Magnetic cartridge
From Wikipedia, the free encyclopedia
A magnetic cartridge is a transducer used for the playback of gramophone records on a turntable or phonograph. It converts mechanical vibrational energy from a stylus riding in a spiral record groove into an electrical signal that is subsequently amplified and then converted back to sound by a loudspeaker system.
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[edit] History
The first electric pick-ups were developed in about 1925. They used a piezo-electric crystal of quartz, stimulated by a stylus made of sapphire or diamond. The magnetic cartridge is presently the most common form of sound pickup used and came into use in the 1950s, following the introduction of magnetic cutter heads around 1945 for mastering records. Compared to the earlier systems, the magnetic cartridge gives improved playback fidelity, and reduces record wear by tracking the groove with lighter pressure, but with lower output voltage.
[edit] Types
In high-fidelity systems, crystal and ceramic pickups have been replaced by the magnetic cartridge, using either a moving magnet or a moving coil.
Magnetic cartridges provide much lower tracking forces (and thus damage the record much less), but also have a much lower output than a crystal or ceramic pickup, in the range of only a few millivolts, thus requiring greater amplification.
[edit] Moving Magnet (MM) cartridges
In a moving magnet cartridge, the stylus cantilever carries a tiny permanent magnet, which is positioned between two sets of fixed coils (in a stereophonic cartridge), forming a tiny electromagnetic generator. As the magnet vibrates in response to the stylus following the record groove, it induces a tiny current in the coils.
Because the magnet is small and has little mass, and is not coupled mechanically to the generator (as in a ceramic cartridge), a properly adjusted stylus follows the groove far more gently and faithfully, requiring less tracking force (the downward pressure on the stylus).
There is a sub-category. Moving iron and and induced magnet types (ADC being a well known example) which have the magnet fixed and move a piece of iron or ferous alloy in the field of the magnet to produce the signal within the fixed coils.
[edit] Moving Coil (MC) cartridges
The MC design is again a tiny electromagnetic generator, but (unlike an MM design) with the magnet and coils reversed: the coils are attached to the stylus, and move within the field of a permanent magnet. The coils are tiny and made from very fine wire, so are even lighter than the small magnet used in an MM cartridge, thus improving the tracking ability of the cartridge. This can give extended frequency response as well as greater fidelity.
A disadvantage however is that moving-coil cartridges generate an even smaller signal than an MM cartridge, because the moving coil cannot be large enough (too heavy) to generate equivalent levels. The resulting signal is only a few hundred microvolts, and thus more easily swamped by noise, induced hum, etc. It is challenging to design a preamplifier with the extremely low noise inputs needed for MC working, hence "step up transformer" are often used instead.
Moving coil systems are extremely small precision instruments and are generally expensive, but are preferred by some audiophiles.
[edit] Moving Micro Cross (MMC) cartridges
The MMC design was invented and patented by Bang & Olufsen. Since it often uses a special mounting, it can be mostly found in Bang & Olufsen turntables (which also cannot use another type of cartridge). Apart from being produced for the Bang & Olufsen mounting system, the SP12 and SP14 were also available in standard 1/2" mount.
The MMC cartridge is a moving iron design. Magnets and coils are stationary while the micro cross moves with the stylus, thereby varying the distances between the arms of the cross and the magnets. The design obviously offers more freedom concerning magnet and coil mass (compared to MC and MM cartridges). For example, the MMC20 (presented in 1978) uses four coils wound around the magnetic cores with 1200 turns each. Minimizing the moving mass also reduces the unavoidable wear on the records.
It is also claimed that the MMC design allows for superior channel separation, since each channel's movements appear on a separate axis.
[edit] Moving Magnet vs. Moving Coil debate
The debate as to whether MM or MC designs can ultimately produce the better sound is often heated and subjective. The distinction between the two is often blurred by cost and design considerations - i.e. can an MC cartridge requiring another step-up amplification outperform well made MM cartridges that need simpler front-end stages? Every now and then a design comes along to re-open this debate. A good example being the Linn K9 (now discontinued) - regarded by some as one of the better MM designs and competetive with MC alternatives costing more. Amongst others, Grace, ADC and Grado also manufactured notably good designs.
[edit] "Decca" Cartridges
The Decca phono cartridges were a unique design, with fixed magnets and coils. The stylus shaft was composed of the diamond tip, a short piece of soft iron, and an L-shaped cantilever made of non-magnetic steel. Since the iron was placed very close to the tip (within 1 mm), the motions of the tip could be tracked very accurately. Decca engineers called this "positive scanning". Vertical and lateral compliance was controlled by the shape and thickness of the cantilever. Decca cartidges had a reputation for being very musical; however early versions required more tracking force than competitive designs - making record wear a concern.
[edit] Stereo reproduction
One reason that magnetic cartridges superseded the crystal pick-up was the relative ease with which it could be made to reproduce stereo recordings, which were introduced in 1958. In a stereo recording, the two channels are arranged to drive the record cutter head at an angle of 45° to the vertical, effectively encoding each channel in the left and right V-shaped walls of the record groove. This system worked well, since it provided full compatibility with a monaural pick-up, so stereo records could be played on older mono equipment. To reproduce the stereo signal, the cartridge simply arranges pairs of coils at 45° to complement the cutting process. With careful design, the coils can be shielded from each other electrically and mechanically such that stereo separation is maximised.
[edit] Comparison with crystal technology
Piezoelectric crystal or ceramic pickups had a few clear advantages. They were much easier and cheaper to make and were more robust than the delicate magnetic pickup. In addition, the output voltage from a crystal pickup is relatively large, requiring less amplification, which helped improve the signal-to-noise ratio. However, the signal from a crystal is not an accurate reproduction of the recording, as there is a lot of distortion introduced. The stylus is coupled to the crystal in a fairly rigid manner, which is also not as good at following rapid changes in the record grooves, so frequency response also suffers. This requires a greater tracking force, which in turn wears the records out faster. (This has earned cheap portable record players, the nickname "portable grinding wheel", in some circles.)
By contrast, the magnetic cartridge is not mechanically coupled, the stylus and lever arm weight can be made exceedingly small. This gives extended frequency response and low distortion. The distortion is further minimised by the fact that there is more inherent linearity in the induction principle than there is in the piezo-electric one. Since the lighter stylus requires very low tracking forces, it requires a more sophisticated counterweighted arm, but reduces record wear.
The output from the magnetic cartridge is only a few millivolts compared to several tenths of a volt from a crystal or ceramic pickup. This requires an additional preamplifier stage. Careful design and shielding in the signal cables and amplifier is needed to prevent unwanted noise (shot noise or EMI). The magnetic induction principle also naturally leads to a linearly rising response with increasing frequency, and this needs to be compensated for to give correct (flat) frequency reproduction. Conversely, the very low bass frequencies are not efficiently picked up, so a strong bass boost is needed. This can amplify unwanted low frequency noise such as that from the turntable motor and drive mechanism itself (rumble). Crystal pickups do not suffer from these drawbacks and give a much better bass performance, so they may be preferred in some applications (such as DJ-ing), where robustness and good bass are favoured over highest fidelity reproduction. The moving coil pickups have an even lower level of output, and so usually require very special preamplifiers to bring these signals up to the level of input that a standard amplifier requires. These special preamplifiers must have very low noise indeed (some have even been cryogenically cooled), and hence tend to be expensive. Many audiophiles claim that the benefits are worthwhile.
The bass boost and high frequency rolloff can be conveniently incorporated into the preamplifier which implements the RIAA equalization curve, a noise reduction technique used on all modern records.
The magnetic nature of the modern pickup means it must be shielded from external fields, especially from the loudspeakers of the same system, or unpleasant and possibly damaging feedback can occur. For this reason, the cartridge itself has a shield, usually of mu-metal, to help screen out unwanted fields.
[edit] The stylus
The stylus, or "needle", is a crucial part of the phonograph, as it is the one part of the system that actually contacts the recorded disc and transfers its vibrations to the rest of the system. It is the part which also suffers the greatest wear. There are two desired qualities in a stylus: first, that it faithfully follows the contours of the recorded groove and transfers the vibration to the system, and second, that it does not damage the recorded disc.
[edit] History of materials
Early phonograph styli in mechanical players were steel, or even fibre, needles, usually with a shank about 1/8" (3 mm) in diameter, ground to a sharp point. These were easily replaceable by the user, as they had a very limited life and wore out fairly rapidly with use. Extensive play tended to wear records out as well as needles.
When the electronic phonograph was introduced, styli were included as part of the pickup cartridge.
In early times, wear of cylinders and glass styli was problematic. By 1908, sapphire styli were being attached to the rice paper diaphragms, and in 1912 Edison began to use the diamond stylus. Typical low cost crystal cartridges of the 1950s tracked at 5-8 grams of force and used replaceable osmium tipped steel styli called needles, which styli might last only five to ten plays before needing replacement. To help make the stylus last longer, sapphire styli for 78 rpm records or diamond styli for 33 1/3 rpm LPs were re-introduced. A 78 rpm stylus typically has a 3 mil (0.08 mm) diameter while a stylus for 33 1/3 rpm microgroove LP discs has a 1 mil (0.03 mm) diameter tip to fit the narrower groove. Sapphire might be good for 40 or 50 hours while a diamond would last at least ten times that long. Typically, these early cartridges were of the "flip-over" type; the cartridge had a stylus on either side, one for 78 rpm discs, the other for 33 and 45 rpm ("microgroove") records. The entire cartridge could be rotated 180° by means of a knob or lever at the end of the tonearm to use the desired stylus.
Later, starting in the 1960s, most manufacturers settled on diamond-tipped styli for all cartridges. Magnetic cartridges lowered tracking forces to 1-2 grams, and with the obsolescence of 78's, diamond became the standard stylus material. Moving magnet cartridges often have replaceable styli; most moving coil cartridges do not offer user-replaceable styli, although some manufacturers offer a trade-in or a re-tipping service.
Magnetic cartridge manufacturers usually provide a specialized range of styli for DJ use. More rugged conical styli are required due to the frequent reversals of direction involved in scratching and back-cueing, see Ortofon or Stanton.
[edit] Stylus shape
The physical shape of the stylus has a bearing on its performance. The most obvious shape is the spherical stylus also (known as "conical"), where the tip of the stylus is ground to a hemisphere for playing monophonic recordings or for rugged use. However, this shape is unable to faithfully track all possible variations in a record groove. Better quality LP styli use an elliptical or "line contact" shape, arranged with its 0.02 mm (0.0007 inch) long axis across the record groove. The short axis may be from 0.005 to 0.01 mm (.0002 to .0004 inch) depending on the particular design. This shape followed the undulations of the groove better than spherical stili because they more closely resembled the triangular cutter used to create the groove.
In later years, bi-radial styli appeared where, in the perpendicular axis, the tip of the stylus was ground to a smaller radius than the main body. The result was a stylus that rode slightly lower in the groove and even more closely matched the shape of the triangular cutter than the elliptical design. There were several variations on the basic bi-radial design depending on the manufacturer who produced them.
[edit] Cantilevers
The cantilever is the arm that connects the stylus to the magnet or pickup coils. Most cartridges have cantilevers made from aluminium or boron; some very expensive models have ruby, diamond or carbon fiber cantilevers chosen for their exceptional stiffness.
Turntablism also places great demands on the cantilever of the cartridge.
[edit] External links
- Cartridge history from a 1958 book
