Use as a guitar pedal.

What type of power supply are you using for the Pi?  I'd recommend some filter caps on both the line out and the power supply.

I would bet the power supply is noisy.

So I tried out a different power supply, as I was connecting it rather sketchily, and it helped a little bit. What ended up fixing it was plugging the amplifier into the same power strip as the RPi. Essentially meaning the distortion was because they weren't sharing a common ground. However, I don't think I necessarily want to plug them into the same outlet every time. So what I think I need to do is ground the Raspberry Pi. I haven't done the research yet, but i'm guessing there are a lot of posts regarding this.

That makes sense.  Also cool project, are you planning to show us some more details?  I always wanted to do my own effects box, I am curious how you are implementing the effects?

If that is happening you have an earth loop problem and you might do better to invest in an audio isolating transformer - you should have one between your guitar and any mains powered (especially experimental) equipment any way. If you are powering the RPI using low cost USB chargers it isn't very safe (some of these chargers don't comply with UK or EU (or anyone elses's) safety regulations. Using a local earth leakage trip (or residual current circuit breaker (RCB) may save your life.

Traditional guitar pedals are battery powered and very safe. Unfortunately a transformer between the guitar and the first chunk of electronics will change the sound, one between the PI and the amp will probably make no audible difference but might get rid of the noise problem.

Between the PI and the amp

Ground Loop Isolator | Rapid Online

Pi power supply plugged into this

R2804 - PRO ELEC - EXTENSION LEAD, 4 GANG, RCD, 2M | Farnell element14

MK

i agree with Michael, you should try an isolating transformer on the output. you can salvage one from an old modem. possibility even use a balanced line(xlr) to further reduce common mode noise.

I gave up on anything other than battery power for pedals and active instruments. a shielded box will no doubt improve things.

I'm a bit confused about the shared ground and ground loop comments.

Isn't the Pi's power supply isolated - Not connected to your house's ground?

when everything is grounded separate points, eg; amp at wall socket, rpi on another socket. there can be a potential(voltage) difference at between these points when they should be the same potential(ground in this case), this will cause current to flow between these points.

the idea of the transformer is to isolate the grounds from each other to eliminate any ground current between these spots. this can be a deadly problem in certain situations.

But is that Pi grounded? Isn't Ryan using your typical isolated power supply for the Pi?

most wall-wart supply's are floating outputs. very very very few have one or the other output tied to mains ground(earth) the cheaper phone charger types are switching and are not guaranteed to be clean, or safe.

Ryan, what power supply are you using?

I'm using this guy: http://www.mcmelectronics.com/product/DISTRIBUTED-BY-MCM-MWS158-0502200UC-MIC-/28-17985?t=2&utm_expid=8634549-27.0czCl7fSQamjILoiAH9mgA.1&utm_referrer=https%3A%2F%2Fwww.google.com%2F

I'm a little confused about why the transformer would help. How would it isolate the grounds from each other, and, I might be reading it wrong, I feel like I want the grounds to be connected. Also I'm not exactly sure what I'm looking for. It seems that a suitable transformer may be over $50 or under $10 (suitable transformer for an audio isolation transformer?). Here is another one that seems more focused on guitars: GuitarCenter

To Joey: if I'm able to get it working I would be very happy to post the info about what I did, however it seems like I have a lot of work to and I'm headed back to college tomorrow, so I'm not sure I'll have a ton of time to make significant progress over the next couple of months.