When looking at schematics of amplifiers, filters etc, I often find that the output to the audio jack is going through a resistor in the 470-680 ohm range and also shunted to ground using a high value resistor, like in this example:
What's the meaning of R2, I just know it's something like a "load" resistor. And how about R1, is it needed for when nothing is connected to signal? What are appropriate values for those resistors? Do I need them when I connect between two op-amps or just to the output jack? I hope someone can clarify those questions or point me to a source where I could read more about these basic topics.
Extra question: what happens when I add a coupling capacitor between R1 and R2? Version with added capacitor: