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Phono cartridges: Moving Magnet and Moving Coil

Phono cartridges: Moving Magnet and Moving Coil

Contrary to the bokeh-esque bonhomie of social media, factual answers to questions in the world of vinyl playback are blurred by confusion, vagueness, and misinformation. Incorrect answers are usually not limited to online forums, hi-fi stores, or local record shops as any collector flipping through the bins can tell you they’ve overheard. While an attempt to tackle the many questions involved when entering into vinyl playback could be considered folly, I wanted to touch on a common query for those getting into the hobby, and that is the differences between a moving magnet (MM) phono cartridge and a moving coil (MC) phono cartridge.

Before we dive straight in, let’s address why a phono input is required to play LPs – regardless of whether the turntable is equipped with an MM or MC cart. People often wonder why one can barely hear a turntable when it is hooked it up to line inputs (and not the phono input) of an integrated amplifier. Most integrated amps usually come equipped with a CD, AUX, and Tuner RCA inputs – not all feature a phono input. A phono input is required for a turntable because the signal output from a phono cartridge is many times weaker than the output of a CD player, tuner, DAC, etc., and requires specific preamplification (a phono preamp) to bring it up to "line level," which is standardized at 0.316 volt RMS, and is the signal level that a tuner, CD player, DAC, etc. is configured to output. So far, so good?

Phono output can be anywhere from (generally speaking) 0.2 mV to 7 mV. The lower rating is usually reserved for moving coil carts, and the higher for moving magnet carts. This higher output allows MM designs to be used into standard phono inputs on modern or vintage integrated amplifiers and receivers. Most integrated amplifiers and stand-alone phono stages/preamps are designed to accommodate MM carts, but some will handle both MM and MC. It’s good to be aware of the differences of the two, as each has specific requirements, not just for voltage, but loading and impedance as well (usually preset at 100 ohms for moving coil and 47,000 ohms for moving magnet). A quick word of caution here for newcomers, never route a line output connection to a phono input. The amplifier’s phono input is designed for that delicate MM or MC signal and if, instead, it is presented with line voltage, it will become overloaded – causing damage to the amplifier, and possibly the loudspeakers.

MM and MC cartridges are miniature electromagnetic generators translating mechanical movement to an electrical signal. This can be categorized as converting one form of energy into another. The goal of the phono cartridge is to perform this conversion as accurately as possible to maintain fidelity to the original recorded performance. The ideal cartridge neither adds or subtracts any sonic artefacts during the transcription process, maintaining “transparency” to the source material. This is practically impossible in the realm of reality (Side note: most manufacturers assert that a flat frequency response is critical for accurate audio reproduction) with every cartridge adding tonal or timbral colouration of some type to the playback due to the very nature of the materials cartridges are constructed with, and the diamond profiles of the stylus being used. Don’t take my word for it, just listen to different cartridges yourself.

This transcription occurs via the physical act of a stylus/generator tracing a record’s groove (the complex, microscopic patterning cut therein) in the horizontal and vertical axis. This is a tortuous, merciless act for cartridge styli that must track the approximately 0.04-0.08mm wide, and 0.02794mm deep “trench” at relatively dizzying speeds. An average LP groove is 460 meters long with the groove wall on the outside edge carrying right-channel information. Both types of cartridge utilize assemblies of magnets and coils of wire (usually copper) to generate a signal, with either the magnet structure or the wire coils sympathetically vibrating to its attached stylus assembly.

The ability to maintain constant pressure and accurate transcription (tracking) of the irregular terrain a record groove presents is a feat of astounding engineering prowess. The instantaneous amplitude changes of the recorded signal cut into a vinyl record’s surface requires a generator with a lightning-fast reaction time. Stylus assemblies must be as non-resonant as possible (or have inherent resonance outside the audible bandwidth – 20Hz~20kHz to not affect sound quality), lightweight (to avoid undue wear to the groove walls) and incredibly stiff to achieve playback accuracy between the stylus and generator coils.

Moving magnet cartridges are the most common type of phono cartridge. Their inherently less-complex design is easier to produce, allowing for a lower price. They are standard on most modern budget or entry-level turntables, and many feature a user-serviceable, replaceable, stylus assembly. Because of their rugged design, moving magnet cartridges lend themselves to professional use in broadcasting, professional and DJ environments. When it comes to vintage turntables, these too, tend to be fitted with MM carts because most were paired with valve, or solid state integrated amplifiers at that time. Most of these integrated amplifiers came standard with an MM phono stage (preamp), but some featured support for both. Moving magnet-type cartridges are equipped with a (stereo) pair of permanent magnets attached to the upper end of the stylus assembly’s cantilever (the “rod” whose lower end sports the stylus – diamond – to trace the groove vibrations). The magnets on the cantilever nestles between two (stereo) sets of fixed-in-place coils of wire mounted on “poles” inside the cartridge housing. This dynamic/fixed structure is the energy converter, or electromagnetic generator. The weight of the magnets means they need to be mounted close to the cantilever’s fulcrum (pivot point) allowing for the quickest, and most accurate tracing of the groove. It is these physical engineering characteristics (a magnet attached to the cantilever, whose movements excite fixed coils) which distinguish moving magnet cartridges.

A moving coil cartridge switches where magnets and coils are placed. Instead of the magnet assembly (and it’s associated mass) riding the cantilever, gossamer copper coils are attached to the cantilever whose movement interacts with a fixed magnet assembly situated within the cartridge housing. This design, with its lower-level signal generation (approximately 20dB less) could be viewed as lacking the robust power generation of MM designs. But with the cantilever freed of magnets, it has advantages over MM; less mass means quicker response by the cantilever to the constantly changing terrain of a record groove. This can translate to an increase in general resolution due to a wider frequency response enabled by its lighter-weight. As mentioned, moving coil variations tend to produce a much lower output, and require a further stage of amplification (or gain) to meet line-level signal requirements. This can be active or passive gain. Active gain would take the form of powered amplification, passive would mean placing a step-up converter (SUT) between the turntable output and the phono inputs of one’s amplifier or preamp. The signal that travels from an MC cartridge through the transformer windings of a SUT to a tubed phono stage creates a synergy to be admired, but that is another article entirely. Lastly, because of the fragile nature of MC cartridges compared to MM cartridges, they usually have no user-replaceable parts or stylus assemblies.

So, moving magnet cartridges do like their name suggests; a magnet moves, the patterned-changes in the field this produces interacts with fixed coil structures within the cartridge housing and an audio signal is produced. A moving coil cartridge eschews the attached magnets for minuscule coils and instead oscillates those within the field of fixed magnets to also produce an audio signal. Is one method better than the other? That, like all things in this weird hobby, is in the ear of the beholder.

Article courtesy of Rafe Arnott - Resistor Mag


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