Why cartridge loading matters
A phono cartridge and the phono preamp’s input form a resonant filter. This can have effects in the audible range.
By matching the capacitance and impedance of the phono pre to the inductance of the cartridge, the resonant peak can be minimized or even eliminated.
The following simplified model is being used:
The cartridge is represented by its inductance and DC resistance, the phono pre by its shunt capacitance and load resistance. The capacitance of the cable will also contribute to the total capacitance and can simply be added.
While more complex models do exist, getting the necessary additional specifications for the cartridge and preamp would be tricky. Currently it is hard enough to figure out the capacitive load of the pre, as many manufacturers ignore this important specification in their user manuals.
The calculator puts out a frequency response graph. The X-axis shows the frequency (20 Hz–20 kHz, i.e. the audible range), but on a linear scale, allowing you to see the problems in the treble region very well. To put things in perspective: the mid-range (around 1 kHz) would be in the middle of a plot with logarithmic frequency scale, which is commonly used for frequency plots).
Other calculators on the internet may give you vastly different resonance frequencies than this one. This is because many of them simply ignore the effects of the resistors. The resonance of the resulting LC filter is easy to calculate, but the frequency response is quite different from the more complicated model that I'm using here.
Please bear in mind that the calculator is based on an electrical model only. It cannot make any any assumptions about about what is happening on a mechanical level. Resonances of the cantilever, which can contribute to the frequency response and sound of a phono cartridge, and other mechanical effects are not taken into account.