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Soldering Fun

From Midi to Control Voltage

I decided to start my project with the Midi2CV functionality. As the mios32 on stm32F4discovery board can already receive midi events from USB it was best to try to turn those into control voltage first. Support for traditional midi hardware input can be added later.

The CV output schematic is heavily based on Midibox AOUT_NG board but I made some tweaks because of component availability etc. For example the DAC voltage reference on my board is Maxim's REF02 because I happened to have one around. I also left the other half of the circuit out for now as I don't need 8 cv outputs (yet!).

I have done lots of soldering during the years but always with the good old through-hole type components. Even a left-handed bass player can easily work with those. Unfortunately I was only able to find the MIOS32 supported d/a-converter MAX525 in a modern super tiny SSOP package. The options were to use another DAC-chip and write a driver for it or learn to solder surface-mount components. As I have written a shitload of device drivers at work lately I decided to choose the latter option this time.
It's difficult to even see those tiny legs!
The first thing to get was an adapter board. The rest of the project will be built on a traditional prototyping board with 2.54mm pitch so the adapter is needed to connect the tiny dac chip to the rest of the hardware. Luckily these boards are readily available in eBay.

Gear Acquisition Syndrome

After asking for soldering tips from my hardware engineer colleagues I found out that apparently a chemical called flux is a must for SMD soldering. With traditional components it's enough to just use fresh solder wire all the time because it contains some flux but with these tiny components more flux is needed. So off I went to the local electronics store. In addition to the flux pen I picked up all kinds of random stuff that seemed useful at that moment. It's already becoming clear that it would have been cheaper to buy a high end synthesizer or at least a midi2cv module instead of starting this DIY-project but who cares. This is much more fun!


geek toys.

Strömsö Time

It was time to turn on my trusty Weller soldering station and try my luck. With plenty of flux I started my first attempt:

omg lol!
I managed to fix that one by removing the solder and trying again but another totally catastrophic failure followed:


Rest in peace, poor chip!
Luckily I had several adapter boards and dac chips and after couple of attempts things started to work out a bit better. Some of the joints looked almost ok!
This one might even work!

Back to the Old-School Electronics

After I got the SMD dac chips soldered I had no idea if they worked or not. It was time to go back to the lovely through-hole soldering and build the rest of the control voltage output hardware. The DAC needed a reference voltage, an op-amp stage for buffering and fine-tuning the output etc. I also needed to get the SPI bus from the stm32 processor to the dac. This all added up to several hours of soldering and wiring work.
A voltage regulator for clean 5V power supply, a voltage reference for accurate DA conversion and the dac adapter card

Also added a socket for the stm32f4discovery board

Lots of tricky wiring in the backside too. Not the prettiest board.

The Moment of Truth

After all wires more or less in place it was time for the most scary moment of any electronics project: First power on! A tough guy would have hooked it up to the synth and started playing funky ass bass lines and mind blowing trance leads right away but I'm not a tough guy so I just turned it on and started measuring the voltages.

5V power supply seems to be about right.

DAC reference voltage too!
And finally the CV outputs:
That definitely is a voltage...

...and it changes when playing the virtual midi keyboard on the screen!

It works out of the box! Great! Still some work needed to get the signals to the analog synth circuitry so unfortunately no sound samples yet. Anyway, after adding a midi input port and some wiring and tuning I should be able to kick the lousy PIC micro-controller out from the wooden box! Yes!

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