Dr. Analogue's Corner

Good Things in Small Packages – Part One

By September 28, 2017 No Comments

One of the least glamorous components in a vinyl playback chain is the phono cartridge, perhaps due to its relatively small size. Like microphones and loudspeakers, phono cartridges are transducers, converting variations in a physical quantity into an electrical signal, or vice versa.

All phono cartridges have a diamond stylus with a specific shape or profile whose polished surfaces contact the left and right walls of the record groove. In our experience, the Shibata EVO stylus profile used by all Acoustical Systems MC cartridges and originally developed to resolve the ultrasonic surround sound information encoded in the JVC and RCA Quadradiscs of the 1970s, provides the very best high frequency reproduction, especially at the inner grooves, resulting in lower distortion, as well as superior imaging and soundstage depth.

The stylus is attached to a rigid cantilever which is often made from aluminium alloy, boron, ruby, or sapphire. The cantilever pivots at a damping element, usually an engineered polymer, and terminates with a very fine tensioned suspension wire.

As the name suggests, in moving coil (MC) phono cartridges the cantilever carries coils of conductive metal, commonly copper or silver, which move within a fixed magnetic field. As a superior electrical conductor, silver coils are always found in the highest fidelity MC phono cartridges. This group includes the legendary Fidelity Research FR-7f (and its rare “z” and “s” variants made exclusively for the Japanese market) as well as the modern Acoustical Systems Archon, Astron, Aiwon, and Palladian cartridges.

In the case of moving magnet (MM) phono cartridges, the situation is reversed, with a tiny magnet attached to the end of the cantilever in proximity to stationary conductive metal coils.
A third design, the moving iron (MI) cartridge, is like the MM type but instead of a tiny magnet, the cantilever carries a piece of iron or ferrous alloy. The lower moving mass gives the MI design a distinct tracking advantage over most MM cartridge types.

In all cases, the relative motion between a conductive metal and a magnetic field induces an electrical current. The stereo record groove’s turns (x-axis) and valleys (y-axis) are translated into a modulated electrical signal by the phono cartridge. The strength of this electrical output varies depending on the type of cartridge and its design choices. MC type cartridges typically produce only a fraction of a mV at their output pins and are therefore considerably more demanding in terms of the connected phono preamplifier. By comparison, MM and MI type cartridges typically produce ten times the output of a MC design, albeit at the expense of higher moving mass compared to the MC cousins.

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