Magnetic cartridge

From Academic Kids


A magnetic cartridge is the most common modern form of pickup used for the playback of gramophone records using a turntable or phonograph.

The magnetic cartridge came into use in the 1950s, following the introduction of magnetic cutter heads around 1945 for mastering records. The first electric pick-ups had been developed about 1925. They used a piezo-electric crystal of quartz, stimulated by a stylus made of sapphire or diamond. Compared to the earlier systems, the magnetic cartridge gives the best fidelity of playback, and tracks the record itself with the lightest pressure.

There are two types of magnetic cartridge, moving magnet and moving coil (originally called dynamic). Both operate on the same basic principle, that of electromagnetic induction. The moving magnet type is by far the most common and more robust of the two, though audiophiles often claim that the moving coil system gives higher quality playback.

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Diagram of moving magnet cartridge, showing essential parts

In either type, the stylus itself, usually of diamond, is mounted on a tiny metal strut called a cantilever, which is suspended using a collar of highly compliant plastic. This gives the stylus the freedom to move in any direction. On the other end of the cantilever is mounted a tiny permanent magnet (moving magnet type) or a set of tiny wound coils (moving coil type). The magnet is close to a set of fixed pick-up coils, or the moving coils are held within a magnetic field generated by fixed permanent magnets. In either case, the movement of the stylus as it tracks the grooves of a record causes a fluctuating magnetic field which causes a small electrical current to be induced in the coils. This current closely follows the analogue waveform cut into the record in electrical form, and may then be carried by suitable cables to an electronic amplifier where it is processed and amplified in order to drive a loudspeaker.

In most designs, the stylus itself is detachable from the rest of the cartridge so it can easily be replaced. The cartridge can either be mounted to the arm using small screws to the headshell, or can be integrated into the arm itself. This latter approach, often used on mass market turntables, means that you are locked into a particular type of cartridge, and replacing it with a better quality one is not an option.


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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.

Comparison with crystal technology

Piezo-electric crystal or ceramic pickups had a few clear advantages. They were much easier and cheaper to make, and were very much more robust than the delicate magnetic pickup. In addition, the output voltage from a crystal pickup is relatively large, requiring less amplification, which helps to give good signal-to-noise ratio. However, the drawbacks were that the signal from a crytsal is not a terribly faithful reproduction of the recording, and hence there is a lot of introduced distortion. 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 good frequency response also suffers. This also leads to a greater required tracking force, which in turn wears the records out more quickly.

By contrast, the magnetic cartridge is not mechanically coupled at all, so 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. Because the stylus is so light, very low tracking forces can be used, which require a more sophisticated counterweighted arm, but do not wear out the record.

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.

Stylus design

The stylus itself is also a very important component. In the early days of recording, sound reproduction was entirely mechanical. Wear of cylinders and glass styluses were problematic. By 1908, sapphire styluses were being attached to the rice paper diaphrams, and in 1912 Edison began to use the diamond stylus. Typical low cost crystal cartridges of the 1950s tracked at 5-8 grams pressure and used replaceable osmium tipped steel styluses called needles. These styluses might last only five to ten plays before needing replacement. To help make the stylus last longer, sapphire styluses for 78 rpm records or diamond styluses for 33⅓ rpm LPs were re-introduced. A 78 rpm stylus typically has a 3 mil (0.08 mm) diameter while a stylus for 33⅓ 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. Magnetic cartridges lowered tracking forces to 1-2 grams, and with the obsolesence of 78's, diamond became the standard stylus material.

The physical shape of the stylus has a bearing on its performance. The most obvious shape is the spherical stylus, where the tip of the stylus is ground to a hemisphere. However, this shape is unable to faithfully track all possible variations in a record groove. Better quality LP styluses use an elliptical shape, arranged with its .0007 inch (0.02 mm) long axis across the record groove. The short axis may be from .0002 to .0004 inch (0.005 - 0.01 mm) depending on the particular design. This shape gives the best possible compromise between tracking ability and high frequency response.

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