Moving magnet cartridges requires a certain minimum of moving mass which helps to limit their performance at high frequencies. Considering the fact that the magnet is not very strong, the number of turns in the coil is likely to be high to provide a healthy output of about 5mV per channel for an RMS lateral velocity of 5cm/s at 1kHz.
This large number of turns results in high inductance which is necessary to give the required output level. The process results in a high impedance that requires a high input impedance in the phono preamplifier, typically terminating the cartridge with a load resistance of 47 kOhms.
A large number of these turns will also lead to the distributed capacitance between the layers and turns of wire. Also, the tonearm wiring and interconnection to the phono preamp usually add to the total value of capacitance, and the preamp also provides some certain values of load capacitance to fine-tune the high-frequency response of the setup, which can exhibit a higher frequency resonance due to the presence of the LCR resonant circuit, capacitance, and resistance.
When there is the interfacing of a moving magnet cartridge to a phono preamplifier, the cartridge will be generally terminated in resistance in parallel with some capacitance. This type of capacitance is typically made up of the connecting cable’s parasitic C, about 100pF, and any capacitance implicitly or explicitly presented by the preamp’s front-end.
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However, the terminating resistance in the preamp is always about the usual 47 kilo-Ohms. If a higher recommended load capacitance is presented to a moving magnet cartridge by the manufacturer, the result of this will be an over-emphasis of frequencies falling somewhere in the mid to upper treble regions, joined with an under-emphasis of frequencies above those frequencies.
It can also cause a correspondent excessive brightness joined with a sluggish transient response in subjective terms. A moving magnet phono cable that has a 400 pF of capacitance is simply absurd, the recommended load capacitance of various moving magnet cartridges ranges from about 100 to 500 pF. So a 250 to 300 pF capacitance is suitable for use with the majority of cartridges, although some models require more or less than this which is why the capacitance should still be within the normal range.