Super VCO: prototype finished


My new “Super VCO” for Eurorack is now finished as a prototype, and it has become “a little more” complicated than planned 🙂 (48 op-amps, 9 other IC’s, ProMini w. I2C display, + lots of ‘discretes’). The main purpose has been to develop and test various design ideas, and then afterwards slice it down to four less complex units. Four internal boards on a small motherboard packs a lot of features, almost a complete synth. I even had to make an ad-on module for linear to exponential converters. Main module and add-on module looks like this from the front:

The features are many:

  1. Main VCO
    • Triangle generator with high linearity and stability.
    • Front panel switch selection of normal/low frequency range.
    • Four inputs for frequency control: Potentiometer, CV from panel or bus, envelope and FM.
    • Five derived waveforms: Ramp, ramp with double frequency, square wave, pulse and sine.
    • All waveforms are “true symmetrical” around zero volt.
    • Four summed inputs for pulse-width control: Potentiometer, PW, envelope and PWM.
  2. Modulating VCO’s
    • Two extra generators
      • one VCO for frequency modulation (FM) of the main VCO
      • one VCO for pulse-width modulation (PWM) of the pulse output from the main VCO
    • Both VCO’s are designed for a lower frequency span than the main VCO, but also have selectable ranges.
    • Both VCO’s are designed with sine wave output, and FM-VCO also with square-wave output.
    • Both modulators have built-in VCA’s for amplitude and AM.
  3. White noise generator.
  4. Sub-harmonic divider from main VCO frequency
    • One-of-twelve dividing steps via rotary switch.
    • Selected sub square wave is open collector.
    • Selected square wave also produce a random level signal.
  5. Envelope generators*
    • Six generators, internally wired for
      • Main-VCO bipolar frequency envelope.
      • Main-VCO bipolar pulse width envelope.
      • Frequency-modulator-VCO bipolar frequency envelope.
      • Frequency-modulator-VCO unipolar amplitude envelope.
      • Pulse-width-modulator-VCO bipolar frequency envelope.
      • Frequency modulator VCO unipolar amplitude envelope.
    • Amplitude envelopes produce start-stop signals for VCOs, to ensure start from zero.
    • Each generator has six envelope steps.
    • Each envelope step is defined by target voltage and time to reach target.
    • Common Gate trigger signal can be analog, and trig level can be programmed.
    • Gate delay can be individual for each envelope.
    • Gate-release mode can be selected.
    • A “next gate” 5V signal can be produced with a delay from the Gate signal.
    • The “next-gate” can be programmed to repeat up to 16 times when triggered by Gate.
    • All programming is done with 4 rotary pulse encoders and one mode switch.
    • All parameters for a selected envelope and its envelope-steps are displayed.
  6. Internal module temperature 0.1V/dgrC output (intended for any future compensation).
  7. DC voltage reference, -10 to +10 volt via 3-turn potentiometer.
  8. Jack-socket attenuation: Internally wired envelope- and modulating signals utilizes the jack-sockets builtin breaker-contacts, in a way where jack-connectors will disable, connect or activate attenuation when pushed in.
    • Dummy-plugs (no internal/external connections) to utilize these functions without using cables.
  9. Linear to exponential converters
    • Two converters placed in extra-module.
    • …one for Main VCO frequency control input.
    • …the other for FM-modulator-VCO frequency control input, to make it track main VCO.
    • Common bypass switch to enable linear mode.
    • Converters based on the THAT2180.
    • 1V/oct.- and offset-potentiometers are installed on front-panel.
    • Linear and exponential FM possible at the same time.


To get a better understanding of how everything is tied together in the prototype, you can study this functional diagram:

VCO functional diagram, ctop_003


Example of strange sounds, generated by the VCO:


Below you can see pictures of all the waveforms:



Working very well from 60 Hz to 6 kHz (and actually much wider up and down):


Envelopes, examples:


Examples of outputs from built-in PWM and FM modulators, when subjected to the envelopes shown above: