Howdy everyone!
I’ve started working on the schematics for a digital Teensy-based synth that I want to be able to have interface with my existing Kosmo Modules. I’ve read up on These Posts, But I’d call myself fairly green to the whole world of modular synthesis.
My primary questions are:
What is the simplest way to convert incoming signals from a bipolar ±10v kosmo format to a unipolar 0->3.3v form and vice versa? There seem to be a number of approaches around
for the conversion back I wouldn’t terribly mind if the output is still unipolar if it means that we can keep the component costs down
Looking around, I can see that I may have taken up a sisyphian boulder, but its my turn on the boulder anyhow
I will update this post with the in-progress schematics as soon as I’ve got it sent up to github
thank you all!
One way would be something like this:
The resistors on the left form a voltage divider that converts a ±10 V signal to 0–3.3 V. It could be done with two resistors instead of three, but then you’d need a 2 V reference voltage; with the third resistor you can use the same 3.3 V as is powering the microcontroller. The op amp is a voltage follower that buffers the divided voltage and the Schottky diodes with the 1k resistor are to protect the microcontroller against voltages outside the 0 to 3.3 V range.
This actually converts +10 V to about 3.25 V, and that’s if all component values and the reference voltage are exactly as shown. If you need something precise, especially if you’re handling 1 V/oct signals, you’d probably want to get the 3.3 V from a precision voltage reference rather than just from the microcontroller power rail, and you might want to replace two of the resistors with multiturn trimmers for calibration — or, given a stable voltage reference, you could tweak the range in the microcontroller software.
If you’re using a DAC, its output presumably will be unipolar. Whether a unipolar output will work for you or not depends on what you intend to do with the output. If it’s an audio signal going into Kosmo modules, some designs expect zero centered signals, some don’t. You can AC couple the DAC output with just a capacitor and a resistor to center it; that in principle high pass filters the output and particularly will distort the shape of a square wave, but if the cutoff frequency is well below the audio range you’ll never hear the difference. Or you can use a similar op amp circuit to boost the output range to whatever you want and center it. That will have the side effect of protecting the DAC against inadvertently shorting the output to a power rail or something; without an op amp you’d want a couple more Schottkys for that.
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You’re a lifesaver and a saint, This is exactly what I was looking for!
I’m not planning to give or receive 1v/oct signals on this project, but the advice is well received and appreciated
To make sure I understand the math about it all, How did you come up with the specific resistor values? I can see the voltage divider between 3.3v and ground and between the input signal and ground but i cant seem to find any texts about how the two of those interact
Would it be worth putting a diode between the 3.3v and the ±10v signal so that it doesnt pulldown the 3.3v power when the signal swings negative, or is that not really a concern?
You can solve it using Kirchoff’s and Ohm’s laws, but what you end up finding is that when you have multiple voltage sources connected together through resistors, then the result is the weighted average of the voltages where the weights are the inverses of the resistances. So in this case, when input is ±10 V, the resulting voltage is
(±10/100k + 3.3/33k + 0/47k)/(1/100k + 1/33k + 1/47k)
= 0 for -10 V input, +3.2478 for +10 V input
The 3.3 V is the output of a voltage source (regulator or reference), so will be unaffected by changes in another voltage connected through a resistance. In principle at least — real world voltage sources are never ideal — but that shouldn’t be a problem here.
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Thank you so much! Here’s the github with the kicad files I’ve been using and working with so far! Will post updates as I work on it <3
My newest question and hurdle for this is in regard to the CV out jacks, I have this configuration going for them, which should allow a DPDT switch to move us from a bipolar to unipolar output
As far as my simulations show, this configuration should work if I power the opamp with +10v and -10v
My question is, what’s the best way to go about achieving this voltage? My initial thought is to just find a wallwart that gives at least 10v, but that leaves me unsure how to go about achieving the -10v for the opamp’s lower voltage; and while its not a requirement, as it stands right now I could just sorta slap at least 4 AA batteries in the finished product and have it be battery powered, which would be nice for convenience.
Sorry if i’m asking a bunch of silly questions; this is my first foray into actually designing one of these systems instead of just building one from schematics I find online, and thank you all for all your help!
Enter “power supply” in the search on this site, you’ll find lots of discussion.
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