[sdiy] Ken Stone VCS -> confusion

[Tim Stinchcombe]

Apologies for resurrecting a thread that went dead a while ago, but I would
like to correct a couple of 'inexactitudes' if I may (I do so hate putting
bum info out...). I have also written up most of my original post, plus more
(including pretty pictures), on a new webpage:

http://www.timstinchcombe.co.uk/synth/vcs/vcs.html

The 'corrections':

> Note that 5/6/7 is *inside* the negative feedback loop of 
> 1/2/3

This was referring to the TL072 sections, and it is really the other way
round - 1/2/3 is inside the feedback loop of 5/6/7. In fact the whole thing
acts like an 'opened out' op amp - slew section+high gain amp+ integrator.

> As for a 'gate' signal being applied to cause a 'sustain', I 
> can't see that is to be applied at the 'trig in' signal, for 
> two reasons: 
..<< snip >>..
> 2. once the 
> envelope has climbed past about 3V and 'end out' has gone 
> low, this (probably) clamps the top-most diode at pin 13 
> about a diode drop above ground

No it doesn't - there is plenty of resistance at LM3900 pin 4 (1k+2k2) to
allow it rise up, so...

>, reverse-biasing it, as pin 
> 13 is _already_ about a diode drop above ground from the 3900 
> input. This should (again I add an 'I think') be enough to 
> stop any further pulse at 'trig in' from having any effect

...this isn't so either - the module is happy to re-trigger if you supply
another pulse before the current envelope has completed!
 
> So maybe the 'gate' has to be supplied at the 'in', where it 
> forces the envelope into some sort of 'voltage following', 
> i.e. holds it where it is (sort of, this is where things are 
> getting fuzzy...).

This is exactly it - to get sustain, input a pulse of desired amplitude to
the 'input' jack, and the envelope will follow it when it falls to that
level (simple enough really).

Tim
__________________________________________________________
Tim Stinchcombe 

Cheltenham, Glos, UK
email: tim102 at tstinchcombe.freeserve.co.uk
www.timstinchcombe.co.uk