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Moving Coil vs Moving Magnet vs Moving Iron

 

As its name suggests, instead of a moving magnet interacting with a stationary coil, this type of generator works the other way around. This still satisfies Faraday's Law because a conductor moving in a stationary magnetic field still experiences a changing magnetic flux. A large permanent magnet is fixed in position and creates a strong uniform magnetic field in the gap between two pole shoes attached to it. Suspended by a rubber damper in the gap is the armature which carries the generator coil and this in turn is connected to the stylus via the cantilever. As the stylus traces the record groove, the coil moves in the magnetic field and a corresponding e.m.f. is generated within it. Because its moving mass is much closer to the pivot point, inertia is reduced and so it has a superior transient response compared to MM types. At the same time it it is easier to accurately position stereo coils within the magnetic field and so there is the potential for improved channel balance and crosstalk figures.

To keep mass low, the coil contains only a few turns of wire - so it generates a much lower output than a moving magnet type (typically 0.5 mV). For this reason MC cartridges require a special high gain phono stage and a lower load impedance and so cannot be used with a normal MM phono input. Because it has both low inductance and internal resistance, the MC generator is not affected by the load capacitance presented by phono stage and interconnects, making for a flatter, more extended frequency response. MC phono stages usually offer a variety of loading options to tailor the sound of the cartridge to taste.

 

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.

 

All transducers of this type depend on Faraday's Law of electromagnetic induction which implies that a conductor placed in a varying magnetic field will have an electromotive force, or e.m.f., induced in it proportional to the rate of change of the field. If a permanent magnet is suspended at or near the poles of a coil with an iron core, the coil will experience the required variable magnetic flux if the magnet is moved. A cantilever transfers the mechanical vibrations picked up from the record groove by the stylus directly into the permanent magnet. The changing magnetic field it produces induces a magnetic “flow” through the pole shoes which, in turn, generates an e.m.f in the fixed coils in direct proportion to the mechanical vibration. In this way, the sound wave captured as undulations in the record grooves has been converted into a tiny electrical signal which can then be amplified and converted back into sound by the speakers.

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 MM cartridges have a relatively high output and require a 47 k load impedance for current to flow making them compatible with the basic 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.

 

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