AW: [sdiy] ASM-1

[Gene Stopp]

This glide circuit will "hunt" for a stable state.

When the input voltage is different than the output voltage, the output will
glide linearly to match the input voltage. Once they match, the thing will
oscillate with a triangle wave output, without any resistance between the
op-amp output and the cap. Amplitude and frequency of this oscillation will
depend on the individual components used, most importantly the op-amps as
Magnus states. The resistor is there to kill off the oscillation and
accomodate whatever parts the builder may choose - to be honest, I'm not
even sure if the oscillation will be there when a TL082 is used!

A 330 ohm resistor will not cause noticeable minimum glide time - to be
honest, there is really no such thing as "no glide at all". Everything
slews. The point here is to get it below the threshold of musical
perception, which would be in the hundreds of microseconds or couple of
milliseconds range. As Happy Harry so correctly pointed out, the linear
glide "chews up" capacitance like mad. An R/C value that would be considered
"long portamento" in exponential mode would be much, much faster in linear
mode.

You can put in a switch to change the first op-amp from a buffer
(exponential) to a comparator (linear), but if you do, it would be a good
idea to use another pole of the switch to add a 10X cap to the existing one
when linear mode is selected.

It's funny to me how early Electronotes portamento circuits described linear
portamento as "inferior" or "less desireable" due to the unequal times
between notes close together versus far apart. At the time I remember
thinking that linear was better because Minimoogs had it!

Best Regards,

- Gene



-----Original Message-----
From: Magnus Danielson [mailto:cfmd at swipnet.se]
Sent: Monday, October 29, 2001 2:39 PM
To: np at inverse-entertainment.de
Cc: synth-diy at dropmix.xs4all.nl
Subject: Re: AW: [sdiy] ASM-1


From: "Nils Pipenbrinck" <np at inverse-entertainment.de>
Subject: AW: [sdiy] ASM-1
Date: Mon, 29 Oct 2001 11:50:07 +0100

> Speaking about the linear glide:
> 
> > Linear Glide:
> >
> >         +-----------------------------------*------+
> >         |2|\    330R     1M                 |6|\   |
> >         +-|-\ 1 ____    _____               +-|-\ 7|
> >          3|  >-|____|--|_____|-*---*------+  5|  >-*---o CV Out
> > CV In o---|+/             ^    |   |      +---|+/
> >           |/              +----+  === 1uF     |/
> >                                    |
> >                                    v
> 
> 
> What's the reason for the 330Ohm resitor here?  I would like to have a
glide
> that I can adust to "no glide". I think the resistor will prevent this.
But
> I'm sure it's there for some reason.. Maybe to make the circuit more
stable?

Actually, it depends on the op-amp there. The TL082 has a 128 Ohm
resistor in series with the output if I recall things correctly.

Cheers,
Magnus