Hey! I hope this is on topic. I have an ADC0804 that I'd like to interface with arduino. It'll be converting analog audio from a headphone jack on a computer. The ADC can't read negative voltage so I'll need to use an op amp to amplify the whole audio signal so it's above 0V. My question is, what's generally the lowest and highest voltage I should be expecting from the headphone jack, so I know which gain to use? Thanks.
About 1V RMS with the volume cranked-up. Of course it varies a lot. Some headphone outputs/amplifiers are capable of more voltage than others, some songs/programs are louder than others, and there is a volume control.
The DAC can't read negative voltage so I'll need to use an op amp to amplify the whole audio signal so it's above 0V.
You can shift/bias the voltage with an op-amp but if you simply "amplify" a negative voltage it gets more negative. 
You can make an offset circuit with two equal-value resistors (a 50/50 voltage divider) plus a capacitor to block the DC bias from the audio circuit. There is a bias schematic at the bottom of [u]this post[/u].
I have an ADC0804 that I'd like to interface with Arduino
The Arduino already has a built-in 10-bit ADC and the Uno has 6 analog inputs. (There is no DAC).
Hey, I just measured the voltage of both, my phones aux jack and my pcs line out.
I measured by playing a 50hz sine wave tone and then probing with my multimeter the GND and Left Channel of my aux cable. The multimeter gives me Rms values, which is why i multiplied them with the Root of 2, to get the peak Voltages. ( Vrms * ROOT(2) = Vpeak )
Values for pc line out: ~1,2Vrms ( = 1,7V peak ), so i think this one might go up to 1,75V peak.
Values for Phone jack: ~0,520Vrms ( ~700mV peak), so this one might go up to 750mV peak.
Cheers