> the listening circuit must also be tuned to resonate at the expected frequency of proton precession, which will depend on Earth’s magnetic field at your location
> the frequency of these tones matches the magnetic field at my location to about 1 percent
I don’t doubt the physics, but I’m not sure about the experiment design. Being able to hear the correct frequency may just mean you’ve built an oscillator and tuned it.
https://m.youtube.com/playlist?list=PLJ04J62_rG0AwQ57wb8Rxp7...
This particular vid is a sort of FAQ
Using cheap bag-of-1000-for-a-fiver Chinese transistors off eBay I was able to get incredibly quiet output, to the point that I needed to add a muting gate because the radio was objectionably noisy. I notice that the exact transistors are not mentioned but any small-signal NPN and PNP ones will do - I used BC548 and BC558s, like I use in everything.
It will be way quieter and way more stable than an LM386.
Edit: I'm a lot better at drawing things in Kicad these days, and would have left the capacitors at the input a lot tidier.
Further, the magnetometer in your phone is a 3-axis device that measures the orientation of the magnetic field, whereas the magnetometer in the linked article detects only the strength of the magnetic field (in fact, is tuned to detect only a single strength/precession frequency).
[1] https://en.wikipedia.org/wiki/MEMS_magnetic_field_sensor
