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Aug102008

How to Choose a Turntable

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Micro_Seiki_SX-8000II

Keep on groovin'

By Roy Johnson, loudspeaker designer, Green Mountain Audio, Inc.


LPs are back. Why have them at all? Because most of the time they sound more musical than a CD. Good pressings played on a good turntable are also quiet and can easily remain that way for decades. While new turntables priced around US$400 -- including the tonearm but with no phono cartridge -- will satisfy most vinyl tastes, there are several benefits to owning an even better model. If one is in your future, our guidance will help you make an informed selection.


The turntable system

The turntable has several important components: The platter and record mat; motor; main bearing, on which the platter spins; plinth, on which everything is mounted; suspension under the plinth; tonearm, and the phono cartridge, with its diamond stylus (needle). Together, these form a turntable system. Each part affects what is heard.

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A turntable may be purchased without a tonearm or phono cartridge. A very experienced retailer must guide you through the selection process to obtain the best sound. In order, the selection process will focus on your music preferences, phono cartridge, and tonearm.


Elements of design

The best turntable designers strive for audible improvements in the construction of the platter, bearing, motor, plinth, and suspension -- assuming they do not also design tonearms and phono cartridges. Nonetheless, they are still hampered by having to judge their efforts by the sound of the end result, since all design elements interact. This means the best turntables are produced by very experienced designers who have had the patience to try many approaches and to examine both the competition and their own assumptions. Every decision is interrelated and hopefully each is based on physics. What follows is an explanation of each of the turntable's components.


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The Platter

The platter (and platter mat on top) serves several purposes. As a flywheel, it smooths out speed fluctuations from the motor and varying drag of the stylus while it encounters changes in the music. As a vibration absorber, it damps the sound of the record which is vibrating under the stylus.

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The platter must firmly support the record, so that the vinyl does not deflect away from the stylus on loud passages. It must also dampen any vibrations that emanate through the main bearing. There are many ways to achieve high performance in each of these areas. The best turntable designers have achieved a balance among them all. You will hear pitch-perfect performance, excellent rhythm and dynamics, and precision stereo images.

Platters are either very heavy or made with a ring of weights around the edge to provide the most flywheel effect. Some designers believe a light platter sounds fine if driven by a strong and smooth motor. We disagree but are always willing to listen to their results.

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Platter materials and the record mat can be chosen to damp the higher-frequency vinyl vibrations (from the stylus), but not allow a record to flex on the midrange and lower notes. Therefore, you would think it is best if the platter mat is neither hard plastic nor very soft felt.

Materials for the platter itself have ranged from cast aluminum and lead to glass platters and acrylics. There are platters layered of lead, plastic and cork. Each material has its own pros and cons that should be weighed against the other design elements. There is no 'best' one.

A record clamp always helps to hold the LP flat to the record. That 'clamp' could be a weight placed over the spindle or an actual clamp to grab the center spindle without adding much mass while pressing down on the record. Any improvements in sound quality would be heard in dynamics and clarity.


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The Main Bearing

There are many theories as to what constitutes the best main bearing on which the platter spins. Some turntables have oil lubrication, a few have plastic sleeves that require no oil, and still other models use air pressure or magnets to keep surfaces from touching. No matter what, the end results are minimum friction, vibration, noise, and wobble.

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Low friction means less drag on the motor, so the motor could be small and low-powered, for less vibration. But that motor then needs a heavy platter to smooth its own speed fluctuations and those created by the varying drag of the stylus. However, does it then produce enough torque to start the heavy platter spinning? Higher friction provides a constant drag on the motor and smooths any speed fluctuations, but the motor must then be larger or spin faster to supply extra turning power. Either way, this can mean more vibration than from a small and/or slower-turning motor.

The main bearing provides support in two directions, laterally and vertically. It cannot allow the platter to move by the smallest amount in any direction, smaller than the record groove undulations themselves. Laterally, the bearing must prevent the platter from wobbling and vertically, against gravity.

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For the lateral support, the main shaft/spindle and the bearing housing could be machined to the highest possible standards of roundness and highly polished, while leaving room for a thin film of oil in between. The clearance could be greater, permitting a thicker film of oil that also allows less precise machining. Some designers report the turbulence in a thicker film of oil creates unwanted noise. The viscosity of the oil affects any decision, such as a thicker layer of oil allowing more vibration if its viscosity is too low (too thin).

The vertical support of the main bearing can be provided by very smooth materials, either hard or soft. One solution is a hardened steel tip on the bearing shaft riding in a bronze cup. Another solution is a polished tungsten-carbide ball (extremely hard) on the end of the shaft, spinning on top of a polished sapphire plate (also extremely hard). Perhaps vertical support is provided by opposing magnets or compressed air, lifting the platter up just a little. It seems like those two choices reduce noise and higher-frequency vibration at the possible expense of less resistance to very-low-frequency sideways vibrations caused by the platter and motor tugging on each other when the stylus encounters complex grooves or the impulse of a bass drum.


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The Motor

Motors can be powered by either DC or AC, and valid arguments are made for each. The motor can be high-speed or low-speed, even direct-drive, where the motor surrounds the main bearing, driving the platter directly with no belt in between.

Regardless, the motor's torque has to be sufficient enough to get the platter spinning, which is much more than what it needs to maintain the platter's speed against the drag of bearing friction and the stylus. Any motor needs to have its rotor balanced for the least vibration and belt-drive motors are usually hung from their own rubber mounts to prevent their vibrations from entering the plinth (the base), where they would travel up into the platter and tonearm.

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An interesting relationship exists between the momentum of the spinning motor, the elasticity of the belt, and the momentum of the spinning platter. They form a resonant system.

A heavy platter and soft belt connected to a massive motor can vary (bounce) in speed at a very low frequency (called 'wow,' but here not audible as a speed fluctuation). That very low resonant frequency is excited not by the usual audio-range signals such as 'bass,' but by the sum-and-difference 'beat' frequencies between two bass notes or the impulse of a drum beating. One hears the rhythm of the music retarded or enhanced, depending on the music and the resonant frequency.

There are other resonant conditions related to how fast the motor spins relative to the 33 1/3 rpm of the platter, and how many 'poles' the motor has. Those not only can induce vibrations, but also will play against the elasticity of the belt and the rotational momentum of the platter, causing higher-frequency speed fluctuations (called 'flutter').

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Some designs use a medium-mass platter coupled to a motor so small and weak that it has no power switch. One just stops that platter from spinning by hand while the motor remains powered, trying to turn but unable to start the platter spinning again. The platter needs a little push to help the motor start up.

A direct-drive motor driving a platter of light or moderate mass constantly varies its speed by just a small percent, centered around that 33 1/3 revolutions/minute. This is because its servo-control system cannot sense very low-frequency or very slow variations in speed. It is always 'looking for' exactly 33 1/3 rpm, by overshooting and undershooting it at a very slow rate. While this is too slow to be perceived directly as a speed variation, it is heard to affect, for example, the 'solidity' of a piano's single notes.

A direct-drive motor can be fitted with a very heavy platter, which will move those slow variations in speed to even slower variations, but this requires a larger the motor. The pinnacle of direct-drive technology is represented in the discontinued Technics SP-10 Series and studio drives from Denon and EMT. All remain popular on the used market.


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The Plinth

The plinth of the turntable (its base), is not supposed to flex, resonate, or vibrate while it supports the motor, platter, and tonearm. The plinth is also exposed to sound coming through the air. The most likely range of tones it will pick up from the air are those in the voice-range, because those wavelengths 'fit' the plinth in size and are of the same tone that many common materials prefer to receive from the air. The solution could be a plinth made from materials exceptionally dead in that middle-tone range, and/or to simply cut away all the extra plinth material that is not directly supporting the motor, main bearing, and tonearm.

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A suspension under the plinth can isolate it from shelf and floor vibrations, allowing less vibration to come up through.

There could be no suspension at all, which would keep the turntable plinth from rocking when the stylus hits a loud passage or a drum beat.

For us, the best-sounding solution has been to use a turntable with a very firm suspension and heavy plinth in order to firmly resist any motion created by the stylus and to place that turntable around the corner in another room on a heavy shelf, away from floor and airborne vibrations. Floor vibrations are at a minimum when the floor is cement, or when the turntable is placed on a shelf hung on the wall.

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Shelf vibrations are of two types: The center of the shelf moving up and down like a trampoline and the materials of the shelf vibrating on their own.

The former is reduced with a stiffer shelf or one supported in the center. The latter is a property of the 'particles' that make up the shelf, whether those are granite or wood. A stethoscope easily reveals their presence. We recommend shelf designs by Harmonic Resolution Systems.

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Any non-metallic cones used under the plinth will isolate it from the shelf, because they touch less of the shelf's surface. We recommend those from Marigo Audio and Black Diamond Racing. Theirs work best when one is placed near the motor; near the main bearing, and near the base of the tonearm. That placement also creates an irregular triangle for contact with the shelf and avoids some of the whole-number-ratio shelf flexations as well as the shelf's center, which moves up and down the most.


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The Tonearm

Tonearms are simple devices but very complicated in design for many reasons. A tonearm must must support the phono cartridge with no flexing or bearing rattle, all while letting the cartridge move across the record, and up and down with virtually no friction. Under no circumstances should it allow the cartridge to torsionally rotate about its axis.

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Because the stylus vibrates, it vibrates the body of the phono cartridge, even though it is seemingly isolated by its own soft rubber suspension. Those vibrations are induced by the changing magnetic fields created by the stylus' vibrations, which is why many manufacturers tout their materials and construction of the body.

The tonearm must absorb those vibrations without reflecting them back into the cartridge body (i.e., resonating). This requires a careful choice of tonearm materials. For example, an arm that is slender, thus somewhat flexible, but highly damped against prolonged ringing, will permit the initial vibration to occur (by flexing a little, perhaps as a torsional instead of a bending mode) and then damping any recurring oscillations. That combination is known to take some of the 'life' out of the music. If an arm has parts made from wood, it often produces a different sound in the highs than arms made from metal. Different metals produce different sounds as well.

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The effective moving mass of the arm and phono cartridge reacts with the softness of the stylus' rubber suspension.

If the arm is too massive for that suspension, the phono cartridge will bottom out on record warps, like a Cadillac on Volkswagon springs. However, that combination yields the smoothest and most dynamic bass response.

If the arm is too light for the softness of that stylus' suspension, the phono cartridge will 'jump' off the record warps and also make the bass 'boomy.' A tonearm should be matched to the cartridge because there is a correct range of effective mass recommended for each.

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The Phono Cartridge. The phono cartridge is a transducer, turning mechanical vibrations into electrical variations. Its many sub-assemblies act together to create the signal. Every one is imperfect in some way, so there are always trade-offs to be weighed by a designer.

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The cartridge's diamond stylus rides in the groove, mounted on the end of what is called the cantilever, supported by a rubber suspension. On the other end of the cantilever, a magnet will be moving between fixed coils of wire that create the signal. The coils (much smaller) can be mounted on the end of the cantilever instead, and moving between the North-South poles of a fixed magnet. The magnet and coils can both be fixed in position, with a soft-iron sleeve fitted around the end of the cantilever becoming magnetized from the nearby fixed magnet and then moving between the coils.

Respectively, these moving magnet, moving coil, and 'induced magnet' (or 'moving iron') designs each have advantages that are traded off against how far the stylus actually swings the other end of its cantilever.

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If it only swings the other end by small amounts, then only small electrical signals are generated which can also be transmitted only by the finest quality of wires for the coils. If it swings by a large amount, then very large signals can become softened as the outer fringes of the magnetic field are approached.

Some phono cartridges require extra amplification through a step-up transformer or a pre-preamplifier ('a moving-coil preamp'). Because their phono signals are 100 to 1,000 times lower than a regular phono cartridge, care must be taken in the design of a step-up transformer and moving-coil preamp to achieve low noise and wide frequency response. Some phono cartridges require special loading via a change in the resistor in the phono preamp stage and of any capacitance.

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Suspension. A stiff rubber suspension for the stylus and cantilever does not like to move on very small signals, which can be called a 'sticking friction.' Then again, a very soft rubber suspension can increase in stiffness at its extremes of motion, constraining large sounds. The suspension can be placed around the center of gravity/center of vibration of the cantilever, where the moving cantilever is balanced from front to rear and the suspension is stressed the least. But then the cantilever is more easily disturbed torsionally (rotated about its own axis), allowing the diamond to twist in the groove.

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That rubber suspension could be located all the way at the far end of the cantilever, with all motion leveraged against it. Mounted in that position, the rubber then provides more resistance to torsional stresses, but the rubber can be more difficult to displace (stiffer). That rubber (actually, a polymer) could harden over time, and at colder room temperatures.

The suspension of a new cartridge requires at least 50 hours of playing time to loosen up (break in). Its sound will become more clear, lively, and with better bass.



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Magnets. The magnet(s) inside the cartridge must provide a uniform and focused field surrounding the pickup coils. Any soft-iron pieces or laminated silicon-steel plates inside those coils or connecting the North-South poles of the magnet(s) will have their own non-linearities and eddy currents which affect the signal. Those metal pieces and the metal coils also have their own mechanical resonances. The coils should be magnetically shielded by mu-metal, which other metals and all plastics fail to do. The magnetic 'stiffness' of the magnet and its resistance to demagnetizing is important. The higher the better.

Cantilevers. A cantilever must be light, rigid, and self-damping against high frequency ringing. The shorter it is, the higher its natural resonant frequency. One cartridge is famous for having an extremely short cantilever made of solid diamond, both for extreme stiffness -- Dynavector's model 17. The London Decca series of cartridges have essentially NO cantilevers and are renowned for their dynamics, but do not like record warps. Cantilevers affect the sound so much that many materials have been used -- solid ruby; hollow sapphire; boron; carbon fiber; titanium; beryllium; diamond-coated boron; multi-tube cantilevers (one inside the next), and the least-expensive, aluminum tubes.

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Styli. The stylus, or diamond tip (needle), is precisely ground then polished to fit the record grooves. The most common shape is elliptical when viewed from above, with its narrow left and right edges fitting into the small high-frequency groove wiggles. Other shapes have been developed to fit into those wiggles even more tightly and also contact more groove up and down the sidewalls of the grooves.

These 'fine-line' styli are often named for their inventors, such as Gyger, Shibata, and van den Hul. They have unique shapes because there are different ways to interpret the mathematics of what shape best fits into 'the groove.' These fine-line shapes require very careful setup of the tonearm for the best sound, especially in the high frequencies.

When a diamond is ground, the best manufacturers choose a natural axis of the diamond crystal to become the L-R edges of the ellipse or the line-contact edge, for maximum lifespan. They also spend more time polishing the diamond for less groove noise and longer life. A diamond stylus does not wear down from friction. Instead, small chips flake off from micro-fractures in the crystal. The L-R edges of the stylus become flattened, creating audible distortion in the highs. Inexpensive phono cartridges have industrial-quality diamonds that last for about 300 hours. The best diamonds are gem grade and therefore clear. They can last for more than 1,000 hours. Keep the diamond clean with the Last Factory's stylus cleaning fluid and brush, and use their Stylast diamond preservative. When a gem-quality diamond is clean, you will be able to see through it to a white wall behind.

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Seldom mentioned is how the soft vinyl groove reacts against the pressure of the stylus. Simply put, the actual bass notes are essentially slightly below the visible surface of the groove because the stylus has more time to press in on the vinyl during the longer-lasting bass frequencies. That pressure applied per square inch differs with each stylus shape and of course, the vinyl's composition. Therefore, the sound of the bass depends on the stylus shape and tracking force.

A diamond tip can be mounted to the cantilever 'nude,' or it can be glued to a piece of brass before it is ground, then that brass glued or brazed to the cantilever. A nude stylus sounds better because it is a more rigid affair and has less moving mass for the groove to throw around without losing contact (mis-tracking). It is more expensive to grind. A nude diamond will also likely be of gem-quality.

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The stylus is kept in contact with the groove walls by the tonearm's tracking downforce. The tracking force applied through proper balancing of the tonearm (and leveling of the turntable) will be recommended by the manufacturer of the cartridge. In the 1970's, a cartridge tracking at a downforce of about 1 gram extended record life. This was for elliptical (also called 'bi-radial') styli.

The newer cartridges with line-contact styli have tracking forces from 1.5 to 2.5 grams, but as that force is spread out over a longer line of vertical contact with the groove's sidewall, the record wear is no greater and even perhaps less.

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Anti-skating. As the tonearm swings across the record, the necessary angle for the cartridge at the end of the arm causes the stylus to be pushed inwards towards the label. A tonearm usually has an anti-skating adjustment for a compensating outward force. The amount of that anti-skating force depends on the shape of the stylus and ideally should vary across the radius of the LP. Anti-skating is best adjusted with a test-tone record and an oscilloscope. Listening on headphones also helps dial-in the correct amount. Check the stylus and cantilever from the front. They should remain centered in the cartridge body, pulled neither to the left or right.

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Alignment. The phono cartridge is aligned from front-to-rear in the tonearm to the specifications of the tonearm's manufacturer or with the use of an alignment protractor. Setting the tonearm's vertical height changes the 'vertical tracking angle' (VTA) of the stylus in the groove and must be adjusted for the best clarity, especially in the highs. If setting the VTA by eye and ear, most cartridges sound best somewhere between the top surface of the phono cartridge parallel to the record and the tonearm's back end lowered by an 1/8th inch (3mm). Using headphones is a faster process. Original Sheffield Labs Direct-to-Disc LPs and those from Mobile Fidelity produce a VTA setting that works well for most other LPs. The stylus must be aligned vertically when seen from the front. Observing its reflection in the vinyl helps, as that reflection doubles any Left-Right lean of the diamond.


Recommendations

To care for LPs, we recommend seeking out the Discwasher D4+ record brush and fluid. To control fingerprints, always handle an LP by cradling its outer edge into the crook between your palm and thumb, letting your fingers of that hand support the label.

As you see, turntable design (and setup) is an art because of the expertise and creativity required to balance all of these variables. We like the engineering and performance of turntables (without tonearms) manufactured by Acoustic Solid, Avid, Basis, Bluenote, JA Michell, Micro Seiki (out of business, but considered among the highest standards), Simon Yorke, and Thorens. Many of these manufacturers offer excellent tonearms or can recommend certain brands. So will your retailer.

Regarding phono cartridges, expect to spend at least US$100 for a cartridge that will not destroy your records in only a few plays. We like ones made by Dynavector, Goldring, Nagaoka, Ortofon, and van den Hul.


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