As mentioned in a previous post, I was working on making an iPhone/guitar interface with a bypass switch, not unlike the recently-announced iRig Stomp by IK Multimedia. I made something that worked, more or less, and wrote a page about it, and then found a lot of information online by others who have looked at the problem and interpreted it far better than I had (the page on this is now private til it’s better).
My error was mainly in thinking this is a simple passive circuit with no components besides connectors and switches. It turns out that’s not true at all, that there is in fact a preamp and some passive filtering going on in the iRig and other phone/guitar interfaces. I didn’t think active circuitry would be present without a battery, but it turns out that the iPhone’s mic connection puts out 2.7 V in order to polarize condenser mic elements. The iRig seems to use this to polarize a JFET in its preamp.
So, long story short I need to spend more time on this. Here’s a thread I found recently I wish I’d seen earlier, it explains a lot.