[sdiy] CV outputs from 3.3V and 5V uC systems

Benjamin Tremblay btremblay at me.com
Wed Sep 16 14:54:24 CEST 2020

I have researched this issue a bit.
A) I tried using a Digilent PMOD DA4 8-channel DAC. I got eight channels and only needed one set of SPI wires. Wicked fast.
B) I tried using PWM pins on the Teensy to drive "non-critical”  (non-pitch-accurate) CV, example filter resonance and mixing the outputs of a state variable filter.
C) I looked into DAC boards with built-in wider voltage range. 

A —> This thing is great but only gives me 0-2.5V range. It’s 12 bits, so I if double the output I get that reasonable range for 5 octaves of CV. (EIGHT OF THEM)
B —> This covers many use cases for CV. Teensy has lots of pins. I was able to automate all the knobs on a home-made drum synth with this approach.
C —> I purchased a break-out board for a Maxim Pixi chip. This crazy thing has 20 bidirectional DAC/ADC ports and you get a 0-10V range for free. I ran out of time this summer to explore this more.

My observations: 

X) Modular synth folks tend to be “purists”, and most of the time I am not a purist. I don’t mind using CV that’s not pitch perfect if I am not trying to control something that does not emit a musical pitch.
I’m not heavy into that laboratory-grade normalization scheme. Not at this time, anyway. 
Y) People are selling MIDI-to-CV products that use PWM.  (Example: https://www.tindie.com/products/Robertron2084/midi-to-cv-converter-kit-midimuso-cv12/ <https://www.tindie.com/products/Robertron2084/midi-to-cv-converter-kit-midimuso-cv12/> )
Z) There ain’t nothing wrong with buying a break-out board for $60 if it saves me hundreds of hours of R&D and micro-soldering. 

Other MIDI/Audio boards are competing with Teensy (http://www.axoloti.com/product/axoloti-core/ <http://www.axoloti.com/product/axoloti-core/> , https://www.electro-smith.com/daisy <https://www.electro-smith.com/daisy> ) and they’re both 5V on the outside but a tender 3.3V on the inside.
In a year we’re all just going to get over the limited V/Octave range of the Teensy. 

Bes wishes,


Benjamin Tremblay
btremblay at me.com
330 Fiske Street
Carlisle, MA 01741

> On Sep 16, 2020, at 5:29 AM, Spiros Makris <spirosmakris92 at gmail.com> wrote:
> Hello list,
> I am working on a (predominantly) midi project, based around the Teensy platform, and I'm trying to figure out the best way of adding a CV output, compatible (and useful) with eurorack.
> Without any additions, I can use any off the shelf DAC to get 4 or 5 octaves of 1V/oct, but I would be interested in supporting the whole MIDI range (~10 octaves) - which means I need some switching converters, perhaps even a voltage inverter.
> The options I can think of are:
> 1. Don't do anything, have 5 octaves of range.
> 2. Use a boost converter to get a 12V rail, then have 0-10V range
> 3. Use both a boost and inverting converter, to get a pair of dual voltage rails (-12/+12 or similar?), and use a (-3,8V) range.
> I know the basic concepts of switching converters, but have never used one myself. I assume I need less than 100mA of current (to supply 4 opamps and a DAC), what parts would you recommend? 
> Regards,
> Spiros
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