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Compact Oscillator

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This Compact VCO started out life as a replacement for the original VCOs in Ken's '73 Serge, so it replicates a lot of the functionality of that module. Of course there have been numerous enhancements to the design as well, making it right at home in a 1V/Octave synthesizer. The core chosen is the well known Electronotes ENS-76 VCO Option 1 as used in the ASM-1, Modulus, etc. This core was an obvious choice due to the amount of information, lists of suitable substitutes and so on that are available on the web.

It has AC and DC linear CV inputs as well as 1V/Octave inputs and scalable/reversible CV inputs. It has sawtooth and pulse/square outputs, as well as a waveshape that is variable between sawtooth and a "sine-like" shape, and a fixed "sine-like" output too. It also includes a simple sub-oscillator for those who wish to experiment with that.

The [FINE] control is wired as a dual function control:-

  • Between its mid-position (12 o'clock) to fully clockwise position (5 o'clock) the frequency range of the VCO is from around 8Hz to over 14kHz
  • Between its mid-position and fully anti-clockwise (7 o'clock) the VCO can be driven down to LFO speeds as low as 1 cyle per 4 seconds

The sub-oscillator is based on that from an LM3900 application note (AN-72), but will not cover the full range available from the VCO core itself.

This Compact version omits a handful of features/functions dictated by the requirement of the smaller panel space:-

  1. There is only one 'voltage processor' input - [VC F]
  2. There is only one 'frequency modulation' input [1V/OCTAVE] (J103 is assigned to the [FREQUENCY] control)
  3. There is no DC copy of the [VARIABLE] waveshape
  4. There is no [PLS] output from the staircase generator
  5. There is no [STEPS] control for the staircase generator. This is fixed to a defined number of steps as part of the calibration process. As such, the [STAIRCASE] output is a fixed sub-octave of the main frequency
  6. There is no external input to the staircase generator, this is hardwired to the [PULSE] output
  7. There is no [SYNC] input to the staircase generator .
Note that the "sine-like" waveform is not a pure sine wave. There is a significant glitch in it that is almost impossible to trim out. As this is the recreation of a module that had such a waveshape, this is appropriate. If you require an accurate sine wave, this module is unsuitable for that purpose.
PCB Dimensions: 2" x 6" with 3.2mm fixing points 0.15" from the edges
Power Consumption:
+12V @ 18mA
-12V @ 13mA

Article, art & design copyright 2003 by Ken Stone

Email:  elby-designs@bigpond.com
© Copyright 2000. All rights reserved.     Revised: September 10, 2023