SN76477 Eurorack Module

Details

This is just the very first prototype of the Board. I printed out a version that followed the suggestion of Texas Instruments on how to get the most out of the chip without having to reconfigure every time. I plan on adding more CV controls and make it a bit smaller to better fit in a eurorack.

In the mean time, check out some highlights from a previous set I played live that shows most of the capabilites of the SN76477 (well...four of them). https://www.twitch.tv/videos/2859688350

On these highlights, only the SN76477s make the sound. the modules on top of them are just for CV and Gate Trigger. All audio is routed to my computer, where i added a Low Pass Filter and a little bit of Reverb depending on the role of the SN76477

Files

GateTriggerSchematics.pdf

Kind of working Schematics for the CV Gate handling

Adobe Portable Document Format - 19.53 kB - 08/29/2026 at 22:49

Preview

BOM.csv

BOM. Note that most of the Capacitors and Potentiometers just define ranges, so they dont have to match exactly

Comma-Separated Values - 3.21 kB - 08/29/2026 at 13:23

Download

PCB_v1.pdf

PCB Layout (since i forgot to put the values on the right layer)

Adobe Portable Document Format - 273.48 kB - 08/29/2026 at 13:08

Preview

Archiv.zip

Gerber Files for the very first Dev-Board

Zip Archive - 75.34 kB - 08/29/2026 at 13:07

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Project Logs

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Gate
decurus • an hour ago • 0 comments

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...
First printed verion
decurus • 10 hours ago • 0 comments

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.

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Build Instructions

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Building the PCB

You can just have the PCB printed and put all the required Parts on then. Nothing more to it.

A couple of notes thou:

The SN76477 Chip supposedly does have a version with a 1.27 pin spacing. However, all the versions that i did buy have the smaller one, even thou it was claimed to use the package with the bigger spacing. So: the PCB has the bigger pin spacing, if you cant find one, you just have to find an adapter plate (or print one)

You will find some places on the PCB where there are a bunch of Capacitors next to each other in a row. These arrays are used to define the range of the correlating potentiometer. Therefore the capacitor values dont have to exactly match what i put on the PDF. they are just usefull ranges. You can exchange them at will and however you need them. E.G. i already recommend using higher capacitors for the LFO because the ones that are on there right now can produce a way higher frequency then the VCO which kind of defeats the purpose of a LOW Frequency Oscillator

Usage

Using can get a bit complicated because there are a lot of setup options where you dont immediately hear something. I myself still haven't figured out all the quirks yet, because for this desing i just wanted to see the extend of whats possible and so i just followed the suggestions from Texas Instruments themselves.

Here are some things to keep in mind when playing around:

First: all the jumpers need to be connected. There are five (or so) jumpers that define ranges for their corresponding potentiometers. these need to be connected. To do so, just put one end of a jumper cable into the first slot (each jumper section has one slot that is NOT marked, thats the starting point) and connect the other end to whatever other port you want (in that section.) a good starting place is 1 or two over from the start.

Power: Just Connect Eurorack power. The Board only uses 12 Volts, so you can also just put GND and +12 to the corresponding Pins.

Mixer: What is being mixed to the output is defined by a binary table. Said table is printed on the PCB but WATCH OUT: i already messed up this table. the first and third column are flipped. So for example. if you want to hear ONLY the voice you have to set all switches to low. If you want to hear Voice and Noise (V/N) you have to set the switches to High, High Low (remember, first and third column are flipped

Envelope: thats the weird one. To fully understand what it does, you should read the manual of the SN76477, everything on this board works as described. but its to much to fit in here.

At some point i will make a video on how to use THIS specific dev board.

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SN76477 Eurorack Module

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Files

GateTriggerSchematics.pdf

BOM.csv

PCB_v1.pdf

Archiv.zip

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First printed verion

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