Eurorack Synth-VCO Design

cTOPconsult have designed and built a novel “Super VCO” conforming to the Eurorack synthesizer standards. It is currently a rather complex prototype (48 op-amps, 9 other IC’s, ProMini w. I2C display, + lots of discrete components), but the main purpose has been to develop and test various design ideas, and then afterwards slice it down to four smaller units.

The main goal has been  to make a brand new VCO design, not copying the usual designs found on the internet. The design should be all-analog/no-trimmers/DC-coupled, and preferable eliminating parameters that can create temperature-instability and frequency nonlinearity. With this accomplished, the various necessary power supplies/references were put on a second board, including a few extra features. Two more boards were added for testing modulation- and envelope-generating ideas, and all four boards are tied together on a little motherboard.

Since the VCO is basically voltage-to-frequency-linear, a matching linear-to-exponential converter had to be added to make it “musical”. This is generally not a trivial task, and was developed as a separate add-on module that sits besides the main module (but still as an integrated part of the design). The “Super VCO” is an almost complete analog synthesizer in one module, and the prototype looks like this from the front:


(If you have seen this before, you maybe will notice that I added yet another “extra extra panel” to the right. This was to enable linear FM of the Main-VCO and the FM-VCO, and still keep exponential pitch control of both. At the same time I added switches for waveform-shape and frequency-ranges for the FM-LFO/VCO, and ranges for the PWM-LFO/VCO)

…and the main module looks like this from the back:

(Two ribbon cables are connections to the “first extra module”, and third one is for power)

The features are many:

  1. Main VCO
    • Triangle generator with high frequency linearity and stability, combined with just as good amplitude linearity over the total frequency range. The waveform itself has very close to ideal triangle shape, with 50% duty-cycle.
    • Front panel switch selection of normal/low frequency range.
    • Four summed inputs for frequency control: Frontpanel potentiometer, pitch control (= CV from panel or bus), frequency envelope and frequency modulation (FM is an Ac input).
    • Five derived waveforms with same good characteristics as the triangle: Ramp, ramp with double frequency, square wave, pulse (pulse width modulated) and sine wave.
    • All waveforms are “true symmetrical” around zero volt, with no capacitors in the signal path.
    • Four summed inputs for pulse-width control: Front panel potentiometer, PW control from panel, PW-envelope and pulse width modulation (PWM is an AC input).
  2. Modulating VCO’s
    • Two extra generators that are slightly cheaper versions of the main VCO
      • 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 modulating VCO’s are designed around a lower frequency span than the main VCO, and are meant to be used as LFOs, both internally or connected to other modules, eg.: as modulating signal for a ringmodulator. Two range switches are later installed to also give *10 and /10 ranges, for super low LFO or FM-synthesis.
    • Both VCO’s are designed with sine wave output, but the FM-VCO can also deliver a square wave (frontpanel switch).
    • Both modulators have built-in VCA’s for amplitude control and amplitude modulation (AM).
  3. White noise generator, based on selected transistor in a reversed base-emitter coupling.
  4. Sub-harmonic divider from main VCO frequency
    • One-of-twelve dividing steps selectable from front panel rotary switch.
    • The selected sub square wave is output via an open collector, but with a 10k pull-up to 5V (via diode).
    • Selected square wave is also used to sample the noise signal to produce a random level output 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.
    • The two unipolar 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 value 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 (pos. or neg. flank).
    • 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 is measured, scaled and put out on the front panel as 0.1V/dgrC. This is intended for any future compensation that would be necessary to avoid frequency- or amplitude drift caused by changes in temperature.
  7. DC voltage reference, +/-10 volt is put to front panel via 3-turn ww-potentiometer and a buffer amplifier.
  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. I have made a six 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.
    • Converters are inserted between summing input op-amps and integrating circuits.
    • A common bypass switch is installed, to enable linear mode.
    • Converters are based on the THAT2180 voltage controlled amplifier chips.
    • 1V/oct.- and offset-potentiometers are installed on front-panel
    • Simultaneous linear control of both FM outputs are now possible from new extra input, or for existing input when switched from front panel.
*Note that the envelope generators are not strictly ‘analog’. Shifting through states of an envelope is in a sense always done digitally, and here it is done by means of a small ProMini processor, with active 3.order filters on PWM-outputs.


The plans for further development is to split the design up into four individual modules, for example containing these functions:

  • Main-VCO super precise all-analog module, only including the six simultaneous waveforms, linear and exponential sum-inputs for frequency-CV and FM,  PWM,  AM and amplitude control.
  • Utility module with noise-source, sub-divided square, random signal, precision DC source, temperature measurement and maybe a small voltmeter.
  • Six (or more)-envelopes module, also including the ‘next-gate’ generator.

cTOPconsult will probably produce a small limited number of these modules, along with a couple of other modules currently in development. I will continue to use only through-hole components on non-complex PCB’s, in order to make the modules “hackable”.


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

This is how the six envelopes and the “next-gate” is programmed from the four encoders on the frontpanel:

Envelope programming


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:


Super VCO, explained shows and explains all the major design principles.