| The reason for this
elaboration is supposed to be that it allows a reduction in
the mass of the moving system, and thereby its inertia (which
is specified as the tip mass). This promises improved tracking
performance and raising of the tip mass/vinyl compliance
resonance higher in frequency. However, the avoidance of
patent infringement has played its part in the development of
these 'induced magnet' designs. The suspension compliance of
moving magnet cartridges is generally high (20cu or more),
which demands an arm of low to medium effective mass. Most moving magnets (fixed coil is perhaps a better generic term) use a large number of coil turns, to provide an output of the order of 1millivolt at 1cm/sec rms recorded velocity. This minimises the gain required in the preamp disc input, but results in the cartridge having a high coil impedance (typically 600ohm resistance in series with 250mH inductance). In consequence the cartridge's frequency response is dependent on the capacitive load presented by the arm cables and preamp input. In the past correct capacitive loading was critical to the tonal balance of most moving magnet cartridges. Today the use of stronger magnets has allowed a reduction in the number of coil turns required for a given output voltage, with the result that capacitive loading is generally a less critical factor. However, it still makes a difference (cartridges vary in their sensitivity to loading), and some amplifiers continue to provide switchable capacitance on their phono input. The standard amplifier input resistance for fixed coil cartridges is 47kohm, and they typically track at 1- 1,5gram downforce, 0,5gram less than most moving coils. For reasons that have never been clearly established, although additional factors such as the structural integrity of the cartridge body probably have at least as much bearing as the generator principle, moving magnet/fixed coil cartridges are generally inferior in sound quality to moving coils, despite their technical advantages in terms of tracking ability and high(er) output. Moving coils dominate the ranks of upmarket cartridges. |
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