[sdiy] Glide Pitch Drop?

[harrybissell]

Hi Chris

There is another issue. I don;t know what kind of opamp is driving the
cap, but some opamps (esp TL07x series) are not stable with a capacitive load.

The usual solution is a small resistor in series with the output, before
the cap.  You've done that already. This may be another reason that you
were successful in this.   It is always a good idea to use a small series
resistor
before driving a cap with any opamp, to avoid drawing too much peak current.

H^) harry

xamboldt wrote:

> Thanks, Harry. I went and reread your cap FAQ, and it made more sense now
> that I had a real-world understanding of how the leakage affected a module I
> was making. I replaced the 10K pot with a 1Meg, and the 10uf electrolytic
> cap with a .1uf mylar (that was all I had other than ceramic in that value).
> There was still a little droop, so I measured the pot to see at which point
> in the rotation the droop started, and put a resistor of that value in
> series with the pot (about 100 ohms). So rather than getting rid of the
> droop, I just made it so it was always there. If you can't beat 'em, join
> 'em.
>
> Maybe on my next mouser order I'll get a better .1uf cap, but for right now
> this solution should work for me...
>
> Thanks for the help,
> Chris
>
> On 9/2/05 5:47 PM, "Harry Bissell Jr" <harrybissell at prodigy.net> wrote:
>
> > Its likely you have a 10uF electrolytic, and it
> > has some leakage.  I'd use a much larger potentiometer
> > and a much smaller cap. Maybe a 100K and a 1uF or
> > (if you have one) a 1M pot and a .1uF capacitor. Once
> > you get to a .1uF you could get a polystyrene or
> > polypropylene cap, which would be MUCH better in
> > leakage etc.
> >
> > Its possible to use a Tantalum if you NEVER have
> > negative voltages. If you do, avoid tantalum like
> > the plague.
> >
> > slight possibility there is a bad TL074...
> >
> > H^) harry
> >
> >
> >
> > --- xamboldt <xamboldt at earthlink.net> wrote:
> >
> >> Hi Everyone,
> >>
> >> I'm modding a sequencer to add a simple glide out,
> >> so I copied the glide
> >> section from John Blacet's old "Digital Pattern
> >> Generator":
> >>
> >> http://www.blacet.com/dpg1.jpg
> >>
> >> I didn't use the 50k output attenuator, just took
> >> the CV out from the
> >> sequencer and fed it through the glide circuit
> >> exactly as shown in the
> >> schematic. I'm using a TL074 for the opamps.
> >>
> >> If I have the glide set to 0, then the voltage out
> >> of the glide CV out is
> >> the same as the non-glide CV out. The problem is,
> >> once I turn the glide up a
> >> bit, the CV out drops a little. Not the greatest
> >> thing to happen if you get
> >> a sequencer set up to the right pitches, and you
> >> just want to add a little
> >> portamento, but in order to do so you have to reset
> >> all the sequencer
> >> controls to compensate for the voltage drop.
> >>
> >> Any ideas on how I could fix this? Or is a different
> >> circuit my only hope?
> >>
> >> Thanks,
> >> Chris
> >>
> >>
> >