VCO’s which creates all of the multiple VCO “fatness” that we know and love.
We have added several features which were not available on the original VCO’s. Firstly, instead of having a square wave and two fixed width pulse waves we have added fully variable pulse width, which can be varied from the front panel PWM width pot or can be modulated by a +/-5V control voltage, the depth of which can be controlled by the PWM CV pot. To emulate the Model D waveforms with the pulse width control position 0 is square wave, control knob position 3 is pulse 1 and position 5 is the narrower pulse 2.
In addition to the 1 volt/octave and exponential CV voltage control of pitch we have added a linear CV modulation input with a variable level control. This applied modulation to the VCO core “downstream” of the exponential converter and it can be very useful for subtle modulation effects which remain more constant with octave changes compared to exponential CV modulation.