[sdiy] Ken Stone VCS -> confusion

Mon Dec 28 05:48:00 CET 2009

Tim,

Your assessment of the way a sustain would work on that circuit is
correct. The In is used as a Gate Input and the Trigger In is used
just to fire a complete cycle. I've been dinking with this circuit for
the last few months (I'm really slow) and your understanding of it has
helped quite a bit, thanks so much for posting.

I think one of the trickiest bits I ran into was getting a handle on
the "End Out" functionality as my initial assumption was that it would
stay at a low state when nothing was going on but instead it hovers
~5V.

Neat circuit though, eh?

-dustin

On Fri, Dec 25, 2009 at 8:58 AM, Tim Stinchcombe
<tim102 at tstinchcombe.freeserve.co.uk> wrote:
> 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
>> _______________________________________________
>> Synth-diy mailing list
>> Synth-diy at dropmix.xs4all.nl
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>>
>>
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