During the development of proton magnetometers I designed a simple "listening circuit" to hear the precession sound of protons in the magnetic field of the earth. I tried to make it as simple as possible, using only cheap and non-exotic parts. It is simply not true that super exotic low-noise OpAmps or instrumentation amplifiers (for 20$+ per unit) are needed. This is the result (click to get a PDF):

Here is a short video how it works, you might also use Youtube for discussion:

Once the electronics are ready the input circuit needs to be made resonant and tuned to the expected frequency. If you are interested in these topics you have found the expected strenght of the earth's magnetic field at your location on the net for sure. But how to measure the resonance frequency of the circuit, especially if not having an oscilloscope? It is pretty easy: Switch to listening mode and move the speaker closer to the coils until the system starts to oscillate. Using a guitar tuner or simimlar application gives the exact value of the tuning frequency as shown in the following short video:

By adding/removing capacitors in parallel or in series the tuning can be done at home, no need to go outside for that. Note that C1..C3 are just shown in the schematics for illustration, you can use as many as you like, of course. Being off by 50Hz or so does not really matter a lot. With some luck the guitar tuner app can also be used when experimenting outside so that the best tuning can be found in an iterative way.

The most fascinating moment might be to try with and without liquid in the coils. The fact that there is no signal without a liquid is a good proof that the signal must come somehow out of the liquid, although it is non-magnetic. Isn't that pure magic?

Note that this design is purely for learning and to experience the exciting moment when you can her the protons singing. Solid state switching, microprocessor control etc. is not included by intention.