[sdiy] VC Glide Circuit

[Guy McCusker]

Seems odd that this simple circuit isn't working as intended. I'm not sure
how adept you are at troubleshooting so for what it's worth here is how I
would start approaching this. Apologies if this is telling you things you
already know.

The basic setup should be that CV generates a current (via the input
resistors), which is fed into the OTA control pin, and that same amount of
current comes out of the OTA output, into the capacitor which integrates
it, giving the slew ramp. When slewing in the other direction the OTA sinks
the current rather than sourcing it. The OTA is driven hard by the
input voltages so for most of the time the current into or out of the
output pin is equal to the control current.

I'd look at the circuit in two parts: the part that turns CV into current
that goes to the OTA, and the OTA output/integrator.

First check that the expected current is being fed to the OTA control pin.
You can do this by measuring the voltage across the input resistors R4 and
R5, and calculating the current flowing there (add the two currents), and
then the voltage across R3, and calculate the current there. If the two
figures you get are not the same or very close, something is wrong in that
section. Transistor pinout would be one thing to look at.

If the control current checks out, then you need to look at the post-OTA
section. That's not so easy to do without reworking the circuit a bit --
maybe someone else has a suggestion for how to verify the OTA's output
current and take a look around the integration section.

Maybe this gets you started, or maybe it's tedious stuff you have already
thought about!

Guy.


On Sat, May 29, 2021 at 9:38 AM ackolonges fds via Synth-diy <
synth-diy at synth-diy.org> wrote:

> Hi All,
>
> I wanted some VC slew for keyboard-style playing from my modular, so I
> searched for a circuit, and decided to try and build one called 'Unopole',
> - circuit diagram at
> https://thonk.co.uk/documents/stroh/Wiring/Unopole.jpg
>
> The graph on the bottom right of the diagram shows a 0V to 10V signal
> slewing in 10ms, and I simulated the circuit in Falstad Circuit Simulator
> (text for circuit pasted at end), and it is slewing even faster. However,
> when I build the real circuit the glide is much slower.
>
> I've tried putting maximum current (2ma) to the OTA control pin, bringing
> down the value of the charging cap C1 to 10nF, and the fastest I'm able to
> get it to glide (at fastest pot setting and CV input together) is about
> 32ms from 0V to 1V, which is about 300ms over 10V.
>
> Below about 10nF for C1, the tuning of the 1V/Oct signal starts faltering.
> I'm using a ceramic C0G cap for this. Should I be using a different type of
> cap? If so, which would you recommend?
>
> Alternatively, does anyone have any ideas as to why I'm having trouble
> getting fast enough glides with this circuit? I've built it a few times
> with the same results. Or, has anybody on here built a different VC glide
> circuit that is more suitable?
>
> Thanks so much for any advice any of you may have.
>
> FALSTAD CIRCUIT
> ------------------------
>
> $ 1 0.000005 11.086722712598126 17 5 50 5e-11
> 174 48 128 48 208 1 100000 0.48020000000000007 Resistance
> g 48 224 48 240 0 0
> R 48 128 48 96 0 0 40 12 0 0 0.5
> w 64 176 96 176 0
> r 96 176 176 176 0 15000
> r 96 240 176 240 0 10000
> w 176 176 176 240 0
> a 240 192 320 192 8 12 -12 1000000 0.000005472331080331692 0 100000
> g 240 208 240 240 0 0
> w 176 176 240 176 0
> c 336 192 336 256 0 1e-10 -0.5472385803642495 0.001
> w 320 192 336 192 0
> d 400 192 400 256 2 default
> w 336 192 400 192 0
> w 176 240 176 272 0
> w 176 272 304 272 0
> w 304 272 304 256 0
> w 304 256 336 256 0
> w 336 256 400 256 0
> w 400 256 464 256 0
> t 432 224 464 224 1 -1 5.814783631368241 -0.5472385803642495 100 default
> w 464 256 464 240 0
> w 432 224 432 192 0
> w 432 192 400 192 0
> w 464 208 464 144 0
> r 464 144 464 80 0 10000
> 402 656 352 768 352 1 0\s0\s40\s-9\s0\s0\s0.5 0\s0\s40\s9\s0\s0\s0.5
> 0\s1\s-12.343529771373202\s0.5472335109924158\s100\sdefault
> 0\s1\s-0.5472335109924158\s0.5469786835162118\s100\sdefault
> 0\s1\s0\s0.5469786835162118\s100\sdefault
> 0\s1\s-8.547498288201437\s-4.257672077077218\s100\sdefault
> 0\s1\s-2.9215945571340436\s0.5469761486367535\s100\sdefault
> 0\s-1\s0.12505886554275492\s-0.14737295745119638\s100\sdefault
> 0\s-1\s0\s-0.14737295745119638\s100\sdefault
> 0\s-1\s17.405000677164992\s-0.12505886554275492\s100\sdefault
> 0\s-1\s0.5469710782877009\s-0.5467162500596725\s100\sdefault
> 0\s-1\s0\s-0.5467162500596725\s100\sdefault
> 0\s-1\s7.72624111758356\s-0.5469710782877009\s100\sdefault
> 0\s1\s0\s0.1634519859667556\s100\sdefault
> 0\s1\s-0.159115513874303\s0.1634519859667556\s100\sdefault
> 0\s1\s-8.857504054228007\s0.159115513874303\s100\sdefault
> 0\s1\s0\s0.10015101242435948\s100\sdefault
> 0\s1\s0\s-4.704497213289612\s100\sdefault
> r 656 320 560 320 0 10000
> R 560 320 512 320 0 2 100 5 0 0 0.5
> w 464 80 752 80 0
> w 752 80 752 320 0
> w 768 352 800 352 0
> c 800 352 800 416 0 8.2e-8 0.18007155406906683 0.001
> g 800 416 800 432 0 0
> w 800 352 848 352 0
> r 672 496 752 496 0 10000
> w 656 384 656 496 0
> w 656 496 672 496 0
> a 912 336 992 336 8 15 -15 1000000 0.1800697533715331 0.18007155406906683
> 100000
> w 752 496 864 496 0
> w 848 352 912 352 0
> w 864 496 864 320 0
> w 864 320 912 320 0
> w 864 320 864 272 0
> w 864 272 992 272 0
> w 992 272 992 336 0
> r 992 336 1072 336 0 330
> g 96 240 96 272 0 0
> o 28 64 0 4098 10 0.1 0 2 28 3
> o 45 64 0 4098 10 0.1 1 2 45 3
>
>
>
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