by Robert Long
What is meant by a "quad" turntable? Generally speaking, it means simply that the leads from the tone arm to the preamp are designed for the 100-picofarad (or less) capacitance that is recommended for satisfactory recovery of CD-4's high-frequency carrier signal with most pickups de signed to play the Quadradiscs. Of course matrixed quad can be reproduced with regular stereo record-playing equipment, so a "quad turntable" isn't al ways needed to play quad.
There are some other wrinkles that may be incorporated by some manufacturers or on some specific models. Dual, for example, has been adding a CD-4 scale (in addition to those for elliptical and spherical styli) to its antiskating dials. Some manufacturers (BSR is one) offer changer-and-pickup packages that include a CD-4 (Shibata or similar) stylus so that you have everything you need, short of the demodulator, for playing Quadradiscs.
Some go one step further by offering the demodulator built right into the turntable unit. Hitachi has a belt-drive single-play model (PS-14) of this type; B&O has a plug-in demodulator board for its automated single-play Beogram 4002. Less expensive models have been offered by companies like Panasonic. But these models remain exceptions; you shouldn't assume that demodulator or other electronics are included in any turntable assembly.
Is antiskating more or less important with CD-4 pickups than it is with elliptical? If anything, it's probably more important. The purpose of antiskating is to equalize the bearing forces on the two groove walls by offsetting the natural "bias" toward the inner wall produced by the total frictional drag of the stylus, acting via the arm's off set as a torque around the arm's pivot. The forces are quite small, and conventional (elliptical and spherical) styli generally can operate near optimum force on both groove walls even without antiskating.
But in CD-4 systems we've examined so far performance can fall off more rapidly-or in more respects-as the force on the groove wall moves away from optimum. Not only does distortion increase (as it does with any pickup), but so do crosstalk and interference effects. Therefore the audible degradation of signal quality can be more noticeable.
The geometry of CD-4 styli varies from design to design and therefore theoretically affects optimum antiskating settings. Some styli look a little like a rounded pyramid, others like a distended elliptical.
The one thing all CD-4 stylus designs have in common is that they present a 'much greater radius of curvature in the vertical plane than they do along the length of the groove. That is, their "size" with respect to the recorded wavelength is very small, and the consequent loss in bearing area is compensated for by spreading the bearing surface up and down the groove wall. In this they carry the concept of the elliptical stylus one step further. And if you are using such a stylus and have no CD-4 scale on your antiskating control, you can get close to the theoretically correct value by using the elliptical scale.
Where some arms made so that they will accept only one pickup? Very few are, though this is the ideal in the sense that the integrated design allows absolute control over resonance. Even the most "perfect" of pickups will per form poorly in an unsuitable arm and vice versa. If each can be designed to complement the other, both will perform at optimum.
This principle has been applied at both extremes of the price range. B&O has two elegant, high-performance models (the Beograms 3000 and 4002) that will accept only B&O plug-in pickups. And console and compact makers have for years been offering similar non-interchangeability. While audiophiles may sneer at the cheap ceramic pick ups found in the latter group, cartridge and changer often are carefully matched to each other so that best possible performance can be achieved within the admittedly low cost and performance ceilings.
What items are included in an "integrated record player"; which should be? Phrases of this sort can mean several things. It has been common for years to speak of a turntable with an integrated arm-as contrasted with a turntable that is delivered without the arm. Thorens and Rek-O-Kut, for example, have traditionally offered separate platter-and-drive units; the draw back for the average home user is that he must carefully measure and drill a mounting board (or pay someone else to do so) for a separate arm like the SME. This requirement was sufficiently unpopular that the vast majority of units offered to day have pre-mounted arms.
Often they come complete with base and dust cover, and this is what may be implied by an "integrated unit." More often, however, this sort of phrase is used to describe a changer with a base and often a dust cover, plus a premounted cartridge.
The advantage of having the housing (base and cover) as part of the package is obvious, since few users today mount turntables directly into cabinetry. The advantage of buying a preselected cartridge as well is explained in the previous answer.
This popular form of integration is most often available as record-playing "modules" (BSR, Garrard, Glenburn, etc.) in the low price brackets and intended for the relatively unsophisticated user rather than, paradoxically, as equipment intended for the knowledgeable user who presumably should understand the importance of getting a really good arm/pickup match.
One reason is the insistence of most audiophiles on making their own selection. Another lies in the fact that few companies make both turntable/arm assemblies and cartridges. There are exceptions:
Elac (which makes Miracord), Empire, Philips, B&O, and Win Laboratories come to mind; some other companies have tie-ins (like Shure/SME or ADC/BSR) between pickup line and arm or turn table line. But beyond these exceptions the "ideal" of designing arm and cartridge for each other re quires cooperation between companies. It also, to some extent, implies the tying of one company's product to that of another--a situation that most dislike locking themselves into, just as audiophiles dislike being told which pickup they "must" use with a given arm. But the technical advantages of this form of integration remain.
There are some technical advantages to the inclusion of preamp and related circuitry (usually built into the turntable base) as well. Problems of hum pickup and capacitance matching are kept to a minimum. With pickups requiring special circuitry there are additional advantages. We've already mentioned CD-4 demodulators in this context; solid-state strain-gauge cartridges (like those from Panasonic, Toshiba, and Win Labs) or photoelectric models (Toshiba) require power supplies that can logically be built into the turntable unit.
But, again, the pickup manufacturers tend to be specialists; and audiophiles generally prefer the option of choosing preamps and related circuitry from companies that are electronics specialists.
Buyer preferences presumably will continue to dictate the degrees of integration available, with platter-drive-arm-base-cover units popular at higher prices and the same group plus pickup a favorite in budget componentry. What should be included is to that extent academic; but a strong case can be made for integration of the pickup and a somewhat less convincing one for the inclusion of at least some electronics as well.
Is there an ideal shape for the tone arm? No--with one possible qualification. Though some ads for turntables with conventional pivoted pickup arms loudly proclaim that the arm is "a modified S" or "a straight, all-metal tube" or "has an off-set angle or so many degrees, these factors mean little by themselves. They do influence the mass and balance of the arm, and they do contribute to its freedom from lateral tracking-angle error; but they do so only in relation to other factors like effective arm length, counterbalance system, headshell design, and so on. For example, arm length plus offset angle determine tracking-angle error; the shape is the means of achieving the offset angle required by the length, but which shape does this best will depend on the distribution of mass within the arm and hence the materials from which it is made.
The possible qualification is that, in terms of lateral tracking-angle error, the "tangent tracking" or "radial" arm is ideal. And it can be a perfectly straight arm (that is, it requires no offset angle since it dispenses with the conventional pivot) and there fore can most easily approach perfect balance of all active forces in every plane.
Why are there so few radial arms available? Here are some. The most famous, perhaps, is the Rabco SL-8E, while that on the Beogram 4002 has received a lot of attention recently. But there has been no discernible rush to compete in this area, largely because the servo drive system used in both models is inherently more complex and expensive than conventional pivoted arms.
Some radial arms (notably that on the SSI turntable, announced but not introduced several years ago, and that on the Rabco ST-4, now discontinued) have simplified the means by which the arm is moved across the record, but only by adding complications elsewhere in the design. One new model is slated for introduction about the time this issue appears: the Rabco SL-7, which includes a turntable and therefore replaces the ST-4.
There is one alternative, though it is available only from a single manufacturer: the Garrard tangent-tracking pivoted arm. It sometimes is called "articulated," because it uses a pantographic subsidiary arm to alter offset angle as the pickup moves across the record, automatically correcting lateral tracking angle as it goes. This design, too, is somewhat more complex than that of conventional arms; its extra pivot points, required by the articulated head and pantographic assembly, require top design and manufacture if they are not to introduce excessive arm friction.
Does the articulated arm literally have zero tracking error? No. But the error is minute--a tiny fraction of that in a conventional arm so to emphasize the error is to split the thinnest of hairs.
What is meant by "bidirectional damped cueing"? It means that cueing action (using what some manufacturers term the "pause" control) is gentle as the arm moves either upward or downward. Most cueing devices work this way today. Some formerly were damped in the down ward direction only, setting the stylus gently on the record but popping the arm up so abruptly that it would bounce out of position and not necessarily return to the same spot on the record when it was lowered once again.
Is there an ideal length for the tone arm? No-partly for the reasons explained in the answer on arm shape. The longer the arm, the lower the maximum lateral tracking-angle error, even with optimum offset angle. This fact has led to the assumption that the arm should be as long as possible.
But (all other things being equal) the longer the arm, the greater its mass; and the greater the mass, the more difficult it is to control low-frequency resonance, particularly with modern high-compliance styli. The possibility of mistracking, in using a high-compliance pickup in a heavy arm on records that are even moderately warped, can be far more disturbing than the rise in distortion caused by in creased lateral tracking-angle error with a short arm. Hence the most desirable arm is not the longest, but the one that makes the optimum compromise between tracking angle and resonance with a given cartridge. Though this fact is well established, some turntable ads still emphasize arm length as a virtue; it is so only if arm mass also is low.
What is arm resonance, and can it be removed? Properly speaking, it is arm-and-stylus resonance, and it is the (normally subsonic) frequency at which the entire arm assembly tends to oscillate physically, the way a guitar string oscillates when plucked to produce the note to which it is tuned. Arm resonance is defined in terms of both frequency and amplitude (severity)-so many deci bels of rise in response when the stylus encounters the resonance frequency.
If it is so high as to be within the range of deep musical tones (which begin at about 30 Hz except for the very lowest pipe-organ notes), the result is unnatural exaggeration of those tones and, some times, bass feedback from the loudspeakers to pro duce a "howl" at the resonant frequency and/or its harmonics. At the lower frequencies where it usually is found it can emphasize rumble and, if severe enough, produce audible intermodulation with the program signal. If it is well down in the subsonic range-say, at 5 Hz or below--it will limit the degree of warpage that can be tracked successfully, since warps generally "look" to the pickup like vertical modulation in this frequency range. When that limit is exceeded, the stylus behaves very much like a Pogo stick (also a spring/mass resonance sys tem) and can hop about from groove to groove.
Resonance can't be eliminated; it's inherent in (and can be calculated from) the mass and compliance of the system. But it can be controlled. The most common means simply lies in choosing design values that will result (with an appropriate arm/ cartridge combination) in a resonance that is not too severe (say, less than 10 dB) and somewhere in the frequency range (around 10 Hz) between that of most warps and that of actual groove modulation. Damping is a textbook approach to resonance control; some of the newer arms (notably that on the Dual 701, though Lenco had earlier used a similar scheme) use what is known as "de-coupling," which produces a damping effect with out introducing friction. Part of the arm (usually the counterweight end) is attached to the remainder with a somewhat compliant coupling. This "breaks up" the mass into subunits and consequently breaks up the resonant peak into sub-peaks. The result is a series of resonances that are spread out in frequency but none of which is nearly as severe as the single resonance encountered with a totally rigid arm. Some manufacturers have used viscous damping to control arm resonance; Win Laboratories, one of the few doing so at present, claims it also obviates antiskating.
HOW about shock mounting-are turntable suspensions changing? Yes, to some ex tent. When most turntables were designed so that they could be built into a permanent installation, the shock mounting generally consisted of springs between the unit's top plate and the mounting surface. Bases are universally offered today as a substitute for this built-in mounting, but most changers continue to use the sort of spring suspension that would be appropriate either way.
An alternative (most closely associated with the AR turntable, though the "seismic" mounting of the new KLH Research X follows the same idea and other companies have used it) is to put the suspension between the top plate and a subassembly that supports motor, platter, and tone-arm mount.
This isolates the actual record-playing parts from external shock. If you press a control button while playing a record on such a suspension, for example, the chances are small that the consequent jarring of the top plate will cause mistracking.
But the virtually universal acceptance of base-mounting (as opposed to a built-in custom mounting) has led some designers to take the opposite route. In many models the shock mounting is in the feet on which the entire assembly, including the base, stands. If this is the only (or at least primary) shock mounting, the model can't be built into cabinetry without sacrificing some isolation from external vibration-speaker feedback, football shock effects, and so on.
Among these options there is no "best" solution.
Like the tonearm, all represent spring/weight resonance systems. To some extent so do the furniture on which the base stands and the room in which it is used. If suspension resonance falls at the resonant frequency of wood-frame flooring, the turntable can suffer from the Pogo-stick effect when you walk across the room, just as the arm can in tracking warps. For this reason some manufacturers add damping to the suspension. Viscous damping has been used, but foam inserted into the springs is an effective and less expensive solution. For similar reasons, other designs use multiple vibration-isolation elements. The Yamaha YP-1300, for example, has an anti-feedback design in the turntable mat, shock-absorbent ribs within the cabinet, and sponge "springs" built into the feet. The result is comparable to decoupling in the arm-though here it is the compliance of the spring, rather than the mass, that is prevented from acting as a single value.
Why is more than one rumble spec given or some turntables; isn't there a measurement standard? There are several, none of which is universally accepted, and hence more than one may appear on a given spec sheet.
We use the ARLL spec devised by CBS Labs be cause we consider it the most comparable to S/N ratio measurements in, say, amplifiers. It is weighted to reflect the audibility of the frequencies present in the rumble-that is, it tells you how much less audible the rumble will be than music at the reference level. Other test methods can vary in both weighting characteristics and in reference level.
The IEC spec, for example, chooses 0-dB reference 9 dB below those of the DIN specs and would therefore deliver rumble figures that are 9 dB poorer with the same weighting curve. But the curves are quite different; IEC measurements are far more sensitive to noise at high frequencies and in the upper midrange than DIN measurements. The DIN A weighting is much more sensitive than IEC to low-frequency (including subsonic) noise, while DIN B weighting is less sensitive than IEC in this region. The ARLL standard chooses its reference level 3 dB above IEC (and therefore 6 dB below DIN), with a weighting curve whose bass end is somewhat similar to IEC and whose treble end resembles DIN.
Ballpark figures for a really fine turntable measured under all these systems (and there are others) might be 60 dB ARLL, 60 dB IEC, 50 dB DIN A, and 70 dB DIN B. When the manufacturer gives a number without specifying the measurement standard you can only assume that one delivering flattering results (perhaps DIN B) was used.
Why are wow and flutter shown as a single number--aren't they different? They differ only in the rate of speed change: Wow is a slow variation in speed, flutter is faster. There is no distinct dividing line between them, any more than there is between bass and midrange frequencies. But they do differ in audibility. Wow that completes its fast-slow cycle about four times per second is easiest to spot in most kinds of music; faster ones (or even slower ones) that represent the same degree of departure from absolute speed-that is, the same unweighted percentage of wow-and-flutter-are less audible. Present testing equipment generally incorporates weighting for this audibility factor, and therefore it no longer is necessary to differentiate between the two.
Sometimes two measurements are shown, however--one showing the average value of the speed variation (the measurement you see quoted most often), and another for the instantaneous peak value. Usually the two are close to each other, with instantaneous values higher. But let's say that poor finish of turntable parts cause a sudden, brief rise in friction once in each revolution. Because of the short speed-change duration the average value might change little while the peak value shoots up considerably. HF test reports emphasize the aver age peak value (though maximum instantaneous value may be given as well), measured with the ANSI/IEEE frequency weighting.
Why are multiple motors used on some newer models? Partly to keep wow low. A motor with the heft to move the arm back and forth isn't necessarily one that will drive the platter with most accuracy. The possibility of undesirable inter action between mechanical functions is the basic reason why manuals traditionally have been assumed to be better than changers. The use of multiple motors is one of the means by which designers seek the best of both worlds, particularly now that an increasing number of single-play units are adopting changer-like automated features. (One thinks immediately of the repeat-play features on the Technics SL-1300 and Dual 601.) Hence some designs, even among automated single-play turn tables (the Philips GA-209 is an obvious example), use separate motors to isolate mechanical functions from each other so that one will not impair the perfection with which another is performed.
What is an "electronic drive system"? Usually such terms are used for DC motors whose speed regulation is achieved by electronics that compare the actual speed with an oscillator reference of some sort and feed corrective voltages to the motor. This kind of servo control can be used with AC motors (BIC does, for example) as well, but the majority of the electronically controlled units (Dual, Sony, Technics, Tannoy, Yamaha, Kenwood, etc.) are of the direct-drive DC type.
Is the use of the more sophisticated motor types justified in a changer? Certainly, as long as it's not a question of installing a fine motor in a so-so turntable that will simply undercut the value of the motor itself. The question arises, in fact, only if we assume (as many audiophiles still do) inherent inferiority in the changer. Elac/Miracord long ago showed that, if the changer already was mechanically excellent, the addition of a good synchronous motor could make total performance even better.
The current trend to even more sophisticated de signs (such as servo control) simply carries this premise a step further, by adding ever better drive systems to existing changer lines.
The opposite approach was taken in the Technics changer (SL-1350), which adds record changing to an existing, sophisticated (direct-drive, among other things) single-play design. And of course the BIC changers fit neither concept since they are to tally new designs.
How important is a strobe speed indicator, and does accuracy vary from one design to another? Assuming a modern super accurate drive system (be it servo DC, hysteresis/ belt, or whatever), the strobe serves hardly any practical function for the typical user once the unit has been set up and checked out. He may enjoy the reassurance of glancing at it from time to time (and that's a legitimate function, if not a particularly practical one), but unless he intentionally detunes (say, to play an instrument along with a disc) he may never need to retune to precise speed. (Most vernier adjustment-controls are calibrated but not very accurately. Tuning to other than normal speed generally must be done by ear, rather than by eye; and in any event the strobe is of no help in the process-only in returning to normal speed.) At least one manufacturer of electronically con trolled turntables (B&O) claims its electronic control to be more accurate than a strobe. Superficially, the accuracy of a strobe would seem to be absolute, and the claim unjustified. But the strobe is only as good as the stability of the line current frequency. To put it another way: The strobe measures how accurately the turntable (that is, the strobe markings) tracks the line frequency (the strobe light)-but line frequency itself can vary by more than the speed-accuracy spec of a good turntable.
Modern "grid" interconnection of power companies in this country requires a high degree of stability in the average frequency maintained by all companies on the grid. (The famous Northeast blackout occurred in part due to a domino effect.
As one power station went out it could put sufficient extra load on a neighbor to slow its generators and get them out of phase with normally operating equipment elsewhere in the grid tantamount to a short between the overburdened plant and the grid, knocking out the second one as well.) If you are powered from the grid system, any momentary straying of line frequency will be corrected in short order. But if not, your power-source frequency could be regularly high or low, causing the strobe to misinform you about perfect speed. In this sense B&O has a valid point to make, since its reference oscillator will not vary with line frequency.
With so much attention being paid to speed "tuning" controls on the better units, why do so few offer the 78-rpm speed today? Simply because so few users seem to want it. To play 78s you need not only the correct rotation speed, but a special stylus (or cartridge) as well.
Some manufacturers (like AR) evidently felt they could reduce costs, and increase both performance and reliability through simplification, by omitting the speed; and when the resulting models sold well other manufacturers followed suit.
DC motors theoretically can be switched to the 78 speed as easily as to any other; that is, they don't require the special pulleys that are needed in conventional drive systems and can be set for any speed over an extremely wide range. So far, how ever, they haven't been used this way. (One exception is on the way at this writing: the Fons turntable from Scotland that is expected on the U.S. market this spring.) The models that have continuous wide-range speed controls (from Lenco and Rek-O-Kut) use mechanical couplings for the purpose. In the Lenco models, the drive shaft is tapered. The position of a narrow idler wheel on the drive shaft determines the speed it will transmit to the platter.
What practical difference does platter diameter make? Not as much as some advertising might lead you to assume. Primarily it's a question of adequate support for the record. If the platter is large enough to extend under the edge-bead of a 12-inch LP, it will support it more firmly than a smaller platter. This can help in trying to play warped discs. The 45-rpm discs have no edge bead, of course, and are supported only in the label area no matter how large the platter.
When an ad says something about "full-size transcription turntable" it generally is talking non sense by referring inaccurately to obsolete radio jargon. True transcription turntables were de signed to handle 16-inch discs. This has nothing to do with disc-playing in the home today, and true transcription platters therefore are beside the point.
The larger the platter, the greater its flywheel effect tends to be, of course, and therefore the smoother its motion. But the same rotational inertia can be achieved with a small, heavy platter as with a large, light one; so diameter alone is not a criterion in this respect.
Are all platters today nonmagnetic; if not, what difference does it make? When magnetic cartridges first came into use decades ago the attraction between their magnets and the steel platters of the day could pose problems. Most platters today are nonmagnetic for this reason, though the magnet structures used in modern pickups are tiny and the potential problem therefore minimal.
There is one combination we know of that has caused problems for some users: the current Decca pickup (which has an unusually strong magnetic field) mounted in a Philips turntable (all current models appear to use a steel alloy platter and hence are not nonmagnetic). If the arm is set for full recommended tracking force (which is relatively high with the Decca), that force plus the magnetic attraction can cause the stylus assembly to "bottom." The obvious (and, we're told, successful) cure is to re duce the tracking-force setting so as to compensate for the magnetic attraction.
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(High Fidelity, Apr. 1975)
Also see:
Kenwood's Automated Single-Play Turntable (review, High Fidelity mag, Apr. 1975)
Understanding Tonearms (Audio, June 1980)--part 1
Understanding Tonearms (Audio, June 1980)--part 2
Tone Arm Damping--The Overlooked Feature (High Fidelity, Jul. 1975)