This Guitar Tuner and Preamp project built by Rob uses a PIC microcontroller and an old stereo to allow visual tuning of a guitar. LEDs indicate how well tuned each string is.
I started by extricating the entire CD changer mechanism, including the spindle drive and laser unit, and the board with the DAC circuits. It was a simple matter to find where the audio inputs on the main board where that the CD audio signal was fed into. I would use these to feed the guitar signal into the stereo amplifiers (so I can still use the tuner and tape decks normally. When I want the guitar on, I just select “CD” for the input source)
The guitar’s input is buffered by an Analog Devices AD620 instrumentation amplifier. This chip is definitely overkill for this application ( I think each 8 pin DIP retails for $12) but I happened to have a few lying around my bench so what the hell. It has a great CMRR and an external output reference which can be used to virtually eliminate ground loops and hum. ”
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