Tinkering question: Injecting a signal into a circuit

[ryangobie]

Maybe some of you can help me. I've been circuit bending some stuff I have laying around in my free time (it takes my mind off school which is nice). I was thinking about trying to turn some circuits into an effects box. So I figure if I can inject a signal through the circuitry where the oscillators signal from the keys might be I might get lucky. Worth a shot maybe. Anyway, here's my questions:

1) When I send a signal in from a guitar or whatever else, I'm gonna have to solder the ground of the cable to the circuit to get it in there? Or can I just attach a probe to the hot line and poke around?

2) The output of these things is pretty hot and at a low impedance to drive the things internal speakers. A voltage divider can calm that signal down but is it gonna up the impedance to where an amp is gonna be happy? Would I have to tack on another stage to help this?

I could be completely off base but I won't know unless I ask eh. Thanks.

[Scodiddly]

The gain stuff is going to vary quite a bit depending on where you tap into the circuit. But at a minimum you're going to want DC blocking between your source and the circuit - just a nice big capacitor, say 10uF, and voltage rating higher than whatever the power supply puts out.