[sdiy] ADSR woes..

[Seb Francis]

Hi,

I've just finished building my 'NoizBox' and I'm rather disappointed to 
find that the ADSR part of it is not behaving itself.  Unfortunately I 
didn't notice this behaviour at the breadboarding stage and now 
everything is densely packed and soldered onto stripboard :-(

This being the case, it's quite difficult to try out different things, 
so I thought I'd run the problem past the group to see if anyone can see 
an easy solution ...

The circuit:
http://burnit.co.uk/sdiy/ADSR.gif

The problem:
When the release pot VR8 is set to 0 (fastest release) and the envelope 
voltage at the release point is above a certain level (e.g. depends on 
Decay and Sustain settings), the flip-flop (U1E, U1F, etc.) doesn't 
reset reliably when the gate goes low (i.e. when U1:D pin 12 goes low).
I'm guessing this is because the current drawn into U1D on release is so 
high that the output is not square enough to generate a sufficient low 
blip on the other side of C19 to reset the flip flop.

I've tried:
Reducing R21 down to 100R
Reducing C19 down to 470pF
Both these things help (i.e. the envelope voltage at the release point 
above which things stop working increases, but the problem still exists 
at extreme settings - e.g. release time = 0, env voltage at release = full)

I thought about reducing the values of R21 and C19 even more, but I 
suspect this still will not be very reliable at extreme settings.  I 
also though about buffering between U1:D pin 12 and R21 with a voltage 
follower opamp (e.g. TL082), but this will be a bit messy to do in the 
board space I have left .. hence over to you guys for your input :)

Seb