MattiasOfTheMetal:
For the AC over DC, that makes me feel better about whether or not it would fry my arduino. The only thing though, I was unable to effectively do capacitive coupling for the guitar's signal. When I would apply the DC to it, it would just stop any sound. I was using a small cap (don't have any 1 farad non-polarized lying around) so that may be the issue, but if not, have I then done something wrong?
Do you have a scope so you can see the what is on the output side (what should be AC) of the capacitor? Also, are you only using a capacitor or is there a resistor going from the capacitor's output to ground? If you don't have a resistor adding one would make the circuit a first order high-pass filter. This might be more reliable, especially if you are using smaller capacitor values.
MattiasOfTheMetal:
I would prefer to utilize a single mono cable (They are easy to come by, and if anything happens on stage I can easily swap it out; To make it backwards compatible without adding a proprietary jack beside the 1/4"; I don't want to use a Stereo 1/4" because I worry a roadie would put a mono in by accident and it would short my 9v to ground/I would have to add short circuit sensing circuitry to my pedalboard, which I find overkill if there is a better solution to just using a mono cable).
There's no technical reason why you can't or shouldn't use a single wire cable, I was just stating a personal preference. Although I can certainly see why a traveling performer, especially when they won't always be the one setting-up the system, would want to keep things simple while using readily available cables. However, have you thought about using custom solution based on a commerically available multi-pin cable? For example, you could get MIDI compatible circular audio connectors and wire up only the pins you need (of the five available, I think you'd need two or three). This way the power and signal can be separate, it would be impossible to accidently connect it wrong, and it's possible to use an normal MIDI cable as a replacement.
MattiasOfTheMetal:
Current question: what would be the correct value cap to use for a 9v powered system(I may use diodes to half/full rectify the signal to stay at about 10v then regulate it before the arduino, or figure something out), that also has a min resistance of 8k ohms, max of 25k ohms?
The math for this is fairly simple, but it will be dependent on the frequency the ripple you are trying to smooth (which in this case will be related to the frequency of your original AC signal). You want a capacitor large enough such that the RC time constant (?) is significantly larger than one full ripple cycle. I don't know what the ripple period, but you don't have to match it exactly just exceed it. Here's an example...
? = R * C
? = 5*10-5 s (I got this by inverting 20 kHz, your target value could be different)
R = 8000 ? (lowest possible load is the worst case)
Solve for C
C = 5*10-5 s / 8000 ? = 6.25 nF