SoftPot-based Ribbon CV/Gate controller

[Taper]

I am building a number of synthesizer parts, mostly very simple circuits, into Eurorack modules so that I can play with modular synthesis on my very limited budget. One piece of the puzzle I've been lacking, however, is a vaguely playable device to provide control voltages and gates. MIDI to CV will likely come later, but right now, I just want something to give me something construable as an analog output and a gate.

One thing I do have is a well-stocked junk box, and some years ago I acquired a couple of softpots. The one I've been testing with is maybe 20cm long, but I have another that's a full 50cm. So I figured I'd whip up some kind of ribbon controller.



Here's the circuit I've come up with. To explain it, first, I've got to say that there's some conflicting information around on these softpots. I've heard they can be used as a straight voltage divider, just like a normal pot, but also dire warnings not to run "full voltage" across them, lest they burn up when you touch the bit that would drop the divided voltage to zero. Deciding to err on the side of caution, I followed the advice of Adafruit's page and connected it to power and ground through 10k resistors on both lugs. Combined with a weak pull-down, this gives an output of 0v when not touched, and a range of 3v-6v when touched (it seems pretty linear, but this is by no means a precision instrument). I was initially a little disappointed by this output, until I realized that this jump on touch would, when run through a comparator, give me a gate output as well as the CV output.

So. For the CV path, the 3-6v is run into the first op-amp; the top Zener diode is generating a 7.5v level that's then divided into 3.75v, and that plus the rest converts the signal back into one that varies between 0 and 5v. The second op-amp buffers that; it's probably not strictly necessary but I have a lot of TL082s and might as well use both op-amps on the chip. Then a series resistor protects the output, which goes to a 3.5mm mono jack.

The gate path takes that same 0v or 3-6v signal, and opens the gate whenever the voltage is greater than the reference voltage of 2.7v, provided by another Zener diode. Protection resistor and mono jack, again.

It's all designed to run off a 9v battery, to be self-contained, and the power switch just cuts the hot connection to everything else.

Disclaimers on the drawing: I neglected to include the bypass cap on the comparator -- and while it's shown as an LM311, I'm actually using an NTE922M, because that's what I have. (I believe they're nearly identical, but...) Also, resistors don't usually come in 60k. (I'm going to use two 30ks in series.)

(The scale-and-offset circuit was designed with help from TI's Application Report SLOA097, "Designing Gain and Offset in Thirty Seconds"; the zener voltage converters were checked using the Android app ElectroDroid, though I'm using relatively large current limiting resistors since they're feeding op-amps.)

Known or suspected problems:
1) This is going to drift a bit with temperature and, in particular, over the course of the battery draining. Which end is high and which is low will be consistent, but I don't think you're going to be able to get really consistant pitches into a VCO without some quantization, and maybe not at all. (Inconveniently, I don't have a good VCO yet, so I'm not sure.)
2) The CV is 0-5v, but the gate is 0-9v. Sloppy.

Both of these, long-term, are probably best dealt with by using a 12v power source and proper voltage regulators, to provide consistent power rails and references. My current modules with gate inputs don't mind the higher levels, but if you really need to limit the gate currently (to feed digital/microcontroller modules, perhaps), you could slap in a pot or voltage divider as an attenuator, maybe swapping the voltage follower to buffer that instead of the offset/scale.

Anyway, I've breadboarded this, but intend to put it all on stripboard in the next couple of days and have a properly boxed-up instrument. Is there anything I missed? Any gaping holes in my logic, or my math? Comments, suggestions, hints? Any feedback is greatly appreciated.

[PHOBoS]

looks good to me.
just some suggestions: put the 1K resistor on the cv output in the feedback loop or leave it out all together. As you said it isn't really a precision
instrument and wouldn't cause much of a problem, but it can cause some detuning if you were to connect more than one device to the output.
Also you might want to add a sample & hold circuit although you could add that externally since you have a gate output. This will keep the CV
level constant instead of dropping back to 0V when you release the ribbon. oh and maybe increase the value of R3 and lower the value of R10.
(although right now it creates 4.5V if you connect it to a 100K input which could be useful)

[Taper]

Thanks!

Yeah, a S&H would be a really good idea on there -- might be a better use of the buffer amp; I'll have to think about that (it might wait for version 2, when I commit the longer softpot -- and I need to check what caps I have that'd work well for that). Do you figure boosting R3 to 1M and dropping R10 to 10k makes sense?

[PHOBoS]

Those are values I'd probably use
Maybe a bit lower for R10 but if you want to have it battery powered I'd stick with 10K. I actually use opamps instead of real comparators
most of the time since they can sink and source and often are protected against shorts too. Unless I have to switch a different voltage
in which case it is very useful to have an open collector output.

For a S&H you might get some inspiration from the Quad S&H circuit I posted here.

[Taper]

Wow, I missed something big in this circuit. I designed the scale-and-offset amplifier, there at U1A and associated bits, to move a 3v-6v range signal to a 0v-5v range. And it works admirably on that. But I forgot entirely about two things.

1) It's an inverting amplifier, so when the input is at maximum, the output is at minimum. That's fine, of course, when you know that -- I just need to note down which end of the strip is high, and maybe swap the lugs if that's undesirable.

2) The input range doesn't range between 3v-6v at all! When pressed, the voltage is between 3v and 6v -- but when not pressed, the voltage drops to 0v (as designed! that's why there's a pulldown! the gate depends on this!). As soon as the input to UA1 goes to zero, the output pegs at the high rail -- 9v, here.

Number 2 is the worse mistake, obviously. That's not, strictly speaking, a showstopper -- it's within Eurorack tolerances, and if this is a VCO input with the gate controlling an associated envelope/VCA, the frequency jump will go unnoticed -- but it's, again, sloppy. I think that a S&H would fix this, if the timing is right; it would have to, basically, open the gate before the 9v jump got through the opamps. But this all complicates the situation tremendously.

PHOBoS, I looked at your S&H, and it looks like a great circuit (I might build the whole thing), though I'm concerned about adding a few more ICs to use very small parts of them. (Though I went down a JFET-switching rabbit hole, too, and clawing through too many simulation systems.) I may see if I can get hold of an LF398 just to keep the thing inside the stupidly small enclosure I'd been mocking this up to fit inside.

At least in simulation, if I float the output of the softpot, I lose the ability to create a gate. Any other ideas?

[Grumble]

any ideas? sure, if you not already have, read:

http://electro-music.com/forum/topic-41078-0.html&postdays=0&postorder=asc&highlight=appendage
and:
http://electro-music.com/forum/viewtopic.php?highlight=upendage&t=70620
and:
http://electro-music.com/forum/topic-49650.html

[Taper]

Ah, thanks for those links! I had come across the Appendage earlier, but got lost in the original thread and decided I was wanting to do something simpler -- but reading through the "Appendage Exposed" document explains a lot that I was groping towards. Likewise, your uPendage looks quite spiffy. I've been trying to stick with analog in my current stuff, mostly because I've been doing microcontroller (Arduino and Teensy) stuff for a long time and am wanting to get away from software.

Stites's document mentions John Simonton's method of a 'lag' circuit to keep the voltage feeding the S&H (well, really a track & hold, but the standard two-opamps-a-switch-and-a-cap circuit seems be that) in place long enough to be triggged by the gate. That should be doable with a slew-rate limiter, and I was just playing with one of those from Ray Wilson's AR envelope generator from the Noise Toaster -- it's just a diode and a resistor. I'll have to do some more breadboarding and report back.

(Long-term, I think I'd rather like to build an Appendage or the digital equivalent for myself, but I'm not sure I need the full complexity at this point. My single "VCO" is an AtariPunk modded to take CV inputs, to show the level of sophistication I'm working at at the moment.)