@bimasta Here is some info translated from Japanese about Air Core Coil of Fidelity-Research PMC-3 :
" Air core coil is adopted as coil, weighing is realized compared to ordinary coil part made of magnetic material as core material, and magnetostriction caused by hysteresis curve of core material is expelled. In addition, by adopting a new special lightweight resin coil bobbin, the total weight of the oscillator is reduced to about 1/3, and the tracking ability is improved.... Samarium cobalt magnet is adopted as a magnetic circuit. This material has an ideal characteristic that it has higher holding power compared with ordinary Alnico type and it is hard to self-demagnetize. Moreover, the yoke part can also be downsized, the cartridge’s own weight is drastically reduced, and the initial characteristics can be kept for a long time.... Precision cutting is applied to the yoke to make the entire magnetic circuit close to a circular shape. As a result, effective magnetic path length can be shortened, size can be reduced, and magnetic flux leakage at the corner portion is reduced to achieve high efficiency.... A 0.04 mm diameter net flow line is adopted as the coil. "
" Air core coil is adopted as coil, weighing is realized compared to ordinary coil part made of magnetic material as core material, and magnetostriction caused by hysteresis curve of core material is expelled. In addition, by adopting a new special lightweight resin coil bobbin, the total weight of the oscillator is reduced to about 1/3, and the tracking ability is improved.... Samarium cobalt magnet is adopted as a magnetic circuit. This material has an ideal characteristic that it has higher holding power compared with ordinary Alnico type and it is hard to self-demagnetize. Moreover, the yoke part can also be downsized, the cartridge’s own weight is drastically reduced, and the initial characteristics can be kept for a long time.... Precision cutting is applied to the yoke to make the entire magnetic circuit close to a circular shape. As a result, effective magnetic path length can be shortened, size can be reduced, and magnetic flux leakage at the corner portion is reduced to achieve high efficiency.... A 0.04 mm diameter net flow line is adopted as the coil. "