[sdiy] Troubleshooting the Morph-Lag

[David G. Dixon]

> I built this circuit to use for as part of a demonstration at 
> my job, and I have run into a little problem. Certain 
> settings of the "shape" knob (close to fully linear I 
> believe) are causing the CV out to hit the positive rail. I 
> am using a TI TL072IN if it makes a difference. 
> 
> Has anyone ever experienced that with this circuit? I found 
> some suggestions from David Dixon in the archives and I may 
> try those to see if the problem goes away, but I wanted to 
> rule any other possible causes out first.
> 
> One of my coworkers made the suggestion that the shape 
> potentiometer could be bad. Sound plausible?

According to my simulations, the comparator will oscillate badly in pure
linear mode.  You might try putting either a small resistor (100R or less)
between the shape pot and ground, and/or putting a large resistor (1M or so)
in the negative feedback loop.

Also, going back on what I said a year ago, I have decided that Henry's
original circuit with the two feedback resistors to the shape pot from the
downstream ends of the diodes is better than taking the feedback through a
single resistor directly from the comparator output.  This is so that an
exponentially lagged output will return to the original input voltage.
However, I have replaced the two 20k resistors with 10k to get a first order
response.  Apologies if anyone actually followed my earlier advice in this
regard.

Bear in mind that I have never built this circuit; I've only simulated it.
However, I still have great plans for it (including adding voltage control,
courtesy of V2164).