[sdiy] lin to log envelope

[harrybissell]

Hi Gregory...

If it has a 'simple' decay envelope, they are usually RC or exponential (log).

Don't be confused by exponential and log... they are usually referring to the
same
response.

If it is an RC time constant... it is an exponential decay.  To make it linear,
you need to discharge
the timing cap through a constant current source, rather than through a
resistor. Not a trivial mod but you 'could'. It would probably be easier to
design a linear envelope and patch it into the existing circuit.

In general, you cannot just swap the inverting and non-inverting inputs of an
opamp... with
negative feedback it usually makes a polite amplifier, with positive feedback it
becomes a
comparator with hysteresis... not what you'd want or expect

H^) harry

(if you have a schematic or link you could contact me offlist and I'll see if
there is any way to
make a simple mod.  BTW you should try and find a way to temporarily 'try' the
effect you want and see if that gets the sound. Its a bummer to spend a lot of
time doing a circuit... to find that
"it ain't that" :^)

gregory zifcak wrote:

> thanks,
>
> they don't have vc inputs, but they're simple decay envelopes based around
> an opamp, a couple resistors, a capacitor and a couple of diodes. the
> schematic shows the output of the opamp feeding back to the inverting input.
> i am wondering if its simply a matter of rerouting this to the positive
> input of the opamp (possible through a pot to allow continuously variable
> decay shapes?).
>
> >From: Peter Grenader <peter at buzzclick-music.com>
> >To: gregory zifcak <zifcak at hotmail.com>, <synth-diy at dropmix.xs4all.nl>
> >Subject: Re: [sdiy] lin to log envelope
> >Date: Tue, 18 Nov 2003 16:49:05 -0800
> >
> >If your envelope phases are VC'able, its cakewalk - output of envelope into
> >VC input.  If it's not, then well...you're going to have to do some cutting
> >and hacking to get it in there.
> >
> >
> >
> >
> >
> >
> >
> >gregory zifcak wrote:
> >
> > > hello,
> > >
> > > i saw in the archives a reference to changing the shape of envelopes by
> > > feeding them back into themselves either positively or negatively. does
> >this
> > > refer to the positive and negative inputs of the opamp?
> > >
> > > my application is this:
> > > i have a simmons sds800 which uses cem3394 chips for osc-vcf-vca.
> >according
> > > to the datasheet, the vcas are approximately exponential, allowing
> > > exponential volume shapes with a linear envelope. i am assuming that the
> > > simmons envelopes are linear or maybe even exponential, because they are
> >not
> > > particularly punchy. i would like to make them logarithmic if possible
> >in
> > > order to obtain punchy, linear dynamic response.
> > >
> > > so basically my question comes down to where do i feed(back) the output
> >of
> > > the envelope?
> > >
> > > thanks for any info,
> > >
> > > greg
> > >
> > > _________________________________________________________________
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> >
> >
>
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