My build notes for the CV Pal.
This is a Euro Format module by Mutable instruments.
CVpal is a DIY USB MIDI to CV interface
It does not need any external power source as it is powered by the USB bus. This is very useful as it doesn't need any connection to your eurorack power supply. You could just put it into an external box.
The official build notes are excellent, with lot's of info explaining what the components do.
It's a great way to learn electronics.
+ Official Build Notes
I think the CVpal is discontinued, but its still a great learning tool.
The CVpal interprets MIDI messages differently depending on the MIDI channel on which they are received. For example, when receiving messages on MIDI channel 1, it behaves like a monophonic synthesizer and outputs a CV/Gate pair. When receiving messages on MIDI channel 10, it behaves instead like a drum trigger converter and outputs a trigger for 4 drum instruments.
So if you can get hold of one it's a great module to control your Modular Synth from a computer, phone or tablet.
The CVpal is a USB device not a USB host! It can be connected to ‘active’ devices such as smartphones, tablets, laptop or desktop computers, but not to USB MIDI controllers.
The CVpal is class-compliant and as such, does not require any driver. It features various conversion/voice allocation modes, covering monophonic, duophonic and drums/triggers applications. 4-HP width.
The CV output is in the 0 .. 4V range - so about of 4 octaves is covered.
Sadily the module doesn't support V/Hz conversion; only V/Oct.
Gate output uses V-trig, has a 5V high level.
If a module or synth requires a higher level, a level conversion circuit such as the CD4504 can be used. Polarity inversion can be implement with a CD4049 or with a software hack.
CVpal works optimally with iOS, OS X >= 10.6.3 or Linux kernel >= 2.6. On other operating systems, the messages might be delayed by up to 8ms, causing a very jittery timing!
BOM
3.6V Zener diodes D1 and D2.
220R resistors (red, red, black, black) in positions R1 to R5.
68R resistors (blue, grey, black, golden) in positions R6 and R7.
2.2k resistor (red, red, black, brown) in position R8.
10k resistor (brown, black, black, red) in position R9.
18pF ceramic capacitors C2 and C3.
100nF ceramic capacitors C1 and C5.
20 MHz quartz Q1. This part is not polarized.
100µF electrolytic capacitor C4
22µH inductor L1
LED (Green)
4 jack connectors and the USB socket.
ATTiny84
MCP4822 is a two-channel digital to analog (DAC) converter.
2x3 header for the AVR ISP port.
----------------------
3.6V Zener diodes D1 and D2.
These diodes serve as voltage-limiters: the USB standard requires 3.3V while the microcontroller uses
5V I/O (the currents involved are so small that the voltages on the diodes do not actually reach 3.6V!).
68R resistors (blue, grey, black, golden) in positions R6 and R7.
These two resistors work along with the Zener diodes for voltage limiting.
220Ω is fine for the green LED provided with the kit.
If you ever want to solder a fancy-colored LED (blue, white, pink…) instead of the one provided with the kit, you’ll need to use a larger resistor.
2.2k resistor (red, red, black, brown) in position R8. This resistor is what allows the host computer/smartphone to recognize the CVpal as a low-speed USB device.
10k resistor (brown, black, black, red) in position R9. This resistor “pulls up” the reset line of the microcontroller - causing it to run continuously after it has been powered.
100nF ceramic capacitors C1 and C5.
They are known as “decoupling capacitors”. Their role is to provide a local reservoir of energy to an integrated circuit (the microcontroller IC1 and the digital-to-analog converter IC2). This improves the stability of the power supply line, and ultimately prevents cross-talk or unexpected coupling/glitches between integrated circuits.
The 20 MHz quartz is labelled Q1. This part is not polarized.
The quartz is the clock that makes the microcontroller beat. It produces a square wave with
a frequency of 20Mhz
This is the 22µH inductor. It's labelled L1.
More about inductors can be seen here:
The 100µF capacitor is also a decoupling capacitor for the digital to analog converter. Teaming with the inductor, it prevents the signals emitted by the DAC to “see” any of the fluctuations and impurities coming from the computer. Digital circuitry needs to be isolated from sensitive analog circuitry!
Jacks next.
Attach panel while soldering.
LED next. Flat side is the cathode. -ve
I'm using a large green LED
Solder this using the panel as a guide to its position.
+ LEDs
ATTiny84
This is a microcontroller with 8kb of non-volatile program memory (flash), 512 bytes of non-volatile memory (eeprom) and 512 bytes of RAM.
You can read more about microcontrollers here
It runs a program implementing the USB-MIDI protocol, sorting notes, emitting gates/triggers on the GATE outputs, and driving the digital to analog converter to produce CVs. You can see the source code of this program here.
MCP4822 is a two-channel digital to analog converter with 12-bit of resolution - it can emit two voltages from 0 to 4.095V by increments of 1mV, under control of the microcontroller.
The finished module
If you have a kit for MI, then the microcontroller is probably already programmed.
Otherwise you will need a programmer.
I used this:
You can also use something like an arduino.
You will need to flash the .hex file to the ATtiny using ISP
Initial tests
As the module doesn't need to be connected to your euro PSU,
just plug it into the USB port of your computer.
The LED should light.
There is no need to install any USB drivers
I think CV/gate 1 is midi channel 1
Links
+ https://sixbyseven.ca/cvpal/
https://core-electronics.com.au/usb-female-type-b-connector.html
https://au.mouser.com/ProductDetail/Stewart-Connector-Bel/SS-52300-001?qs=gZXFycFWdAM098VuPZGg2w%3D%3D
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