Turntables, Arms & Cartridges

Nagaoka is a stalwart of the MP (moving-permalloy) pick-up principle, a refinement of the MI (moving-iron) method introduced by Pickering some 80 years ago. In practice, MP is a subset of the broad moving-magnet (MM) category, but while the latter uses a small magnet attached to the rear of the cantilever, inducing a voltage in fixed coils as the stylus moves through the groove, MP replaces the magnet with a sliver of ferromagnetic metal such as iron. These days, a lighter nickel/iron ‘permalloy’ is used while both the coils and magnet are fixed – as the permalloy element slices through the fixed magnetic field it induces a voltage in the coils.

Compared with an ‘equivalent’ MM, the lower overall moving mass of the MP’s stylus/cantilever [see picture above] has the potential to improve tracking and HF response, a potential realised in the MP-700 [see PM's Lab Report]. That said, there’s a trade-off in cartridge output level against distortion and lateral/vertical response uniformity. Designers cannot simply beef-up the fixed magnet size or coil windings to compensate for the reduced mass of the ‘moving permalloy’ or the latter may become saturated by too high a magnetic flux density. Hence the MP-700’s ‘medium’ 4mV rated output... PM