[Re: VC ADSR (was Pulse VCO Help)]

[Harry Bissell]

Tom: I just saw your mods. Couple'a comments...
1) Why the feedback on the 3080? Chris' circuit really isn't a sample/hold its
a "charge pump" and an integrator. Are you relying on the feedback to stop the
circuit from drifting? I have to think on this one more... there may be a
"hidden weenie" in there. What about offset trim for the 3080? Needed?
2) The fet is probably redundant. The TL074 has plenty high enough impedance
for this circuit. You might need it with a .001uF cap, like in a filter, but
not with a 10uF.
3) Chris needs the sustain buffer because of the low impedance of the charge
pump. You don't need it. you have 220K input impedance which should have
almost no effect on the sustain drive...
4) If you can stand the offset of diodes in your voltage inputs, it would
eliminate the backfeeding of the CV from the pots back into other circuits

I like your transistor input circuit. Was Chris' schematic wrong about the
trigger in on the 555? I thought it needed to go negative...

On your reset to the 555, I like a switch so that there are two modes, 
1) with a short trigger, the unit gives AR response, ie if the attack peak
hasn't been reached by the time the gate is gone, the release starts right
away
2) same as above, but the attack time goes to full peak no matter what. This
is real useful for filter sweeps where you want to trigger and walk away. I
have manual triggers on all my AR or ADSR units.




"tomg" <vco at mindspring.com> wrote:

> Anyway, I had a lot of fun figuring this one out and I am hoping someone
> will have suggestions, improvements, can point out areas of bad design,
> etc.  :-)
> 
> 
> Thanks,
> 
> Chris MacDonald


Actually, it inspired me so much I started messing around with it and before
long I had completely butchered your design...I though you might like to
look it over. The 4052 is really very clever Chris. Thank you for your effort.
Maybe you can find something here to help you with your idea.

btw this works really well!

Changes to Chris's design...

I used a transistor/cap to trigger the 555 - removed the opamp trigger.
I added a pnp  transistor to reset the 555 when the gate is removed.
I used a 3080/fet as a sample & hold - removed the 4066
I used a pnp exp-converter instead of the pulse vco
I added a opamp mixer to the sustain cv in 
I added a opamp comparator in the threshold loop
I added a opamp inverter for inverted out


http://www.mindspring.com/~vco/cbook/pics/vcadsr1.gif

-tg






____________________________________________________________________
Get your own FREE, personal Netscape WebMail account today at http://webmail.netscape.com.