[sdiy] more sh-101 mods?

[megaohm]

"The 'voltage controlled potentiometer' is one of the Holy Grails of s-diy
design. There
are so many ways to do it, and none of them is quite perfect."

That reminds me of an idea to use a motorized pot. Electronic Goldmine has
them for cheap but I keep forgetting to include it in my orders. Has anybody
experimented in this area?

peng



On 12/2/05, harrybissell <harrybissell at prodigy.net> wrote:
>
> john mahoney wrote:  [censored]
>
> > -Modding the ADSR is probably asking too much. However, there are VC
> Slew
> > Limiters. You could insert one of them at the ADSR's output, then you'd
> have
> > VC decay/release; it would affect both decay and release times. Thing
> is,
> > you need voltage control *and* independent up/down lag times (you only
> want
> > down, actually), and only MOTM seems to have that feature set. (Someone
> > prove me wrong, please.)
> >
> > The MOTM also has VC Bypass. You can run an inverted Gate signal into
> that,
> > so the lag is bypassed on "note off", thus the Release will not be
> > affected -- only the Decay will. Tricky, eh? ;-) One more reason to like
> the
> > MOTM: it allows VC of Attack time, too.
> >
> > So, how do you make a voltage controlled slew limiter? I really want
> Harry's
> > Morph Lag
> > (http://www.wiseguysynth.com/larry/schematics/Morph-Lag/Morph-Lag.gif)
> with
> > voltage control.
>
> The MorphLag relies on the fact that a potentiometer can be used for a
> really
> good er... potentiometer.
>
> It is VERY easy to make a lag that works with independant rise / fall
> times.
> Check
> out Jurgen Haible's VC_HADSR schematic. The core is very similar to the
> DBX
> VCA core (Blackmer patent).  You can just use the core and delete all the
> control
> logic. This will have expo (log) response.
>
> Another version with voltage control can be hoisted out of  "Muffy III"
> hex
> guitar
> that is still (at last look) at www.wiseguysynth.com (the late Larry
> Hendry's
> site -
> may he be long remembered !).   This uses a 3080 in a feedback loop and
> gets
> linear response. It can be configured to be an attack-only delay... switch
> the
> diode polarity and it can be a 'decay only' delay as well.
>
> On the ADSR issue, I'd be inclined to look at modifying the circuit by
> directly
> influencing the main cap... maybe a VC reset circuit, or a way of
> switching in
> a new value of decay resistance.   An analog switch, PWM'd really fast
> could
> make the pot look like a different resistance.
>
> > As for the filter, I believe that you can add VC resonance with a VCA.
> > That's probably why it's not found on a lot of synths, because it's not
> > considered worth the cost of another VCA. Some filters require an
> inverted
> > signal to be fed back, so you may need an inverting op amp, too.
>
> On something slow like resonance (no need to move that really fast, you
> won't
> hear it)
> a simple Vactrol is a good choice.  I did one with two photocells, a copy
> of the
>
> AN-20 "analog multiplier" from National Semiconductor app notes.  Use an
> LED
> instead
> of the incandescent lamp.  I made a VC resonance for the Aries filter
> using
> this... when I was 'noob' and did not have the chops to debug a VCA
> version.
> The photocell method
> has excellent signal handling, and ZERO offset voltage problems.
>
> The 'voltage controlled potentiometer' is one of the Holy Grails of s-diy
> design. There
> are so many ways to do it, and none of them is quite perfect.
>
> H^) harry
>
>
>
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