Glide circuit of the ProphetV

[Harry Bissell]

Roel Das wrote:

> <snip>
> > > 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.
> <snip>
>
> Do you mean, use the TL082 at the bias input?

No...

> This confuses me... You just
> mean this is THE opamp to use after the cap, right?

Yes...

>
>
> However, I read Gene Zumchak's OTA primer, learned alot, thanks. But some
> things still bother me about OTA's. How do you use the diode bias input on
> the 13x00? The diodes don't have any influence when you don't connect the
> input or something?

You don't need the internal "diodes" they are used in some apps but thet aren't
needed here. usually you leave that pin floaring if not used.

>
>
> So, if I replace the 3280 with the LM13700, I do still need the current
> mirror and the diodes?

Yes you still need the current mirror. JH says the diodes are for protection
(he's right)... the 3280 only allows +-5V differential input voltage. (that'll
teach me to look it up in the data book. I checked the 13700 and figured...
nah.... don't need protection here.... WRONG!)
The 13700 allows +-13.5 (on a 15 volt supply) so the diodes "might" not be
strictly necessary. But I'd leave them in.

>
>
> I'd also like to add some control to make sure the glide is off for
> portamento 0. Adding a trimmer before the current mirror, would only affect
> the maximum glide time, so maybe a MUX to choose between the original and
> the portamento output?

You don't need to have a bypass for this circuit. You cannot perceive the
fastest glide time... It is sub millisecond. The next note will be there "right
away"... You could prove with a scope that there is a finite "glide" but you
will NEVER hear one.

>
>
> And one more common question, any reason why to use the 1N914 diodes here?
> What criteria are used to select a diode? The current it can handle and the
> junction voltage, or more (and availability, offcourse)?

The 1N914 was the generic "signal diode" of its day. Its reasonably fast which
you want here... The 1N4148 is the common replacement for it today. Junction
voltage is sometimes a criteria, but most diodes forward drop (silicon .6-.7V)
are similar. Usually bigger diodes are slower... rectifier diodes like 1N4002
etc. are much slower than 1N 4148.

Germanium diodes are sometimes used for lower forward drop (.3V)... usually
only in Guitar fuzzbox circuits these days...

Shottky diodes have low forward drop also (.3-.5V) and are fast... but have
poor reverse leakage esp at high temperatures.

>
>
> Thanks
>
> Roel