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I think an improvement might be to add a second pickup coil and separate the inputs for stereo use, and maybe a band pass filter to eliminate noise.
Very Nice! I like the concept, and I see lots of potential other uses such recording audio from other sources where a speaker is used. I'm old school and a bit of a purist when it comes to listening to music. I prefer to listen to true stereo, hardwired through separate channels. (I feel there's too much of a trend these days to sacrifice quality for convenience.) With that said, I would put this up against any Mono channel blue tooth speaker design.
Home Made double layered circuits
This would seem to have potential use in making printed circuit boards. Attach one of those resist pens and let the cnc draw the circuit.