I am building a synth with a library called Mozzi and it has a DUAL PWM OUTPUT with MIDI. Here is the schematic:
The synth by itslef works fine, the audio output is very good. The problem starts when I connect another device via MIDI INPUT, you can hear a very low frecuency wave that is made from the other synth's MIDI clock pulse. Also when a note gets received via MIDI in, it makes a click or crackle sound.
Here is a sound sample, first half with notes being received, last half with just clock (amplified): https://dl.dropboxusercontent.com/u/103115555/autoobst%20midi%20crackle.mp3
the MIDI signal interfering with the audio output is very thin but it's there. tryed to do a RC low pass filter on VREF and AVCC but it does nothing to mute those click sounds.
Also if I take out the Atmega chip, the sound's still there if MIDI is being sent.
Any idea?? Thanks :)