[sdiy] New Thomas Henry VCO-1 Page Up

[Ian Fritz]

Hi listers --

Hmmm ... This is the old, old EN #46 design (from 1974) tricked up with a 
bit of lipstick. A somewhat odd mixture of old and new ideas.  I built the 
original when it was first published and have made a number of 
modifications since.  The first set of these may be seen in EN # 112 (1980).

One of the first and most significant mods was to replace the discrete 
Schmitt trigger with a fast opamp.  For the 1980 redesign I used an LM318, 
at that time one of the really fast new devices.  With this simplification 
I was able to get tracking withinin 0.5% up to 80 kHz.  (I use this VCO at 
10x audio frequency and follow it with a 10-step waveshaper.)   I'm 
disappointed this simplification and improvement was not made in the Henry 
design.

Additionally, I added a Franco compensation resistor in series with the 
integration cap, so that both kinds of high-frequency errors could be 
compensated.  I kept Bernie's FET cap buffer, since this is in a feedback 
loop and the offset doesn't matter.

I also found that the s-shaped tuning curve could be significantly improved 
by driving the OTA input below saturation (~60 mV).  I've never figured out 
the reason for this, but it probably depends on OTA nonidealities.

In 2000 I made additional improvements, including using good modern opamps 
(OPA27 for the CV summer, OPA134 for the CV servo and the Schmitt 
trigger).  At the same time I added added temperature compensation for 
linear temperature drift and tweaked up the scale factor drift.

My version of this VCO is now reasonably stable, although the use of the 
supply rails for critical voltages is something I would avoid in a new design.

So I'd say this is an OK design for non-critical applications or as an aux 
VCO, but it could be made much more accurate and stable with just a few 
simple improvements.

   Ian



At 08:14 PM 5/26/2007, scottnoanh at peoplepc.com wrote:
>Hey list,
>
>I've put up a page detailing a Thomas Henry designed VCO, based around a
>CA3080 with a transistor based Schmitt trigger.  It's a simple but very
>powerful little workhouse of a VCO.  I've put on there an article Thomas
>wrote about the VCO, and there's also a resource page for parts procurement,
>sound samples, stuff like that.
>
>Complete schematics, PCB layout, etc. files are available free for download.
>
>It's here:
>
>http://mypeoplepc.com/members/scottnoanh/birthofasynth/id28.html
>
>Cheers,
>Scott
>
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