I need a little help with a project I'm working on. Here's the gist of it; I am taking two coils, hooking one into the input of an amplifier circuit and the other to the output. Both coils will be over an electric guitar string. The guitar string when vibrated will have a changing induction field which results in an induced voltage in the input coil, which is then amplified and goes through the output coil which creates a stronger induction field increasing the vibrations of the guitar string. The result is a feedback loop causing the string to make a volume swell. The circuit will also pickup background induction fields from the pickups of the guitar so the string doesn't have to be initially vibrating. Anyways, I've developed the amplifier portion of the circuit using the data sheet for the LM386, but I don't know what inductance I should have for the coils. I'm guessing I'll need some way to measure the changing flux of the guitar strings when they vibrate, but I'm at a loss there as well. I don't have my circuit diagram with me at the moment, but if it is helpful I can attach it later.