[sdiy] more sh-101 mods?

[harrybissell]

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