Phonograph: Encyclopedia II - Phonograph - Turntable technology

Phonograph - Turntable technology

Phonograph - Turntable construction

Inexpensive record players typically used a flanged steel stamping for the turntable structure. A rubber disc would be secured to the top of the stamping to provide traction for the record, as well as a small amount of vibration isolation. The spindle bearing usually consisted of a bronze bushing. The flange on the stamping provided a convenient place to drive the turntable by means of an idler wheel (see below). While light and cheap to manufacture, these mechanisms had low inertia, making motor speed instabilities more pronounced.

For the serious listener, turntables made from heavy aluminum castings were offered. Typically, they were machined on a lathe and balanced, operating with negligible vibration. Like the stamped steel turntables, they, too, were topped with rubber. Due to the increased mass, they usually employed ball bearings or roller bearings in the spindle for low friction. While some used idler wheel drive, most were intended for belt or direct drive. The high mass and inertia of such turntables helped keep the speed constant, even if the motor exhibited cogging effects.

Phonograph - Turntable drive systems

Most turntables employ an idler-wheel drive, belt drive or direct drive system to rotate the turntable platter:

• Idler-wheel drive system

• Belt drive system

• Direct drive system

Phonograph - Pickup systems

Another major component is the pickup or cartridge. 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.

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.

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.

Phonograph - Arm systems

Basic arm design has changed relatively little. S-Type tonearms can be found on even the 1925 Victor Orthophonic phonograph. Originally, even though the tonearm was light for earlier electric pickups, the full weight rested on the record. Right through to the crystal pickup, this was required to create sufficient tracking force to follow the grooves adequately with relatively stiff styli. Naturally, record wear was not given much consideration. With the advent of the better technologies, including more powerful rare-earth magnetic cartridges, far lighter tracking forces became possible, and a balanced arm came into use. Most use a counterweight to offset the weight of the arm. A calibrated dial on the weight provides for quick change of stylus pressure. Stylus pressures of 1 to 2 grams are currently the standard.

Tonearms are prone to two types of tracking errors that can affect the sound. As the tonearm tracks the groove, the stylus drags tangent to the disc surface and resistance along the arm combines to create a horizontal skating force towards the center of the disc. Modern arms provide an antiskating mechanism, using springs, hanging weights or magnets, to offset this force, so as to make the net horizontal force near zero. The second error occurs as the arm sweeps in an arc across the disc, causing the angle between the cartridge head and groove to change slightly. A change in angle, albeit small, may have an audible detrimental effect by creating a differential force on the groove walls. Making the arm longer to reduce this angle is a partial solution, but less than ideal, because the arm would need to be of infinite length to reduce angular errors to zero. Some arms (such as the Garrard "Zero" series) have been manufactured with a parallelogram arrangement which pivots the cartridge head on the arm to maintain a constant angle.

If the arm is not pivoted, but instead travels horizontally along a radius of the disc, there is no skating force and no cartridge angle error. The arm is driven along a linear track using an electronic servomechanism to position it properly. Bang & Olufsen developed the first practical system with its model Beogram 4000 in 1972. Early Edison phonographs had utilized similar spring-powered drives to carry the stylus across the record at a pre-determined rate. In practice, the linear tracking system is not widely used today due to its complexity and attendant expense. However, some of the most sophisticated systems do employ this technique. It is nearly ideal, as the stylus replicates the motion of the recording lathe when the master recording was cut.

Phonograph - Front-loading systems

A brief mention could be made of one attempt to make the use of records more convenient, in the dawning age of the compact disc. In the early 1980s, one manufacturer designed an upright (front loading) record playing music centre, in which the record was placed in a door which hinged downwards to accept it. The door retracted automatically and the record was spun in the vertical plane. A pair of linear-tracking arms traversed the disk, one on each side, meaning that the whole record could be played without stopping and turning it over. The whole system was mechanically and electronically exceedingly complex, and while it worked, the system as a whole was aimed at the mass market and had only mediocre sound quality. The large physical size of the hinged door made it vulnerable to damage, and the retraction motor was barely able to lift its weight, especially after some years of use.

Adapted from the Wikipedia article "Turntable technology", under the G.N U Free Docmentation License. Please also see http://en.wikipedia.org/wiki