5 pin Midi key presses cause click on DAC output (Teensy 3.2)

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Hi all,
I am working on a midi synthesizer using the Teensy 3.2, and I have encountered a small problem. The USB midi input works just fine, but when I use the 5 pin midi connector (on TX 1), I get an audible click from the DAC pin every time I press a key. It's usually masked under the intended sounds of the synthesizer, but when I turn the synth down and the speaker up, it is definitely audible. I used the opto-isolated Midi input circuit shown in this tutorial:
http://www.notesandvolts.com/2015/02/midi-and-arduino-build-midi-input.html.

I have stripped my code to the bare minimum, but the click persists even when the software oscillator itself isn't making any noise. It seems to happen no matter what, as long as the DAC is initialized as an audio output.

I am simply using a 10uF capacitor between the DAC pin and the audio output, which might be the culprit, although I'm confused as to why there would be interference since the Midi input is optically isolated and shouldn't be causing any big voltage spikes. Again, this problem does not occur with the USB Midi input.

Has anyone else dealt with this issue, or have any tips? I would also love any input on how best to utilize the DAC output, other than just throwing a capacitor on it.

Thanks all!
Best,
-Wesley
 
Indeed, Paul's schematic seems to have some differences to the one I used, thanks for pointing that out. I will breadboard it out and see what happens.
Thanks,
-Wes
 
Ground loop issues are the most common cause of audio problems!

Check that Teensy and whatever is sending MIDI are separate grounds. Serial MIDI uses an optocoupler circuit so there's no common ground, but it's all for nothing if you power both the sending MIDI gear and Teensy (or whatever creates audio output) from earth-grounded power or the same floating power supply.
 
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