With a stock, official Arduino Due and Pedalshield Due, I was hearing a lot of what sounded like CPU-dependent noise in the output when using the Due's ADCs. Running just the DACs produces a very noise-free output.
I took a look at the AREF pin on the SAM3X8E on my oscilloscope, and sure enough, there's at least 10mVpp of noise. The Due gets AREF from the 3.3V rail and uses some basic filtering - not enough apparently. It needs a better AREF source!
I chose an MCP1501-33. It only needs a few external parts and has the advantage over a shunt regulator of lower noise and a buffered output. It is rated for 27uV noise, which is a huge improvement. There's a 0R chip resistor used as a jumper on the Due which can be moved to route AREF on the MCU to the AREF pin, It's a very tiny part, probably 0402. I did not feel like re-soldering that, even with my hot air rework station, due to all the other small parts nearby, so I just removed it and soldered the output of the MCP1501-33 to the center pad. It's probably lower-noise that way, since the trace to the AREF pin may pick up noise along the way.
How does it sound???
Awesome! What was before an obvious whirling background of CPU sounds and heterodyning whistles is now dead quiet!
MCP1501 is a SOIC-8 part. It is surface mount, but pretty easy to solder. I used a breakout board and tacked on the caps and resistor to make the circuit as shown in figure 5-4 of the datasheet. I only had 75R chip resistors, not 50R, but it seems to make no audible difference to me.
This could be incorporated into a future Due Pedalshield. All the user would have to do is move the AREF jumper on the Due. You could just let it stick to the soldering iron tip to remove it, then blob some solder to bridge the pads to connect the external AREF. Or sell the Due pre-modded for users.
Anyhow, this mod turns the Pedalshield Due into a pro-sounding effect. Try it!