[sdiy] ADSR woes..

[Scott Gravenhorst]

Seb Francis <seb at burnit.co.uk> wrote:
>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 ...


Been there, done that, heh, this is why I like solderless breadboards,
especially for something like an EG.



>
>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
>
>

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