[sdiy] Ken Stone VCS -> confusion

Fri Dec 25 15:58:34 CET 2009

I've been looking at this, and other related Serge circuits, off-and-on over
the last few months, and it was only a few weeks ago on another forum I
mentioned that I believe this circuit to be amongst the trickiest synth
circuits I've looked at in the 8 years or so I have been involved in synths.
(I even picked up a second-hand module some weeks ago, but haven't had the
time to look at it at all.)

I might not be able to help with the 'gate' aspects, as I've not sussed that
part of operation yet (more below), but here is what I do know.

First, the easier stuff, which to my mind actually turns out to be the
LM3900 operation (!), starting with the 'end out' signal. If the output pin
4 were at the positive rail, then there would be approx 10uA into the pos
input, which would keep the output high until the envelope ('out') goes
above about 3V, at which point 10uA or more is delivered to the neg input
pin 3 (via the 330k), which will switch the output low. Thus the default
state is for pin 4 to be high; once triggered and the envelope climbs above
3V, the output will switch low, where it will stay until the envelope has
fallen back to near ground, and the output will flip high again - hence the
'end out' signal.

For the other pair of LM3900 amps (8/9/13 & 10/11/12), the default with no
trigger and envelope low appears to be pin 10 at pos rail (i.e. high), which
feeds current into neg input 8, and with no trigger/no current into pin 13,
output pin 9 is low. On trigger input, a pulse of current into pin 13
switches output 9 high; with no envelope/little current into pin 12, and now
lots of current into 11, output 10 goes low. As the envelope climbs, the
current at pos input 12 will exceed that into neg input 11, and so output 10
switches high, in turn causing output 9 to go low. Thus pin 9 is 'attack',
and pin 10 is 'not attack'.

When generating an envelope (and not 'following'), TL072 1/2/3 is acting as
an inverting integrator; the 4 transistors at the virtual ground at its
input are actually wired as two pairs of differential pairs (re-draw it!),
an NPN pair, and a PNP pair; the tails of each are wired throuh  the 22k to
output 7 of 5/6/7 - 5/6/7 *sometimes* (it is not the whole story!) acts as a
basic comparator, with pin 7 either at the pos or neg rail, and this turns
the appropriate diff pair on or off. When pin 7 is the neg rail, the NPN
pair is on, which sinks current from the virtual ground, and so pin 1
increases, hence we have 'rise' (and v-v for pin7 pos rail/PNP pair/fall).
(The voltage at the bases of the diff pairs sets the rise/fall rate.)

Thus operation is: trig in makes pin 9 high; the PNP connected via 220k/68k
is off, so TL072 pin 5 is pulled to -5V or so by the 2 220ks; (assuming the
47p acts as an open-circuit) pin 7 will be neg rail, so we are in
rise/attack phase, output pin 1 rises (pin 5 will 'track' envelope with an
offset - it is negative and increasing). When envelope gets high enough,
3900 pin 10 flips to high, and pin 9 goes low; the PNP turns on, making the
diode between the 220ks reverse-biased, and now '072 pos input pin 5 tracks
the envelope exactly, which being positive, ensures the output pin 7 goes to
the positive rail, which turns the NPN pair off, and the PNP pair on, and so
we go to 'decay'/fall phase, and the envelope falls. Finally, when it has
fallen enough, 'end out' goes high.

Note that 5/6/7 is *inside* the negative feedback loop of 1/2/3 - when there
is a slowly-changing input at 'in', this is what makes output 1 merely
follow the input; if the input signal starts changing too rapidly, the
slew-limiting effect caused by the max currents in the NPN and PNP diff
pairs comes into play. (This is one area of operation I'm not entirely clear
on...).

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:
1. the 10n at the 'trig in' will only pass a pulse of a certain length, and
after that there is simply nothing that can be used in a 'sustained way'.
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, 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 until 'end
out' releases this clamp by going high (i.e. it cannot be re-triggered once
set in motion).

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...).

I'm hoping that when I get the time to fire my module up, I may be able to
sort this missing piece of the puzzle out!

Tim
__________________________________________________________
Tim Stinchcombe 

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

> -----Original Message-----
> From: synth-diy-bounces at dropmix.xs4all.nl 
> [mailto:synth-diy-bounces at dropmix.xs4all.nl] On Behalf Of ghetto synth
> Sent: 23 December 2009 04:12
> To: synth-diy
> Subject: [sdiy] Ken Stone VCS -> confusion
> 
> 
> 
> Hi All,
> 
> Between my limited electronics skills, the confusion of the 
> LM3900, and the overall busyness of the circuit, for the life 
> of me I can't figure out what's going on in Ken Stone's VCS.
> 
> http://www.cgs.synth.net/modules/pic/schem_cgs75_vcs.gif
> 
> Typically when I'm at this point I just go breadboard up some 
> pieces of it until it clicks but my shop is out of commission 
> right now. Could anyone describe the circuit operation in 
> terms of basic building blocks given the two use cases of 1) 
> a trigger input, and 2) a gate signal applied to the input. 
> In particular, what's going on with the 3900s, the TL072 on 
> the input and the 2N3906 just to the right of the input TL072?
> 
> tnx
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> 