Turntables, Arms & Cartridges

The origins of moving-iron or 'variable reluctance' pick-ups and cutters extends back over a century with contemporary evolutions on the theme introduced by the likes of Decca, Grado, Stanton, Goldring and Soundsmith in the last 50 years. Moving-iron (MI) pick-ups are similar to moving-magnet (MM) types except here the permanent magnet attached to the cantilever of an MM is replaced by a sliver of some ferromagnetic metal, iron or a 'permalloy'. The coils are fixed, as they are in an MM, but there is also a fixed magnet as employed in a moving-coil (MC) pick-up. As the MI's stylus traces a groove its cantilever moves the small iron sample towards the fixed magnet, the sliver becoming magnetised and inducing a voltage in the fixed coils.

On the face of it, this technique would seem to offer numerous advantages, not least the possibility of an MI 'generator' offering a lower overall moving mass, and inertia, than the stylus, cantilever and magnet or coils of either an MM or MC. A lower moving mass may improve both tracking and HF response, but the output of an MI type cannot simply be increased by beefing up the fixed magnet and coil windings to compensate for reducing the size and weight of the 'moving iron'. In practice, if the local magnetic flux density is increased too far then the minuscule iron sliver will become saturated and its induced magnetism highly non-linear as it moves towards and away from the large, fixed permanent magnet. As ever, implementing a successful MI solution is as much a balancing of the 'magnetic circuit' as faced by designers of MM or MC types. PM