MOVING IRON CARTRIDGES
What is a Moving Iron Cartridge?
This type of generator is similar in operation to the moving-magnet type except that a very light, hollow, temporary magnet or armature is used instead of a small permanent magnet. This has the advantage that the moving parts of the generator can be made from light magnetic alloys and therefore have less mass, responding to the the groove wall more accurately. The cantilever transfers the mechanical vibrations from the record groove to the hollow armature. The magnetic field induced in it by the large fixed permanent magnet suspended above it induces a changing magnetic “flow” through the pole shoes which, in turn, generates an electromotive force in the fixed coils in proportion to the mechanical vibration.
The coil must be connected to a special phono amplifier input via the turntable leads so that the low voltage generated by the cartridge (typically 5 mV) is boosted to several volts to drive the speakers. All MI cartridges have a relatively high output and require a 47 kΩ load impedance for current to flow making them compatible with the basic MM phono input found as standard on many integrated amplifiers. If your amplifier does not feature a MM phono input, these can be purchased separately at a reasonable cost.