SergeModular

Hi Steve,

Many thanks for the response - that has more than answered my question!

> > Ken runs a Yahoo group (cgs_synth) for the discussion of the stuff
> > on his site. You may want to post over there.
>
> No. The schematics on this list were produced by Ken, Dean
> and me, and are not normally to be distributed outside this
> group. Ken and I are still active on this group, so no need
> to cross post.

Dang! Too late, unless Ken sees this too and merely doesn't pass my message there!

> > I'm trying to simulate the Negative Slew from this schematic
>
> That's brave of you. Have you found a good spice model
> of the LM3900 / MC3401? There are some homebrew
> models which I don't entirely trust, but I've never seen a
> manufacturer's model.

Homebrew mine is too, as I don't believe there *are* any manufacturers models for the 3900/3401 out there. I have slowly been refining it as I go, and at the moment it is not too bad, except the cap for the main pole causes grief sometimes between DC sweeps and AC analyses, but I might yet get it working better. It has: a dominant pole; slew rate limiting (which again I can't quite get right - the ideal'ish diodes I'm using at the input cause problems...); output current- and voltage-limiting; negative voltage 'protection' on inputs; output resistance (which at 8k from the datasheet seems waaaay too high to make some sims run like the real thing). (I did try a component level simulation, but then it becomes much, much harder to 'remain in control' - you're at the mercy of which ever transistor you use...)

> > I also think the (kind of) 'diode connected' transistor 9/10/11
> > would cause grief to the CA3086 if the 'diode' could in fact
> > conduct: this would effectively put the transistor into saturation,
> > which might be bad news for the chip, as I think it would turn
> > on the parasitic PNP transistor (formed from base-collector-
> > substrate), causing current to flow out of the substrate.
>
>
> This confused me for a while too. I believe Serge is using the
> E-B junction as a (7v) zener diode. When the input is higher
> than the output, the transistor 3/4/5 switches off, the 10K at
> the top pulls pins 10 & 11 high enough to make the E-B junction
> zener, switching on transistor 6/7/8 and discharging the cap.

Ack, that's it! I wondered if it was going to be something yeuchy along those lines. It's unfortunate that the standard SPICE BJT model doesn't model such things as reverse junction breakdowns, as it does for diodes. And the annoying thing is I was referring to Camenzind's excellent book for the stuff about parasitic devices, and if I had read the bit about his standard integrated NPN model, fig 2-11 page 2-12 here:

http://www.designinganalogchips.com/_count/designinganalogchips.pdf

I might have thought about the zener myself. In any case, that has cleared up the mystery, and the simulation does now look like it is producing usable results (now I have to figure out how it manages to _follow_ positive going edges...).

> The other surprising thing is that switching on transistor
> 6/7/8 doesn't cause excessive current to flow into pin 8
> of the MC3401 and cause blowupedness

Presumably the big discharge current merely circulates cap-33R-transistor, so the input pin isn't really involved.

> Have you found the almost-exponential converter yet?

That's the bit I'm interested in! I'm actually looking at improving the 'controlability' of the Plan B M10: it seems the core of that circuit is virtually a direct copy of the Serge envelope generator, and when I saw the arrangement between the summing amp and the integrator (pins 3 & 13, separated by the 18k here) I initially thought it was a mistake, and had to double- and triple-check my circuit tracing, finally concluding it would *really* help to see the initial source of the circuit, which someone kindly helped me out with. The greater part of the action controlling the length of the envelopes occurs within maybe 15-20 percent of pot travel, and the relationship between rise/fall/timebase pots means there are large portions of travel which simply have no effect. Having measured and plotted the time of different length envelopes, I do get a similar-looking curve from my sim, but I really wanted to play with something simpler in order to experiment with a more obvious arrangment between those two amps (i.e. linear), plus perhaps log pots, to see just how 'tractable' that might be, so when I was pointed in the direction of this negative slew, with the exact same set-up, I thought that is what I'd use. It would be interesting to know if people with any of these original Serge modules find that the pot travel is useable, or if it 'could be better'.

Once again, many thanks for your help.

Tim
__________________________________________________________
Tim Stinchcombe
Cheltenham, Glos, UK

www.timstinchcombe.co.uk