Glide circuit of the ProphetV

[Harry Bissell]

Roel Das wrote:

> I've got a little trouble figuring out this circuit. (I don't have any
> experience doing this...)
>
> My guess:
> The Iabc input of the 3280 limits the current provided to the output? (Or is
> it the opposite?)
> And because this is the current provided to (un)load the capacitor, it
> controls the glide...
> The voltage follower buffers the capacitor.

Yes thats right.

>
> And the transistors? I suppose buffering?

No... the transistors are a PNP current mirror. Gheck out Gene Zumchak's OTA
primer. A list member has it (I forget who-- ask on list) on his website.

briefly OTA needs to be sourced a current... Gene called it you have to ":Blow"
current into it as opposed to "Suck" current out of it. The more positive the
"glide cv" the more current is sourced to the OTA... the more current availbale
to (un)load the cap.

>
> And, when you use the 13700, you don't need the buffer, because you can use
> the one on the chip right?

No... the darlington buffer can only "pull up" and has a passive pull-down. so
it can source lots of current, but sink is fixed... this would cause asymmetry
in the glide. The
TL082 has a higher input impedance and symmetric drive current... very good.

>
>
> And the last question, R3121 4,7K in the feedback loop. Is it needed, or is
> it just additional gain?  But what is the use of the diodes?

Consider a step change in keyboard voltage. The non-inverting OTA input goes to
(oh...)
10 volts (so its a big step...). The input will have a huge differential voltage
across it until the cap charges....

The diodes clamp this and the resistor limits the voltage to the clamp. as a
side effect this makes the ramp linear for large steps (input differential is
clamped at .7 volts) and sort of exponential for small steps. In practice you
get a linear glide.

If you remove the diodes and resistor and ground the negative input you will get
exponential glide (and problems of offset voltage which you must trim out for DC
accuracy) The feedback assures DC accuracy.

I have built this circuit with a CA3080, and used it for a smoothing filter in a
guitar synth application. I took the "envelope follower" output from the guitar,
and the pitch to voltage out, and applied them to this circuit. At high
amplitudes, the glide time is minimal. as the guitar decays and tracking gets
more.. iffy.... the glide gets longer. As the amplitude reaches zero I
effectively have a sample and hold....

Tried Tested Proven... its a keeper !!!

H^)

>
> Thanks,
> Roel
>
> ----- Oorspronkelijk bericht -----
> Van: "Harry Bissell" <harrybissell at prodigy.net>
> > You got it.... The circuit works real well, you could also use a CA3080,
> or
> > LM13600 or 13700 here... I's use the more available 13700 since the
> darlington
> > buffer isn't needed.
> >
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