The ENS-76 Home-Built Synthesizer System - Part 7, VCO Options:

Page 4

        One of the things about our Electronotes circuits and presentations, as opposed to the more usual "How to Build" publications, was that we always assumed our readers were knowledgeable, flexible, and willing to experiment. Indeed, it was the readers who provided most of the ideas for modifying and improving circuits. We expected different individuals to do things in different ways. In total, our presentations perhaps resembled musical compositions where we showed the official score, but at the same time, we encouraged different variations and interpretations of the score. This resulted in many interesting individual features and improvements. Looking back from 2001, when certain parts are difficult to obtain, and where fewer and fewer persons can offer useful help, the need to fully understand the circuit operation, and to understand parts substitutions, is of paramount importance.

        Despite our encouragement of experimentation, one should not assume that arbitrary substitutions and modifications will not at times result in a substantially degraded performance. In regard to the Option 1 VCO, we note that there are three parts: the exponential converter, the sawtooth relaxation oscillator, and the waveshapers.

        The exponential converter relied on the AD818 matched pair, which is probably hard to obtain today. This component was "brute force" and worth every penny. If you can't locate this, you really should go back to Terry Mikulic's original CA3046 array based converter. Do not assume that two unmatched transistors, even hand-matched and glued together, will be acceptable in terms of accuracy and stability.

        The relaxation oscillator is Terry's original with a few modifications. As a general rule, for the entire circuit, you should be able to substitute LF351's or similar for the 556 , 301, and 307 op-amps. You can also use an LF351 for the CA3140. You can also use an LF351 for the CA3130, but it would be run on the bipolar supply, and without the 39p compensating capacitor. But keep the 311 comparator. The right FET could be very important. We only had success with the KE4859. Probably lots of other FETs would work, but of the very few more commonly available ones we tried, the sharp reset was missing.

        The waveshapers are not too critical. If you are showing off the waveshapes with an oscilloscope, you might want them to look nice. But it is generally true, that for musical purposes, there is no requirement of even using this particular set of waveforms, let along a need to have these waveshapes without small imperfections.

        As mentioned, there are many variations on this circuit which have appeared in various places. Here are two you should be aware of:

  • Terry Michaels, "Modification to a VCO Design from 1976," Electronotes, Vol. 20, No. 194, April 2000

Bernie Hutchins, April 2001

For more information, contact Electronotes at:

1016 Hanshaw Rd
Ithaca, NY 14850

[Page 1] [Page 2] [Page 3] [Page 4]