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I have an electric guitar, and I'm planning on making a wireless FM transmitter for the guitar to pick it up on my stereo. First thing to do though is to match impedances, add tone balance and make the gain adjustable. This is what the circuit does. First stage uses bootstrap to match the high output impedance of the electric guitar to the rest of the circuit. Bootstrap gives some good qualities to the op amp which a simple high resistance bias won't give. For instance the input has considerably low resistance to DC (around 30k), but immensely high resistance to AC (mostly due to the 1Mohm resistor). This gives the circuit the quality of resisting blocking distortion, because DC bleeds easily through the 30k resistance, while AC signals remain at high impedance. Second portion of the circuit is an inverting unity gain tone balance. Nothing fancy about it, most people might skip it, but my electric guitar doesn't have tone balance, so I added one here. Third part of the circuit handles the gain. Since the signal is going to get fed through an FM transmitter, it's good to be able to match the maximum input signal the FM transmitter can handle before it starts overmodulating. Since guitars can have quite some peaks, this might take some experimenting and variable gain is a good idea.
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