Question: What are the differences between moving magnet and moving coil phono cartridges?
Answer: Both moving magnet (MM) and moving coil (MC) cartridges are tiny electromagnetic generators, converting the mechanical movement (vibration) created by their stylus riding in a record’s groove into an electrical signal that will be amplified and processed by a sound system for us to enjoy. Both use magnets and coils of wire to generate the signal, with one or the other vibrating in unison with the stylus assembly to which they are attached, and each type has its advantages and disadvantages. To fully understand these advantages and disadvantages, let us first discuss how the “ideal” phono cartridge would need to function.
The goal of most every audio transducer, whether it be a phono cartridge, microphone, loudspeaker or other device, is to convert one form of energy into another as accurately as possible so that the original sound of the performance is retained. The device should have little to no “sound” of its own and add little to no noise to the signal. It is important to note however that these devices can be designed to be either as transparent as possible, or tuned to have a certain degree of coloration. This coloration is typically intended to achieve a certain effect or to address a certain problem. A vacuum tube microphone can add a degree of warmth and depth to an otherwise thin-sounding vocal performance, for example. A microphone that has a rolled-off high-frequency response can help tame a vocalist with excessive sibilance. Overall, most phono cartridges are designed to be as transparent as possible with many achieving remarkably flat frequency response for accurate audio reproduction.
Navigating the rugged terrain of a record groove is no easy task. The rapidly changing amplitude of the signal that is cut into the groove requires a generator that can react very quickly to these hills and valleys, yet stay firmly planted in the groove at the same time. A stylus assembly needs to be light in weight, yet stiff, to perform this task. Light to respond quickly and to keep record wear to a minimum, and stiff to quickly and accurately transmit the vibrations from the stylus tip to the generator. The assembly should have very little resonance, and any resonance that it does have needs not only to be kept out of the audible band of frequencies (20 to 20,000 Hz) so it does not affect the quality of the sound it produces, but also be low enough in frequency that it does not emphasize any inherent low-frequency noise, such as turntable rumble. Emphasis of such frequencies can cause stylus tracking problems, affect sound quality and attribute to feedback problems between the speakers and cartridge.
The Moving Magnet (MM) Cartridge
Audio-Technica brand moving magnet-type cartridges carry a pair of small, permanent magnets on their stylus assembly’s cantilever. The cantilever is the tiny suspended “arm” that extends at an angle away from the cartridge body. The cantilever holds the diamond tip that traces the record groove on one end and transfers the vibrations from the tip to the other end where the magnets are located. These tiny magnets are positioned between two sets of fixed coils of wire located inside the cartridge body via pole pieces that extend outward from the coils. This arrangement forms the electromagnetic generator. The magnets are the heaviest part of the moving assembly, but by mounting the magnets near the fulcrum, or pivot point, of the assembly the amount of mass the stylus is required to move is minimized, allowing it to respond quickly and accurately to the motion created by the record groove. In addition to enhancing response, the low effective tip mass reduces the force applied to the delicate record groove, reducing the possibility of groove wall wear and damage. The moving magnet-type cartridge produces moderate to high output levels, works easily into standard phono inputs on a stereo amplifier or receiver and has a user-replaceable stylus assembly. These cartridges have a robust design, making them an excellent choice for demanding applications such as live DJ, radio broadcasts and archiving.
Moving Magnet Cartridge
The Moving Coil (MC) Cartridge
The moving coil-type cartridge reverses the position of the magnets and coils. Two coils are attached to the cantilever and move within the field of a fixed permanent magnet located inside the cartridge body to form the generator. The coils are much smaller than the coils used in the moving magnet design, and they are made from very thin copper wire. The lower effective tip mass of the MC stylus assembly gives it some significant advantages over the MM-type assembly. The electrical signal that is generated by this arrangement is a low-impedance, low-output-level signal. This may not appear to be desirable at first, but this very lightweight assembly allows for much wider frequency response, improved transient response, more detailed reproduction overall and, in particular, the reproduction of signals that are low in level, which a heavier moving magnet assembly may miss. Moving coil cartridges are extremely small, precision devices that are typically more expensive than their moving magnet counterparts. The increased cost is a result of such precision and the increased difficulty in manufacturing. They are preferred by audiophiles due to their measurable and subjectively better performance, however. The moving coil cartridge produces a low output level which requires an additional stage of gain to play at sufficient volume. Higher quality stereo amplifiers and receivers may provide this additional gain by providing a phono section designed specifically for this purpose. Often there will be a set of dedicated phono inputs labeled “MC” for use with turntables employing a moving coil type cartridge. If the amplifier or receiver does not have an MC phono section, a separate step-up transformer or MC phono stage can be used between the turntable and the amplifier or receiver to provide this gain. Moving coil-type cartridges are delicate compared to moving magnet cartridges and typically do not have user-replaceable stylus assemblies.
If you have any additional questions about cartridges, please contact the Audio Solutions Department.