well, on my cheap scope, using a 10x probe seems to increase the apparent input impedance...so i can now get a decent reading of the CV for MG Delay. When set to zero, it's value is -4.73V (as measured on Pin 2 of IC14).
So, if I could make the other leg of the comparator (the leg that is affected by the keypress signal) never go below -4.73V, I would avoid this behavior with the MG that I don't like.
Chip
--- In PolySix@yahoogroups.com, "chipaudette" <chipaudette@...> wrote:
>
> Thanks for the confirmation!
>
> I can't believe that I didn't notice this behavior until now. Now, I'm starting to look at mods to avoid this behavior.
>
> Probing around this part of the circuit with my oscilloscope, it looks like the "problem" is that the output of the sample-and-hold circuit that generates the "MG Delay" CV doesn't go all the way down to -5.0V even with the knob turned to zero. This value is used in a comparator at IC14. The value it's being compared to does drop to -5.0V with every new keypress. So, even with MG Delay set to zero, any new keypress trips the comparator, which trips the MG Amount down to zero. When the MG Delay is set to zero, the MG Amount is nulled for a very short time (25-50 ms), but it's long enough to be quite noticeable.
>
> Based on more probing around makes me think that the trip line (with MG Delay at zero) of the comparator is something like -4.6V to -4.4V. I'm still not quite sure of the exact value because I can't directly probe the CV representing the "MG Delay" because, even at 1MOhm input impedance, my oscilloscope trains sample-and-hold cap too quickly.
>
> I'm churning on a couple of ideas: (1) Connect the sample-and-hold cap for MG Delay (ie, C18) to -5V instead of to ground, or (2) change the DC reference of R82/Q5/C33 to -4.6V instead of -5V, (3) put a diode with a forward drop of 0.4V to 0.6V on Q5 so that it drains C33 only to -4.6V or -4.4V instead of to -5V.
>
> Anyone got any thoughts?
>
> Chip
>
> --- In PolySix@yahoogroups.com, "djangowes" <djangowes@> wrote:
> >
> >
> >
> >
> >
> > Hello Chip,
> >
> > I've done the same test and my P6 behaves as yours... at each note, the cutoff goes quickly from the value it is set by the cutoff knob to the value set by the MG.
> >
> > rgds,
> >
> > Ed.
> >
> > --- In PolySix@yahoogroups.com, "chipaudette" <chipaudette@> wrote:
> > >
> > > Hi All,
> > >
> > > I have a question regarding the MG Delay knob and whether your Polysix responds the same way...
> > >
> > > I set up a patch using a basic saw wave and with the filter cutoff set to half-way. The filter envelope is set to zero (neither positive nor negative. I then set the MG for a very slow sweep with zero delay and moderate intensity (say 3-5). I set the MG to effect the VCF.
> > >
> > > Now, when I manually play a series of individual notes, I would expect to hear the notes as effected by the slowly opening and closing filter (opening and closing due to the slowly sweeping MG). I would expect it to be smooth and nice.
> > >
> > > Instead, what I hear is that the beginning of each note is quickly swept from mid-filter (ie, the setting on the VCF cutoff knob) to whatever cutoff value is being set by the MG. So, you hear the filter either quickly open or quickly close, depending upon where you are in the MG cycle.
> > >
> > > If the MG Delay were set to anything but zero, this is exactly what I would expect. But, my MG Delay is set to zero, so I would not expect to hear any transition.
> > >
> > > Does your Polysix do this? I'd like to know whether I'm seeing a symptom of a broken component, or whether this is a design "feature" of the Polysix.
> > >
> > > Thanks,
> > >
> > > Chip
> > >
> >
>