Pickup systems for turntables

Introduction

Another major component is the pickup or cartridge.

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Quartz criystal cartridges

Early electronic phonographs used a piezo-electric quartz crystal for pickup, where the mechanical movement of the stylus in the groove generates a proportional electrical voltage by creating stress within the crystal. Crystal pickups are relatively robust, and yield a good level of signal which requires only a modest amount of amplification. A crystal's output tends not to be very linear, that is, it introduces unwanted distortion. It is difficult to make a crystal pickup suitable for stereo reproduction, as the stiff coupling between the crystal and the stylus prevents close tracking of the needle to the groove modulations. This tends to increase wear on the record, and introduces distortion.

Ceramic cartridges

The next development was the ceramic cartridge, which was also a piezoelectric transducer like the crystal, but because it was more sensitive, could be made with greater compliance (the ability to ride the undulations of the groove without distorting or jumping out of the groove). This also allowed ceramic stereo cartridges to be made. The ceramic cartridge became standard in most phonographs, except for the better high-fidelity (or "hi-fi") systems.

Magnetic cartridges

In high-fidelity systems, crystal and ceramic pickups have been replaced by the magnetic cartridge, using either a moving magnet or moving coil. In the moving magnet system, the stylus carries a tiny permanent magnet, which is positioned between a series of fixed coils. As the magnet vibrates in response to the stylus following the record groove, it induces a tiny current in the coils. This current, now a weak alternating current representing the original sound wave from the recording session, is fed to an amplifier which boosts the signal, and then to a loudspeaker where it is converted to sound waves. Because the magnet is so light, and is not coupled mechanically to the coils, the stylus follows the groove far more gently and faithfully, requiring less tracking force (the downward pressure on the stylus). Moving coil systems are generally more expensive and are preferred by some audiophiles. Here a tiny coil is attached to the stylus, and moves within the field of a permanent magnet. Magnetic cartridges provide a much lower output than a crystal or ceramic pickup, in the range of a few millivolts, thus requiring a preamplifier stage, as well as additional equalization to correct the response of the cartridge over the audio frequency range. Moving-coil cartridges generate an even smaller signal, of a few hundred microvolts, and require additionally a transformer or pre-preamplifier stage. Electrical noise induced by power lines or other EMI are attenuated by various methods, including electromagnetic shielding in the signal cables connecting the pickup to the amplifier.

Almost all stereo high-fidelity component systems (preamplifiers or receivers) that accepted input from a phonograph turntable had separate inputs for both ceramic and magnetic cartridges (typically labeled "CER" and "MAG").

The stylus is typically a conical diamond tip on an aluminum tubular cantilever for a monophonic sound or rugged use, and an elliptical diamond tip for a stereo or binaural signal. Some very expensive styli have ruby, boron, or carbon fiber cantilevers chosen for their exceptional stiffness. DJs use the more rugged conical (sometimes inaccurately called spherical) styli due to the frequent reversals of direction involved in scratching.

Phonograph recordings are made with high frequencies boosted. This reduces background noise, including clicks or pops, and also conserves the amount of physical space needed for each groove, by reducing the size of the larger low-frequency undulations. During playback the high frequencies are rescaled to the original level. This is accomplished in the amplifier with a "PHONO" input that uses a standardized RIAA equalization curve.