SN76477 Eurorack Module

After finishing the prototype I realized that the One-Shot works differently from what I understood (or rather assumed after looking at the schematics and barely skimming the documentation). A normal gate works like this. You apply high voltage, the attack starts. For as long as the voltage is high, the sound will be sustained. After putting the gate low the release of the sound will start.

On the SN76477 however, the attack starts when putting the enable pin of the chip to low. It will remain active for as long as this pin is low. The relase however will not trigger when the enable pin is put to high but rather when the capacitors on the release pin of the chip are pulled to ground. The enable pin has to be low for the entire duration of the sound. As soon as the enable pin is put to high, sound generation will stop, regardless of whether the release is already finished or not. 

So.... what the CV-Gate module does is the following: 

The enable pin is always low. If the CV gate is high, the enable pin will be put to high for a very short amount of time, just enough so it starts the attack. furthermore, the transistor is set to low. this way, the capacitors are not connected to ground. If the CV gate releases, the capacitor goes back high and the capacitors connect to ground. 

There are still some quirks to this...it can happen that the release already triggers even thou the gate is still open, but i also had that happen when i used the original method from Texas Instruments, so i am not entirely sure if thats just how the circuit works (maybe because the capacitors are still somewhat charged and dont fully discharge if you trigger one shots to quickly?) or if its a fault in my design. I will look at this at some point...

For testing out the chip and the capabilities in general i already had built a version of what Texas Instruments suggested to use for Barks and Chirps. Worked great, but to expand from there i knew that it wouldnt fit on a breadboard anymore. or at least that would not work if i actually wanted to perform live and had to change stuff qucikly and didnt want to fix broken connections every 2 minutes. So I recreated the circuit and printed it out. Apart from me forgetting to put some of the lables on the Lables layer everything worked out kind of great and I can now take the time to understand how the chip works.

The first thing i immediately noticed was, that the Envelope trigger does NOT work as i expected. So, after i understood how that worked I worked out a insert in place prototype for a module that converts a Gate Signal to what you would actually expect to do on the SN76477.

I will upload the Schematics soon, but I already know there are some quirks and I want to redesign the board anyways, so this is not gonna be the final version, its just a make shift solution for a new performance planned in a couple of weeks.