…and there is. Ohm's.
Ever wondered why phono cartridge loading is so important? We want to make sure a moving magnet is properly loaded at 47KΩ, the moving coil perhaps 100Ω.
Get those input impedances wrong and the system won't sound right.
A phono cartridge is little more than a coil of wire. This coil of wire is where the electrical signal is generated. A magnet attached to the end of the cantilever is surrounded by this coil in a moving magnet. This coil is attached to the end of the cantilever and surrounded by a magnet in a moving coil.
In either case, (the magnet moving closer to the coil or further away) a voltage is generated in response to the movement of the needle in the record groove. Apply this tiny signal voltage to the input of a preamplifier equipped with the proper equalization, and voila! we get music.
Seems simple enough. But there's a catch. When we use magnetics (coils of wire) to either transfer or generate energy, we run into a hitch in the get along. These devices are not flat in the frequency domain. At their frequency extremes, they work differently than in the middle of their range. At the lowest frequencies, they don't work at all. At the higher frequencies, they lose steam and begin rolling off—but not before they get louder—sort of a last gasp before their swan song.
This louder signal at higher frequencies is called a resonance, a peaking. It is to this peaking we apply our buddy, Mr. Ohm.
In a moving magnet cartridge, we want to terminate with a 47KΩ resistor and a small capacitor. This damps out the peak and makes for a smooth transition as the cartridge rolls off on the top end. Too high a terminating resistor and we get an unnatural boost at the top end from this peak. Too low a resistance and we lose the top end. The same applies to the moving coil, just on a different scale.