How to make a Bluetooth Cassette Adapter | DIY Bluetooth Cassette Adapter

[@stoojinator]

The reason it sounds like shit is because you're overdriving the tape head. You should have between 1.2 and 2 millivolts going into the tape head, and that bluetooth receiver is trying to drive the head as a speaker and probably outputting over 2 volts. Add a small trimmer to the ground (so you don't lose balance) between the bluetooth receiver and the tape head and adjust it until you get no distortion. Make sure you have your phone turned up all the way so you can adjust it based on that level.