[sdiy] Glide question

David G. Dixon dixon at interchange.ubc.ca
Sun Sep 5 09:15:44 CEST 2010

> > A few examples of glide circuits, based on Harry Bissell's "Morphlag"
> > which allows independant up and down rates, and can vary the glide
> > shape from linear to log.
> > http://www.wiseguysynth.com/larry/schematics/Morph-Lag.htm

I've just been playing with a simulation of this circuit, and I have a few
questions (Harry?):

1.  Is there a good reason why the shape-controlling feedback resistors are
twice the input resistors (i.e., 20k instead of 10k)?  If they were equal,
then the whole circuit just be a first-order lag with TF = 1/(1 + sRC) at a
100% (i.e., fully exponential) shape pot setting.

2.  Is there a good reason why one doesn't simply return the opamp output
directly to the shape pot through a single resistor, rather than the two
diode outputs through two resistors?  My simulations suggest that this would
work equally well, if not better.

3.  According to my simulations, a small resistor (say, 22 or 33 ohms)
between the shape pot and ground helps to damp oscillations at the opamp
output which would otherwise occur at a zero shape pot setting (i.e., fully
linear).  These oscillations are worse if the diode outputs are fed back
rather than the opamp output directly (see comment 2).

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