# Voltage controlled resistor circuit

I think I got everything you mentioned (part numbers may have changed slightly because I had to reannotate)

But with all these changes in place, you can now insert a 2164: Vin to Z, Vout to pin 6 of the integrator.

as breaking

• Pin 6 is connected to Z.

basically allowing a variable amount of current between 6 and z.
Was that assumption correct?

I’m working on PitchCV with @brianleu now. I think what I want is this circuit from ripples (F_CV = PitchCV in my schematic above), but I’ve learned it’s slightly more complicated because I’m just using 9v and don’t have access to + and - voltages to do this. Hopefully we will have that part figured out tomorrow and I can share.

You almost got it right (though in a way that is a bit hard to parse visually!). The R26/R32 divider should be connected to pin 7 of U3B – you’re showing to the comparator a fraction of the voltage at the integrator’s output, not some current!

Ah yes, you’ll have some work to do here! I don’t remember how the V2164 is wired internally – if the control voltage has to be Vbias for an attenuation of 0dB to be obtained (I think so!), or 0V.

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EDIT 5:

Small update, I was missing connection to A3 on the 4067

And so far as the inverter/attenuator, here’s what we (well really what @brianleu) came up with:

simulator

In terms of supply pins for the 2164…we’re not sure if it should be this V/3 bias (necessary to get the math right) or the V/2 bias the rest of the gerassic organ circuit uses, but that’s easy enough to experimentally figure out.

The CV for the new pitch input responds to 0…+9v, which we think is fine? In terms of euro land, it seems more common to accept -5v…5v, but that could be achieved with a negative offset if needed.

Four observations.

1. I assume the 40 Hz sine wave is your CV source. Then you don’t need the buffer. It’s actually doing more harm than good since it has to be a fancy rail-to-rail part to work as expected and output voltages as low as 0V. Obviously the circuit simulator simulates ideal op-amps! A benefit of removing that op-amp is that you can adjust the circuit for a wider CV range that will extend below 0V and above 9V.

2. Related to the above: it’s very rare for a CV input circuit to be something other than a 100k resistor going to the inverting pin of an op-amp.

3. I checked the V2164 datasheet, and the voltage on the VC pin must vary between 4.5V and about 6V for an input range of 9 octaves. 4.5V for the 2164 to let all the current flow through, about 6V for the 2164 to attenuate the current by 2^-9.

4. I would recommend buffering VBias before sending it to strategic places - at the very least the 2164 pins. It’s going to be used in many places in the circuit. You’ll lose some of the original tone character, though, since in the original circuit, many subcircuits are loosely coupled and interact with each other imperfectly through that wobbly voltage.

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The only reason it may be needed here is that this is intended to use banana jacks, which makes it a similar case to this where switched jacks (connected to GND when no jack inserted) can’t be used.

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Bias does very slightly go up when one of the buttons is depressed and back down when it is released (it goes from like 4.1 to 4.25 as measured on my old scope). I can investigate buffering the whole circuit, as well as just buffering the 2164 pins.

I do really like the tone of the organ in the original circuit! fingers-crossed making the circuit better doesn’t lose the vibes…I’m really excited to able to mess with pitch changes in time with modulations doing the gate in stuff, I think it could potentially make some pretty strange and interesting sequences.

So far as the buffer goes, I was just thinking and we could add a physical switch that grounds the input in addition to the jack. Not sure about the tradeoff there.

Also apologies it wasn’t clear about the banana jack thing. the plan is actually to do all the jacks with this combo thonkiconn/cinch banana part that electric noodle box has shared with schematics for their modules https://electricnoodlebox.com/diy-pages/

with the idea that people could have a choice if they want to build this with 3.5mm TS jacks or bananas (like the rest of the Ciat-Lonbarde stuff)

for that to happen i need to figure out how to get their eagle part to work with kicad and i was trying to take it one step at a time (getting the schematic more or less figured out before i start importing custom parts and doing the PCB layout)

Here is what I’m thinking w the physical switch idea

simulation

Ah thanks for pointing that, I’m way too much Eurorack-minded

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I’ve made these circuits on the breadboard and am getting no sound or any sort of voltage change out of J3. Things I’ve attempted to troubleshoot:

• I’ve went over the circuit traces several times
• I attempted to buffer all of v bias because it kept going up in voltage as things were connected to it.
• I’ve tried excising the 2164 part of the circuit, connecting pin 6 directly to z

I’m going to try to redo this whole part, but I want to check to double check things are right around U3 and Q6?

There’s something wrong around R26/R32.

It should be a divider that takes the voltage at U3B’s output, and the divided voltage is applied to the base of the transistor.

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After a period of burnout and taking things a part/putting them back together again several times, I’ve finally got it working. Need to dial in the response a bit, but it’s working pretty good. Thanks for all the help!! Side note, really need to figure out a more organized method of breadboarding than the mess of wires I’ve got going on, makes it very difficult to debug.

I currently have bias buffered to all destinations and the sound is still pretty gerassic organ-y. One interesting thing I noticed is that fast sequences can cause some build up in the amplitude which results in “clipping”. Raising C3 to 100uF helps mitigate this problem, but the response is too slow for normal playing…will probably put that on a switch. It seems like C5 doesn’t seem to really do anything any more. maybe that is due to the bias voltage being buffered.

As I get into PCB designing mode, is there any good resources out there for figuring out the best SMD equivalents to the through-hole stuff I’ve been using on my breadboard (particularly op-amp ICs, transistors and diodes…I feel like resistors and caps should be more straightforward.).

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Got things figured out for v/oct kind of. The response is not perfect, it seems to overshoot and then undershoot as cv goes up, so that it sort of horseshoes around accurate tuning. this is 2 octaves. I did buffer that bias voltage so that’s constant now.

I tested the ripples 2020 with the same signal and it seems to be much more consistent (it actually overshoots a little right now, but in a very consistent way so prob just needs to tweak the calibration trimmer slightly).

I started reading a bit more about this stuff and it seems accurate v/oct exponential conversion can get pretty complicated. I think I’m going to try grounding the other input/output pins of the 2164 like the datasheet recommends and then calling it a day.

kind of answered my own question here. I’m gonna just go with the SMD package of the same parts I’m using, which are:

• LM358/TL07X opamps
• various 4000 series ICs
• 4148 for all the diodes in the circuit…something 4001/7 for power
• 547/557 transistors

I’ve learned the 358 is a pretty old design, but I couldn’t really find definitive answers on what is better and don’t understand enough about this yet to figure out for myself from datasheets.

There are several things to take into account here:

• The op-amp comparator and the switch have a non-zero switching time. The higher the frequency, the more important this switching time is relatively to the period of the waveform. Ie, there’s an annoying constant term in the relationship between the period of the oscillator and its control current. This can be compensated by adding a small amount of current to the input of the integrator.
• The 2164 response is temperature dependant. Check the Anushri’s schematics (page 3, cells A/B, 3/4) for a scheme that eliminates some of that dependence. (rough idea: we scale the CV by a small temperature-dependent amount).
• The response of the 2164 is not perfectly exponential. At this stage, this gets tricky to compensate but we’re in the diminishing returns zone
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So I’ve been working on this decay (x resistor) voltage idea, and you were right, it does do some really cool stuff with the circuit. But I’m having a problem, with the op-amp voltage follower…I believe to properly work, XBiasVoltage needs to be the full voltage of the positive rail to work properly and decay to silence. (It does cool stuff, like infinite droning and weird phantom note appearing when under-biased). That seems to work great simply passing the voltage to it (bottom), but with an op-amp follower, I seem to be losing about a volt (top…measuring +8.4v on the power rail, +7.4 output of the op-amp).

So my questions are:

Does that voltage drop nearr the rail sound like a normal thing, or is there something weird going on? Is there some way I could make the buffer version work?

Would it be okay to just do the simple circuit…or am I setting myself up for potential damage of the circuit or equipment being plugged in?

One answer: the LM358 does not have rail-to-rail outputs (but the input and output voltages can go as low as Vee). This means that the output voltage will clip below Vcc. So it’s normal that your buffer output can’t reach 9V.

Questions:

• I don’t understand the role of the second op-amp with the diodes in the buffered circuit.
• I don’t understand why there are three adjustable resistors involved (RV26, RV6, RV5).

Ah got it! So I need to figure out an opamp that can go rail-to-rail and doesn’t have the TL072 thing where it flips high at < than a certain voltage (I think that’s what “but the input and output voltages can go as low as Vee” means

Yeah I don’t think I need it actually…I thought I needed it to invert the inversion. But I think with single supply, you might not need to do that. I need to read more about these buffer circuits and actually understand them.

• I don’t understand why there are three adjustable resistors involved (RV26, RV6, RV5).

So RV6 was my mistake…I made XBiasVoltage CVable after setting it up (it was just 9v) and I forgot to get rid of that pot…

RV26 = variable voltage going in and RV5 = variable resistance…in practice they are very interactive. Like if both are low you get some weird droning. If the voltage is high, the variable resistance acts as an envelope decay control. If the voltage is middling variable resistance acts sort of like a volume control. There are also what sounds like vaguely LPF cutoff effects.

The buffer circuit made with U7A is non-inverting, you don’t need anything else.

Ha I see, it’s one of these situations where you need to CV-control a current flowing to a node that needs to be held at a certain voltage. There’s a solution for that, it involves an op-amp and a transistor! I’ll post about it later!

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