How Turntables Work
Turntables – Part 1
Last time we looked sat how a CD player works, so I guess it is only fair to look into the workings of record players / turntables, as well. After all, they are making a comeback plus a lot of the youngsters out there might be interested!
Records are made in a similar way to CD’s in some respects; they have a master version made by a record lathe, basically a record player in reverse. Heated vinyl plastic is stamped; the centre hole punched out and the edges are cut so everything is hopefully centred. (Often this is not the case and can cause some playback issues!) The thickness of the record has an effect on the playback, you may see 180 gram or high numbers quoted, denser and thick records are generally reserved for better quality recordings and are in much demand amongst enthusiasts.
Most turntables operate in much the same way. There is a platter upon which you place the record. Platters vary enormously as to materials and thicknesses, heavy platters are designed to reduce variations in speed by having a lot of momentum, so small changes wont be noticed. These variations are called wow and flutter. The different materials used also are designed to reduce unwanted noises, isolating the playback. Noises coming through this way are referred to as rumble. These days, well-designed systems should not have issues with either wow and flutter nor rumble. On top of the platter is usually a mat. The mat offers protection to the record surface from the usually hard surface of the platter plus can add some of its own characteristics; the better ones offer even more isolation. In DJ circles, the mat is used to allow the record to slip, allowing the “rapping” of music.
The platter is driven by a motor, either direct drive or by belts or wheels. Direct drive systems mean the motor is directly attached to the platter. This allows for fast start-up and direct speed control. This method is most common in DJ systems as the isolation between the motor and the platter is not so great. The most common type in hifi turntables are belt driven types. The motor is mounted or sits away from the platter and is connected via a rubber or string belt. Some systems have two or even three motors for even better control; some have extra pulleys in-between and have two drive belts. Belt systems sometimes have to be hand started, you give the platter a push to start it as the motors are designed to be very quite and stable but don’t have the so-called starting torque required. Sophisticated electronic controls are used to keep the motor spot on speed, irrespective of fluctuations in mains power. Speed is switched between 33 1/3 and 45 RPM. Some have speeds around 78 RPM (even adjustable to match various recording methods of the time) that cater for older records. You may see rows of small dots around the outside edge of a platter, these are speed markers, under lights that are usually built in and you can match the exact speed. This is not so common on well-regulated high-end turntables though.
No we move to the tone arm. At one end is the pivot system and the other is the cartridge. The tone arm is designed to move as freely as possible as it travels from the outside of a record to the inside. As it does this by following the groove, it must hold the stylus in position and not allow it to jump out nor to exert too much pressure. Weights and balances are put into play here to offset opposing forces. It is a tricky thing to do and setting up a system perfectly is not so easy. There are even special devices used to measure angles and weights. The weight the tone arm and the cartridge / stylus exerts at the very end is called the tracking weight. It varies with different set-ups, most manufacturers advise their recommended levels. Too low and the stylus will jump very easily, too much and you will wear out the stylus and records very quickly!
Other settings include VTA, which is vertical tracking angle. This is the angle the stylus sits in the groove. If you imagine the platter becoming thicker or thinner (or the arm moves up or down at the pivot point); the angle will change. This has to be set correctly so the stylus is able to read the signal most effectively. Again recommended angles are often supplied. Anti-skating force is an opposing force to the tendency of the arm to want to move towards the middle since it is following a groove spiralling inwards. Due to the distance covered, there is no exact setting for this; some people will say it does not need to exist at all. It is often best to follow the recommendations as advised by the manufacturer.
The tone arm itself must be very stable. Any movement from the stylus should not be transmitted along it and visa-versa. Achieving this at a top level is one of continuing challenges and many designs appear all the time. Exotic materials are used with complex mechanisms and counter balance systems, magnetics, ceramics and other space-age concepts. Even wood is making a comeback! One device that is hard to argue against is the arm lifter, a simple lever that gently raises and lowers the arm; it is much safer than doing it by hand!
Written by Leon Gross, originally published in Audio & Video Lifestyle magazine.
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