This article is in a series pertaining to vinyl setup. For the other articles, see the top-level Vinyl Guide.
The pickup cartridge is the device mounted at the end of the tonearm which holds the stylus (usually diamond) onto the record's groove. The groove vibrates the stylus, which transfers this movement into the body of the cartridge via a short rod (known as the cantilever). The cartridge then has the task of converting the mechanical vibration into an electrical signal. Different types of cartridge use different methods to generate the electrical signal.
The vast majority of cartridges use electromagnetic induction to generate the signal (and are therefore known as "magnetic cartridges"). This method involves moving a permanent magnet and a coil of wire (a pair of coils for stereo) relative to one another.
As the name implies, in this type the coils of wire are fixed inside the cartridge assembly, and the magnet, mounted on the cantilever, is moved relative to them. Most DJ and lower-price home audio cartridges are of this type. There are some advantages to this type:
- Since there is no physical connection between the sylus and the wiring, the stylus can easily be made replaceable. Most (but not all) modern MM cartridges offer replacement stylii, at 50-80% of the cost of a new cartridge.
- Since the coils of wire are fixed, they can use a large number of turns, thereby increasing the output level from the cartridge (typically around 5mV), which will then require less subsequent amplification and hence lower noise compared to moving-coil cartridges.
- Moving magnet cartridges are generally much easier to manufacture and are less expensive than moving coil carts. MM models usually range from $20 to $300.
The large number of turns on the coils give moving magnet cartridges a high output impedance (typically a few kiloohms), much of which is inductive. If the cartridge outputs are wired directly into a high input impedance preamplifier, the cartridge will form an RL filter, which can lowpass the signal in the 10-20khz range and compromise performance. Furthermore, the high mass of the cantilever/magnet assembly generally causes a resonance in the high frequency audio range, although not enough to complement the coil inductance. Two elements are added to alleviate these issues.
* A capacitance is added to the circuit to form an LC tank circuit. This boosts the high end response substantially, but also creates a large resonance, potentially in the audio range. Preamps usually come with 50-200pF of capacitance at the inputs, but often, the cabling from the cartridge to the preamp contains up to 100pF of capacitance. * To reduce the size of this resonance, a relatively low resistance is added between the signal and ground, to dissipate the resonant energy, smoothing out the high frequency response. This resistance should be low enough as to reduce the HF peak, but high enough so as to not compromise the HF response completely. Most cartridge manufacturers have standardized at 47kOhm as the "usual" preamp input resistance.
In a moving coil cartridge, the magnet is fixed and the coils are wound onto arms at the far end of the cantilever. Therefore the stylus cause the coils to move relative to the magnet. At first sight, this construction would appear to have a number of disadvantages:
- Since the coils must be moved, they must also be very light and hence can only have a few turns of wire. This is turn leads to very low output levels (typically less than 1mV) and the need for additional subsequent amplification, and thus greater noise.
- Since the internal wiring is physically connected to the stylus, the stylus is not user-replaceable. (A very few moving coil cartridges have been made with a replaceable stylus, but they were not generally successful designs).
Despite these disadvantages, as a general rule moving coil types yield higher fidelity than moving magnet types.
The small number of turns on the coils results in a very low output impedance (typically under 50 Ohms). This means that the amplifier input must also be very low impedance (typically 300 Ohms or less) to prevent serious loss of signal level. The difference between moving coil and moving magnet inputs is not therefore limited to the amount of amplification they provide: the input impedance is critical to achieving an accurate and efficient transfer of the signal from cartridge to amplifier.
Crystal and Ceramic Cartridges
These types of cartridge use the piezo-electric effect. The vibrations of the stylus are used to apply pressure on a crystal or block of material which has a piezo effect, which generates a voltage. The generated voltage can be quite high (up to 100mV). And unlike magnetic cartridges which generate a signal proportional to the velocity of the stylus deflection, crystal/ceramic cartridge signals are proportional to the amplitude of the stylus deflection. This effectively forms a lowpass filter of the velocity-based signal, requiring little to no additional RIAA equalization. As a result, crystal/ceramic cartridges do not require any phono preamp, are very easy to manufacture, and are still used today for very low cost turntable systems.
Crystal/ceramic cartridges have many major disadvantages. First, they tend to have much higher levels of distortion and noise compared to magnetic cartridges. Second, because the stylus/cantilever system is mechanically coupled to the cartridge, these cartridges tend to have extremely low compliance. This tends to compromise the high frequency response. Third, their purported compatibility with line level stages generally comes at the expense of any sort of close accuracy of the RIAA reproducing curve at high frequencies.
Other Types of Cartridge
Grado, a pioneer cartridge manufacturer that was the primary innovator of Moving-Magnet cartridges, calls their technology "Moving Iron". It is occasionally seen in other brands.