PS/2 MIDI Eurorack module edition

 

I made a Eurorack version of the PS/2 MIDI project I did a while ago. It features a PS/2 input (which can accept USB keyboards that comply with the PS/2 protocol as well, which my most recent mechanical keyboard does), a MIDI output (now with 5 channels: 1 direct play, 3 recorded and 1 for drums), a 1v/oct output of the main notes and a gate belonging to that 1v/oct. There is also an LED that will show that the gate output works when there is no plug in the jack (this is done to prevent unnecessary energy use by the LED, given that my own power supplies can only supply 300 mA).

The source code is available, and I will try to keep it updated with future improvements. The source code can run both on a 20 MHz PIC16F690 or a 8 MHz one, depending on the presence of the MODULE definition.

F1-F4 can be used to record the different tracks. It can play 4 different drum sounds at the same time, each with a selection of 3 different drums for a total of 12 options. A future module will expose the recorded channels as 1v/oct as well.

Originally I used a Sallen-Key filter for the PWM, but it turned out this created some spiky behavior that especially my Chipz module didn't like on the input, so I switched it to a plain two pole low pass filter.

This is the first module for which I made by own panel out of aluminum, which was an exciting and slightly scary thing to do, involving sawing, drilling, dremeling, sanding and painting. The painting was the biggest struggle, as you can see the bottom part has a slightly different hue.

Schematic of the Decay and the Metallic oscillator

 Turns out I had a schematic made of the decay, which is included here:

As well as one of the metallic oscillator, which is here:

Building my own Eurorack module

 For my birthday I got a NiftyCase with various modules, and I also bought a Mr Phil Ter and a Monsoon for it. However, it lacked envelopes. I started looking for possible solutions, On my youtube channel you can see a bit more about the synthesizer, here I will describe the design and making of the module. The basic circuit I came up with is as follows:

There's two inputs and an output, one of the inputs is the gate signal, which starts the decay, and the other is the control voltage, which uses and LED to control a photoresistor which is in parallel with a potentiometer, both of which control the length of he decay. The output is multiplied by a certain amount (currently 1.55x, but I may go back to the idea of multiplying by 2x as in the circuit above.

The 85k resistor was changed to 1M, and I added a LED and 470 resistor to show the decay working after the OPAMP (an TL072). This circuit worked, but unfortunately most gate signals are terrible, so I had to add a special PIC10F206 circuit to clean that up. This circuit also discharges the capacitor quickly between uses, avoiding a wait time that was necessary otherwise.

The only improvement still needed is handling of triggers: if the gate signal is too short I want to change the behavior into a release instead of a decay, possibly with a check that the output signal has grown small enough.

Here is what it looks like on the outside, which required some drilling, which was definitely a new experience, and quite scary.

By now I learned that the CEO of the company that makes these specific panels isn't a very nice person, so I may have to reconsider my purchasing strategy here.