THE CARTRIDGE: The Electromagnetic Generator

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Nearly all cartridges produce an electrical signal through electro magnetism. The rare exceptions include ceramics, which are used rarely now except on the cheapest of kids’ turntables. These operate on the piezoelectric principle—certain crystalline materials, such as barium titanate ceramic, will when twisted or bent produce a voltage at their surface. To withstand the necessary twisting force, the stylus assembly must be quite strong so these cartridges generally have an undesirably high moving mass. Ceramics produce a much higher voltage than magnetic cartridges.

Moving magnets are the most common type of cartridge. They (and their variations—moving iron, induced magnet, and variable reluctance) move a magnetic field through a coil to generate electricity. A tiny permanent magnet is attached to the end .of the moving cantilever and a coil of very fine wire is secured inside the cartridge body. As the groove modulations move the stylus/cantilever, the magnetic field cuts across the coil and induces a voltage. The advantage is a high output level; the disadvantage is the weight of the magnet on the end of the cantilever. It takes force from the record to move this mass, and some transient information is lost by the relatively slow response time of the moving mass. In fact, the magnet must of necessity be kept fairly light and so relatively weak.

Moving coils reverse the positions—an extremely lightweight coil of wire is attached to the cantilever while the heavier magnet structure is housed in the body. The benefit is excellent retrieval of transient and high-frequency information because the movement of the stylus has to drive only light coils of wire, not a heavy magnet. This low mass can respond quickly to rapid changes in the music and so can retrieve music more cleanly and delicately than most MM designs. Also, the magnet housed inside the body can be heavy and so the coil can be low impedance.

The disadvantage of MCs is significantly reduced output level, typically a third less than that of a moving magnet. In an attempt to improve output, sometimes the coil windings are increased, but this also increases the moving mass and so reduces the MC’s advantages. These higher-output MCs suffer some loss of fine treble detail and transient attack.

The MC’s very low output generally requires an additional stage of amplification: either a head amp (a.k.a. pre-preamp) or alternatively an additional stage in the preamp itself. In addition to adding to the expense of these cartridges, which often already cost more than a comparable-quality MM, the additional amp stage complicates the circuit and can itself degrade sound quality. For there to be any sense in using an MC, the cartridge plus extra circuitry combined must sound definitely better than an MM of comparable price.

Some MCs are designed to have a high output and so do without a head amp. These are also generally considered to sound less good than low-output MCs. As always in audio, compromise and tradeoffs are required. Deciding on the basis of theory is tricky—use your ears as your guide.

While many preamps are now being designed to accept the minuscule output of an MC cartridge directly without a head amp, a lot of these really just amount to a head amp grafted onto a preamp, which neither minimizes the circuitry nor necessarily improves sound quality. GSI’s 5TP preamp is a rare exception, carefully designed to have extra gain without extra circuitry. Preamps that do add an extra stage of gain should allow you to bypass this stage when using an MM—since you don’t need the extra gain, you don’t want to be forced to go through the additional circuitry.

MCs have been steadily improving since they were developed back in the 1950s by Joe Grado, who is the father of the MC design and holder of the U.S. patents. They fell out of favor for a number of years and only began to come back again in the mid-1970s, largely picked up by the Japanese designers. Grado himself hadn’t made an MC in many years until recently commissioned to design one. Grado’s cartridges, including his finest hand-built Signature series, employ his own moving-iron variant of the MM design.

Next: The Sound of Moving Magnets and Moving Coils

Prev: Basics of Cartridge Design

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Updated: Friday, 2016-05-13 19:17 PST