by Peter W. Mitchell
Should one choose a turntable with belt drive or direct drive? Both slams have advantages and disadvantages.
IN the words of the old song, "The music goes 'round and 'round, and it comes out here." To play an LP, the phono cartridge has to follow the spiral groove around the record from beginning to end while the stylus traces its microscopic undulations. Taking this description literally, we might imagine a record player consisting of a phono cartridge mounted in a small toy car running on batteries and dragging the stylus behind it.
The stationary LP disc would be placed on a flat surface and the cartridge would drive around the record, following the groove like a farm tractor following the furrows in a plowed field. Implausible as it may seem, Sony actually built a toy record player, the Soundwagon. that worked this way.
We can only speculate about how well this approach might work if it were refined. All of the other record-playing systems in the century-long history of the phonograph have taken the opposite approach, holding the phono cartridge in a semi-fixed position while causing the groove to pass beneath it on a turntable. Upon closer examination this turns out to involve three distinct, if interrelated, tasks.
(1 ) The cartridge must be "semi fixed"-it must be precisely tangent to the groove and rigidly fixed longitudinally (along the direction of the groove), yet it must have enough lateral freedom of motion to follow the average position of the groove as it spirals in from the edge toward the center of the disc, plus enough vertical freedom to maintain a constant height above the disc and exert a constant downward force on the stylus despite surface irregularities and warps. But since the tone arm and cartridge have mass and are supported by the compliant cantilever suspension (a spring), they form a resonant mechanical system that has an unavoidable tendency to vibrate.
(2) The groove must pass beneath the cartridge at absolutely constant (and correct) speed, despite any variations in driving force, drag, and so on. Most of the jargon of turntable design is related to how this function is performed-and, more important, to how well it is performed.
(3) In modern microgroove records (unlike the 78-rpm shellac discs of yore), significant audio modulations may involve groove-wall undulations as small as a wave length of light. The stylus assembly must respond to such small motions and transform them into a usable electrical signal. While we would like the stylus to vibrate only in response to the engraved modulation in the groove walls, the cartridge is an exquisitely sensitive detector of vibrations of all kinds. Any separate motion of either the record or the cartridge will tend to stimulate tone-arm/cartridge resonance.
And from the point of view of the stylus, any vibration of the disc or cartridge appears to be just another waveform that must be traced-one that may inter-modulate with the desired audio signal. Therefore, control of external vibration is the "hidden agenda" that every turntable designer must cope with, whether he does so willingly or not.
In principle, a turntable platter could be rotated by any of the methods that have been used to power clocks and mills throughout history-hard cranks. foot pedals, the tension of a wound-up spring, the pressure of water falling against paddles, falling weights (as io a grandfather clock), etc. In modern times, of course, virtually all clocks and turntables are powered by electric motors. In fact, some turntables actually employ motors that were designed and mass-produced for use in clocks. The principal differences among turntables today have to do with whether (and how) the motor's speed is regulated, how its torque is coupled to the platter, and what means are used to suppress un wanted vibration.
Not many years agc a turntable was a simple product. Typically its electrical part consisted only of an a.c. power cord, a motor, and a switch to turn the motor on and off.
The rest of the turntable was mechanical: bearings, pulleys, a rubber belt or idler wheel to rotate the platter by friction, and an assortment of springs, levers, and gears providing whatever automation was desired.
A few turntables today--including some of the highest--performance models-still fit this description. But in recent years the great majority of turntables have been transformed by a massive infusion of high technology. In today's record players you can expect to encounter such exotica as Hall-effect magnetic sensors, phase-locked-loop feedback control, quartz-crystal oscillators, optical serves, CMOS logic IC's, in some cases as much electronic circular!, as in a typical FM tuner-all to achieve the ideal of perfectly uniform and accurate rotation. And in place of the simple materials of yesteryear (aluminum, wood, stainless steel, and a bit of rubber) you will find fiber glass, Delrin, Teflon, calcium carbonate, acrylic-butadiene-styrol resins, carbon fiber, titanium, and hydrodynamic suspensions--a I selected for their low friction, low weigh, high rigidity, or effective suppression of vibration.
WHILE many means of transferring the motor's torque to the platter can be imagined. only two are common today. In a belt-drive unit a motor spinning at several hundred rpm is mounted in the turntable's base and is corrected to the platter by means of a pulley and a thin rubber belt. Usually the belt runs around an inner platter or sub-platter that has a circumference precisely nine times that of tie pulley this ratio provides the required step-down of speed from a typical 300-rpm motor to the 33 1/3-rpm platter.
In a direct-drive turntable, the platter rests directly on the motor.
The turntable's central spindle is actually the motor's main shaft, so the motor must run at 33 1/3 rpm, less than one revolution per second.
--- Belt drive. In a typical high-quality belt-drive turntable, the
platter and tone-arm assemblies are well isolated from the motor, the base,
and the dust cover, which are sources of vibration.
The decade-long advertising battle is finally over between adherents of these two techniques for making the platter turn. Neither technique is the clear winner: excellent and mediocre turntables can be produced-and have been!-using either kind of drive system. Each has its strengths and weaknesses.
The Belt Drive
The belt drive has the advantage of simplicity, meaning that a belt-drive unit can be made at very low cost without having to sacrifice performance. Using a synchronous clock motor allows consistent, accurate speed without the need of any costly speed-regulation circuitry in the turntable; the designer simply relies on the fact that electric utility companies maintain the average frequency of the a.c. power line precisely at 60 Hz.
The thin rubber drive belt serves as an efficient mechanical filter to prevent the motor's vibration from reaching the platter. Thus even a budget-priced belt-drive turntable can have a low rumble level.
Perhaps the greatest advantage of the belt drive is the ease with which good isolation from external vibration can be obtained. Since the mo tor is coupled to the platter by a naturally flexible linkage (the belt), there is no need for any rigid linkage between the platter and the rest of the turntable mechanism or its base.
The platter can be mounted on a separate subchassis within the base, floating on soft springs, immune from whatever vibrations may be reaching the base from any source the turntable's own motor, acoustic feedback from the speakers, etc.
A turntable's plastic dust cover (which is large, thin, and stiff) tends to act as a sensitive microphone diaphragm, vibrating when struck by sound waves from the speakers.
This airborne acoustic feedback is a significant source of coloration in many turntables, but in belt-drive floating-subchassis designs the dust cover's vibration is coupled into the turntable base and never reaches the platter and stylus.
The belt-drive system does have some disadvantages, however. A belt drive has no rigid connection between the motor and platter; it operates only through the friction of the belt passing around the pulley.
Therefore, it cannot apply the large amount of starting torque that would be needed to accelerate the platter quickly. (Attempting to apply more torque, by using a larger motor in place of the usual low-power clock motor, will only result in belt slippage.) Typically it takes from 2 to 10 seconds for a belt-drive turntable to get up to speed after it is turned on.
Even when operating at speed, the belt can slip if there is any drag on the platter. Many belt drives will immediately stall if a record-cleaning brush is applied to the disc while it is turning. The friction of, the stylus in the groove produces a slight retarding drag that varies with the musical signal and may slow the platter by a measurable, though not audibly significant, degree. (Some belt-drive turntables are designed to run about 0.2 percent fast when unloaded, so that the drag of the stylus will bring them down to exact speed when playing a record.) It is possible to design a belt-drive turntable with a servo system that constantly adjusts the motor speed to maintain exact platter speed, but only a handful of relatively expensive models have been built this way. Most belt drives are "free-running," relying wholly on a synchronous motor (running in step with the power-line frequency) and a precision-ground belt to obtain correct platter speed.
In some floating-subchassis de signs the frequency of the suspension resonance is below 5 Hz. While this provides excellent isolation from acoustic feedback and normal vibrations, it can make the turntable more susceptible to groove-jumping when people walk or dance on a springy wood floor.
The Direct Drive
Not surprisingly, some of the ad vantages of a direct drive are the converse of the belt drive's disadvantages. For example, since a direct-drive system doesn't have the frictional losses of slipping belts, it can be designed to get up to exact speed almost instantly when it is switched on. This (and not any presumed difference in sonic performance) is the reason why direct-drive turntables are almost universally used in radio stations for air play.
Direct-drive turntables usually employ electronic circuitry for speed regulation rather than de pending on the power-line frequency. Once the speed-regulation circuitry is present, further refinements can easily be added-such as a quartz-crystal oscillator for absolutely exact speed, a variable-pitch control to fine-tune the speed, or a servo system to measure platter speed and automatically correct any variations in speed that may occur during play.
-------- Direct drive. A good direct-drive turntable isolates the platter,
motor, and tone arm on a separate subchassis. Lower-priced units often rely
only on shock-absorbing feet for platter isolation.
Since the direct drive relies mainly on electronic rather than mechanical parameters to achieve correct speed, consistently excellent performance is routinely achieved in production with very little sample-to-sample variation. Direct-drive turntables usually have very precise speed control and are able to maintain it despite the drag of disc-cleaning devices.
Perhaps the greatest practical ad vantage of the direct drive is the design freedom that it accords the manufacturer-facilitating the development of record players that stand vertically on edge, "clam shell" record players no larger than an LP jacket, mini-size turntables with a slide-out platter in a drawer, and so on. And with electronic control it is relatively easy (and inexpensive) to implement additional convenience features such as push button operation, wireless remote control, and synchronization with a tape deck for dubbing.
But a direct-drive system also has its disadvantages. In any electric motor the spinning rotor tends to jump from one magnetic pole to the other, delivering its torque in a series of pulses rather than a smooth flow of power. Without a flexible belt to isolate the vibration from the platter, a direct-drive motor re quires sophisticated engineering to minimize this "cogging" and the associated vibration that would be picked up as a rumble by the stylus.
In practice this means that good direct-drive motors are expensive to manufacture, with their large and elaborately interleaved copper windings, precisely machined rotor, and complex control circuitry. As a result, direct-drive turntables tend to cost more than belt drives that have comparable performance and features.
A direct-drive turntable could be designed with a highly compliant suspension to isolate it from external vibration, but very few have been built this way; on the average, therefore, direct-drive turntables tend to be more susceptible to acoustic feedback than floating-sub-chassis belt-drive systems.
The Vibration Problem
Regardless of the pros and cons discussed above, belt and direct drives both work so well that in most turntables the platter drive system has no direct effect on sonic performance. It seems likely that the most important practical difference among turntables is not the drive system itself but the methods taken to control and suppress unwanted vibration from internal and external sources.
Unfortunately, this aspect of performance is very difficult to generalize about. For example, one popular and effective method of reducing a turntable's vibration sensitivity is to make it very heavy. But light weight models are not necessarily inferior; some, such as the classic Acoustic Research turntable, are among the best ever produced.
Every turntable has a compliant suspension that is intended to function as a mechanical filter, isolating the platter and stylus from trouble some external vibrations. Theoretically the ideal approach is to mount the platter and tone arm on a softly-sprung floating subchassis so as to isolate the platter and stylus from the motor, base, and dust cover as well as from the outside world. But that makes it wobbly, and it can even lead to groove jumping if you have a springy floor.
In nearly all direct drives and in budget-priced belt drives, the usual practice is to fasten the platter and turntable mechanism solidly to a dense base and then to support the entire system on compliant feet consisting of springs encased in rubber. In principle this is less effective than a floating subchassis, since the natural resonant frequency of the feet is typically around 15 Hz in stead of the 4 Hz of a floating sub-chassis. This implies that a broader spectrum of vibrations will pass through the feet into the turntable yet some turntables made this way have been among the most vibration-resistant on the market.
There is a relatively simple way to evaluate the vibration resistance of any particular turntable. Set the amplifier's volume control slightly higher than normal, then place the stylus in the groove, turn off the turntable to stop the rotation of the record (in some models this can be done only by unplugging its power cord), and lightly tap the shelf the turntable is resting on. The weaker and briefer the resulting thud from the speakers, the better the turntable's resistance. (If you use this test to compare turntables, you really should use the same cartridge in each one, since the result is influenced by arm/cartridge resonance.) It may be an ear opener to try the test on your current turntable even if you have been totally satisfied with its performance. If you've owned it for quite a few years, try to listen with a more critical ear. You may be hearing things from your turntable that aren't on the records you play. And if you get a substantial boom from the speakers in this test (or, worse yet, a sustained roar), then you definitely need to install more compliant feet or an isolating sub-base beneath the turntable. Or maybe you just need to start shop ping for a better turntable.
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Also see: Tonearms (Jan. 1985)
Source: Stereo Review (USA magazine)