VC ADSR

[macdonald at evenfall.com]

Hi Colin,

> This VCADSR is almost identical to my own design from a couple of years ago.

Wow that's great, I would love to see the schematic!


> I built a dual VCEG using a LM13700 instead of the 3080, and instead of the
> 555, I used a 4013 dual d-type flip-flop.
> The flip-flop is set by the leading edge of the gate pulse, and reset by a
> comparator that detects the attack peak.
> No re-triggering there.

Interesting.  BTW I don't think my circuit will retrigger as long as the
gate input is not left floating.  It should work fine if the gate input
always sees either 0 volts or +5 volts.

In my design there is no state change in the 555 at the leading edge of
the gate pulse.  Instead, the 555's threshold comparator is to detect
the attack peak, which resets the 555 flip-flop, and then the trailing
edge of the gate pulse sets the flip-flop back via the trigger
comparator.  This way you get a flip-flop and a comparator in one 8 pin
DIP, although the envelope's attack peak voltage is limited to 2/3
supply voltage.


> The inputs to the 4052 also have op-amps in front of each that mix a front
> panel control with a modulation input for each parameter.

Good point, my circuit lacks these.  But I had it in mind for use in a
polyphonic voice card so I didn't mind so much.


> However, the use of OTAs as the voltage control element in VCADSRs was
> discussed at length here a while back, and generally established to be 'a
> bad thing' due to the changes in output offset voltage of the OTA with
> changing bias currents as the EG switches between phases.
> IIRC that was the impetus for Juergen to design his nifty 4 transistor
> vc-element design.

I have read the archives on this. Previous circuits I have tried did
seem to suffer from the offset voltage changes.  This circuit seems to
perform acceptably to me though.  ???


> I'm still using the OTA VCADSR though, and the offset problem isn't *that*
> noticeable - I haven't had time to built JH's design yet.
> I may not either, because I've been turned on to the idea of processor
> generated EGs by the Waldorf Pulse...
> More on that subject later, I think.

After having experimenting a bit with VCADSR circuits, I see why
manufacturers went to computer EGs!  :-)

-Chris