Moritz Klein’s EDU DIY VCO, based on a Schmitt Trigger.
- 1V/OCT input – control voltage for pitch, using the exponential 1V-per-octave standard to match output of MIDI module, 0-10V.
- TUNE & FINE knobs – adjust oscillator pitch, coarse & fine.
- BEND knob, CV IN input – modulation of oscillator pitch by control voltage, ±5V.
- PW knob – pulse width of SQ output, centre-detented at 50:50.
- PWM knob, CV IN input – modulation of pulse width by control voltage, ±5V.
- SAW output – sawtooth wave audio signal, ±5V.
- SQ output – square/pulse wave audio signal, ±5V.
This 8HP module will fit a shallow skiff case:
Circuit
This is not my work, so I will not reproduce the circuit. Please see Moritz Klein’s VCO MANUAL and this series of videos for a full description.
Note that the implementation of the square/pulse wave output differs between the manual and the description in the videos; this version followed the manual schematic.
Some resistor values were selected during breadboarding to set the correct output level.
Construction
The whole thing is built on a single 15 hole x 42 hole piece cut from a 12cm x 8cm perfboard. All components are wired point-to-point with green 30 AWG kynar wire.
The 10-pin boxed power connector and the low-profile electrolytic capacitors (directly soldered to the pins of the power connector) are fitted across the top of the board to locate the knobs down from the very top of the panel. These should be soldered first for easier access before the other components are added. Here, yellow and blue wire is used for +12V and -12V respectively.
Five Alpha 9mm vertical pots and five PJ398SM mono jack sockets are placed directly on the board, mounting directly to the aluminium front panel. The other circuit components squeeze into the spaces between these.
The preset pot for 1V/octave scaling is mounted on the reverse side of the board, so it can be adjusted when the panel is assembled.
This is an experiment to fit knobs and sockets on a compact module width, using a similar layout to the previous VCO module which was 50% wider.
A constraint: the input sockets are sited next to their associated knobs, which puts them too close together. The solution was to move the sockets slightly lower than the knobs, increasing the distance between centres from 13mm to 15mm.
Ideas & rethinks…
First, I’m grateful to Moritz Klein for making his designs available to us all, thank you.
The easiest way to build this module is to buy the complete DIY kit from Erica Synths or perhaps from Thonk in the UK.
This is a simpler oscillator than the previous CEM3340-based VCO module, which has better tuning over the whole range, better temperature stability, additional triangle-wave output, FM modulation, hard & soft sync.
There were a few mistakes on the way:
- A planning error led to me trimming the perfboard after soldering on some of the components. Risky, but that actually resulted in a better front panel layout.
- The decoupling capacitors dangling off the power socket is a technique borrowed from some 4HP modules, but that isn’t necessary with more space on that part of the board. And the power socket could be centred horizontally.
TheSlowGrowth’s VCO Tuner was used to set the 1V/octave scaling, sending a sequence of MIDI notes while it listens to the audio input.
The exponential convertor is quite crude, and I didn’t match the pair of transistors so I was only able to set a reasonable scaling over about 3 octaves.
ShedSynth 