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.
Most magnetic cartridges are of the type known as "moving magnet". As the name implies, in this type the coils of wire are fixed, and the magnet is moved relative to them. 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.
- 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.
The large number of turns on the coils give moving magnet cartridges a high output impedance (typically 47kOhm). This requires that the loading at the amplifier be carefully matched to avoid high frequency losses. Fortunately pretty much all phono inputs on amplifiers have been standardised at 47kOhm.
Moving magnet cartridges also typically have a mechanical resonance in the upper treble (above 10kHz) leading to a rising frequency response. This is compensated for by the use of a small amount of capacitance (typically between 100pF-500pF, depending on the cartridge) at the input. This capacitance shunts some of the high frequency signal to ground.
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, which is enough to directly drive a line level input). However, the accuracy of the voltage generation is typically rather poor, so these types of cartridge are rarely seen nowadays.
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.