Everything you need to know about cartridges: Principle of Operation

PRINCIPLES OF OPERATION

Magnetic cartridges (Moving magnet, Induced magnet, VMS)

In number, magnetic cartridges for Hi Fi use are clearly dominant. Their fundamental principle of operation comprises the effect caused by the movements of the stylus in a magnetic field, generating a current in the nearby coils.

Moving magnet

The most common magnetic cartridge is the moving magnet. Here, the magnetic field emanates from a tiny, fixed magnet that is fastened to the remote end of the cantilever. When the magnet is set into motion, which occurs when the stylus follows the modulations of the record groove, the magnetic field through the coil changes, and electrical voltages are generated in these coils.

Although a moving magnet cartridge uses a tiny, powerful magnet on the cantilever, the weight of the magnet will still be a strain on the cantilever, increasing its mass and reducing its ability to react precisely to transients in the music. (Fig. 3)

Fig. 3. Moving Magnet cartridge principle. The movements of the magnet produce
currents in the coils by induction.

Induced magnet

This type of cartridge must be considered a further development of the moving magnet type. With the induced magnet cartridge, it is possible to use a smaller and lighter armature on the cantilever because a larger, powerful, fixed magnet increases the moving element’s magnetic strength.

Compared to a moving magnet cartridge, the total mass of the cantilever can be reduced, thus improving transient reproduction. On the other hand, the cartridge’s weight is increased by the fixed magnet, which may render this type of cartridge unsuitable for use with tonearms which already have a high mass. (Fig. 4)

Fig. 4. Induced Magnet cartridge principle. The field strength in the small magnet is increased by the larger adjacent magnet. The cartridge functions otherwise as a Moving Magnet cartridge.

VMS

The latest development so far in the magnetic cartridge principle is the VMS, or Variable Magnetic Shunt.

The cantilever in a VMS cartridge can be made extremely light because it is not weighed down by a magnet. Here, the magnetic field emanates from a fixed ring magnet that encircles the cantilever, the rearmost part of which consists of a thin-walled armature of magnetic conducting material. When the cantilever is set in motion, the armature short-circuits part of the magnetic field, and a voltage is generated in the coils. The VMS principle not only makes it possible to reduce the mass of the cantilever to an absolute minimum, but the construction is such that the weight of the entire cartridge can be reduced making the VMS cartridge suitable for use with a large number of tonearms. The special construction of the VMS principle means that the magnetic operating point in the cartridge may be placed at the origin of the induction curve, where there is no risk of non-linearity that can lead to distortion. The VMS principle has made it possible to produce Low Mass cartridges with a weight of only 1.5 grams. (Fig. 5)

Fig. 5. VMS (Variable Magnetic Shunt) principle. The tiny armature of magnetic conducting material short-circuits the field from the ring magnet, and voltages are generated in the coils.

Moving coil cartridges

The first cartridges with Hi-Fi specifications were developed in the mid-forties and were of the moving coil design. In general, moving coil cartridges are more expensive than magnetic cartridges and they do not, therefore, enjoy the same popularity. However, the most demanding Hi-Fi enthusiasts remain loyal to the moving coil design because of its great linearity and lower distortion.

Compared with a moving magnet cartridge, the reverse principle is applied in the moving coil cartridge. Here, a powerful fixed magnet is used and the coils are mounted on the cantilever itself. When the coils move in the field of the magnet, they cut the flux lines of the magnet and voltages are generated in the coils. (Fig. 6)

(Fig. 6.) Moving Coil cartridge principle. The coils on the cantilever move in a powerful magnetic field, and voltages are generated by induction.

Once again, with a moving coil cartridge it is important that the moving parts be as light as possible. Therefore, there are only relatively few windings on the coils, and the output voltage from a moving coil is generally so low that it cannot feed a conventional amplifier without special steps being taken.

Nowadays, it is becoming more common to equip amplifiers in the higher price range with a special input stage for moving coil cartridges, but otherwise it will be necessary to purchase a transformer or pre-preamplifier before a moving coil cartridge can be connected to the phono inputs of an amplifier or receiver.

Other cartridge types

The aforementioned four operating principles for Hi-Fi cartridges are the most common and by far the majority of cartridges on the market today belong to one of these groups. Many attempts have been made to use other electrical transformation principles, and some of the prototypes have - albeit to a limited extent - found their way into the Hi-Fi specialist shops. These are electret, condenser, and Strain Gauge cartridges, which require a special transforming section and have so far not enjoyed much popularity.