[sdiy] simulating low voltage LM13700 stuff

[Ben Bradley]

This is not the answer you're looking for, but may be a substitute I
did long ago. I recall your earlier question and was thinking of it
then. I've meant to try it (again) but haven't gotten to it.

Many decades ago (early 80s, just out of college) I made a crude VCF
circuit with a twin-T RC network in the feedback look of an op-amp
(might have been a LM3900 current-mode thing I was playing with that I
got from Radio Shack at the time - I think these will run as low as 3
volts or so). Here's a basic schematic:
http://fourier.eng.hmc.edu/e84/lectures/ActiveFilters/node4.html
This says it's a notch filter, but as it's in a feedback loop, the
gain goes higher at the notch and it's effectively a band-pass filter.
I replaced R3 with a 1N914/1N4148 diode with cathode to ground, and
had a resistor (maybe 2k to 47k, I forget) connected to the
anode/two-caps node. The other side of this resistor had a varying
positive voltage (from an AR or ADSR), The higher the voltage, the
more current through the diode, the further up the diode forward
conduction curve it went, and the lower the effective resistance to
ground.

I knew this was going to have a lot of distortion due to the AC signal
dynamically changing the diode conduction point, thus the signal level
needed to stay low to not have outrageous distortion, but I tried it
and was amazed at how well it worked. I also had no idea how the
Minimoog filter worked, and was amazed so many years later when I
found out.

On Tue, Nov 14, 2017 at 3:54 PM, Chris McDowell <declareupdate at gmail.com> wrote:
> Howdy list,
>
>
> a little bit ago, I posted here asking about low voltage filter designs. I later found that the 13700 works just fine at 3.3V, after designing a circuit for 5V and just giving it a shot.
>
> I'm finding that LTSpice will not simulate these designs at 3.3V. Anything with a 13700 seems to fall apart around 3.9V, from what seems to be some small constant offset. I get signals stuck on the rails in a filter design, 0.4V offset and no gain on a VCA design. I'm not experienced enough to see the pattern here of why it's falling apart this way, or why it would fail in LTSpice but work so well in real life. I've now tried three different LM13700 models, all with the same behavior. Does anyone here have any experience with this? Any idea why this might be happening?
>
> Cheers,
> Chris
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