This is a eurorack “clone” of the 4027-1 (VCO-2) Oscillator used in the ARP2600.
Some substitutes needed to be done due to the no longer existing parts.
However, great care has been taken to not only match the functionality of those parts, but also that raw power in sound.

You also have the option to choose a High quality German made panel. Anodised in matte black, with a white printing on top. (very durable)
The anodising is made after drilling, leaving no bare metals vulnerable for any possible current/grounding to be spread.

Additions & changes on the original circuit are a few recalculated resistor values for the eurorack standard, removal of Low-Frequency option, Buffered in & outputs, Precision Reference Voltage and added Hard-Sync inspired by the Odyssey. (though realized differently)

The 10V Reference Voltage keeps the Frequency part of the Oscillator immune from any noise or cross-modulation that might happen thru the power-rails.
This is particularly useful when having several different VCO’s on the same rail, as they have a tendency to intervene/latch up with each other. However, if this is a desired effect, it’s easy to bypass the Ref Volt section.

Having everything Buffered means the FM-inputs wont cross-modulate, multing the in/outs will not have any signal-loss, and generally keeps the Module protected from the outside world. To add on that, the Power input is also protected from any mistakes when connecting it to your PSU.

No other modern improvements/simplifications have been made. Authenticity was of highest importance. The cost of having a few more parts results in a living organic sound, with all the analog oddities I personally love in the original.

See the V1.5 Build-Document for more detailed info/options regarding the build.


*V1.0 Build-Document (former version, kept for reference)

Main controls

  • Frequency & Fine-tune
  • PW
  • Attenuators for PWM & FM

Inputs

  • 1V/OCT
  • 2x FM
  • PWM
  • Hard Sync

Outputs

  • Saw, Pulse, Triangle and Sine

Technical

  • 10V Precision Reference Voltage
  • Power protection
  • Buffered in & outputs
  • Tracking well over 10 octaves
  • FREQ control 10hz – 10KHz
  • FINE control +/- 5 semitones
  • Width 12HP
  • Depth about 45mm, depending on choice of components