[sdiy] How to make a continuously variable wave LFO or VCO

[Michael Bacich]

On Sep 26, 2006, at 11:09 PM, Dave Manley wrote:

> The circuit is similar to what Paul describes.

Well, yes and no.  The Micromoog's octave doubler circuit has a  
center-tapped pot as Paul described, but it pans only between two  
suboctave square wave outputs (full CW and full CCW) and the main VCO  
wave (in the center of the pot).  That's R435 in the drawing -- it  
has a label that says "DOUBLING".  However, that's not the  
continuously variable waveshape control that is so unique on the  
Micromoog (and Multimoog).  The actual waveshape knob is R414 (a few  
inches to the left of the other pot), and it's labeled "WAVESHAPE".   
As you can see, it's a voltage controlled waveshaping circuit, and  
turning the pot changes the wave from Triangle, to Saw, to Square, to  
Narrow Pulse (or maybe it was Saw to Triangle to Square and Pulse?  I  
can't tell from looking at the circuit).  It's kind of like a super  
voltage controlled Pulse Width Modulation circuit.  You'll note that  
modulation sources (such as LFO, etc.) are also applied to the  
waveshaper via R416.  It's a way clever circuit, and as anyone who  
owns a Micro will tell you, it's very fun and musical to use.  I'm  
surprised that we don't see this kind of thing more often in VCOs.