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The HMVCO1c is a 0.7" pitch DIL module providing a temperature compensated accurate exponential voltage controlled oscillator (VCO) intended for audio and synthesizer applications.  The design is adapted from a VCO design by Terry Mikulik published originally in the 'Electronotes' series


  • Wide sweep range 10,000:1
  • Excellent logarithmic conformance
  • Temperature compensated
  • Simultaneous sawtooth, pulse and triangle wave outputs
  • PWM Input.  0% to 100%
  • Hard sync input
  • +/-12V* to +/-15V power supplies
  • Low external parts count.
  • Above and below audio frequency operation.

The excellent performance of the HMVCO1b is delivered through careful component selection and the matching and grading of devices used.  The thermally conductive potting compound assures a good thermal connection between compensating elements, a large thermal mass to avoid short term temperature fluctuations and good mechanical strength.

The module measures only 30mmx20mmx7mm and when using SMT boards the external components can be soldered to the opposite side of the PCB underneath the module footprint meaning the entire VCO circuit occupies no more PCB real estate than the module itself.


The clip below was produced by Matt J using 2x HMVCO1c to construct the oscillator modules.

Galactic Horizon - HMVCO1c Demo

"..Here is my first quick test of the VCF, ENV and VCA from the great I am using 2 x Hearn Morley Oscillators fed into the VCF. I also add a TRI osc into the second input of the VCA. The ENV is controlling the VCF cutoff."

Note: HMVCO1a/b has been superseded by HMVCO1c.  This has a different exponential converter pair and removes the 10V reference that although provided constant amplitude outputs over the 12-15V supply range resulted in a DC bias on the outputs.  A better system of making pins has also been employed.  These are now 0.6mm square gold plated.  HMVCO1c is pin and function compatible* with HMVCO1a/b.  Shortened pin identifies pin 1.

*When using 12V supplies the waveform outputs will 16Vpp.  Frequency remains constant over the 12-15V supply range.
George Hearn,
25 May 2010, 14:44
George Hearn,
25 May 2010, 14:41
George Hearn,
25 May 2010, 14:43