TURNTABLES: The Isolation System

An isolation system may provide some protection against such gross vibrations as passing footfalls and the like. But its most important job is to isolate the record and cartridge from the tiny microsonic vibrations emanating from the motor, bearing, stylus/groove interface, and environment.

The suspension acts as a filter that allows very low frequencies (i.e., subsonic frequencies, below the audible range) to get through, but will not pass frequencies higher than its tuned resonance. Any micro sonic information that’s in the audible band, and so would directly interfere with the music, is therefore filtered out. The lower the frequency that the suspension is tuned to, the less mechanical energy is allowed through and the cleaner your music.

With the cartridge effectively a microsonic detector, it is essential to prevent spurious sonic resonances from being picked up along with the music signal. Just consider the minute scale of the stylus and the record groove—with groove modulations as small as a wavelength of light (a millionth of an inch), the tiniest vibrations imparted to the platter or arm can contaminate and obscure musical information. It’s the same kind of thing as preparing tomato sauce in an aluminum pot—you do not expect to single out the taste of the aluminum specifically, but the sauce’s flavor will nonetheless definitely be affected by the pot.

And while any one of the many sources of distortion, taken alone, may seem insignificant, their accumulation causes significant muddying of the music, greatly lessening the pleasure of listening and leaving you with nothing more than a canned performance.

A stethoscope can show you just how much microvibration your table must deal with (stethoscopes are expensive, so try to borrow one from a friendly local health-care professional or termite exterminator). With the motor both off and turned on, listen to the table support, the platter, the chassis. Do this at varying hours of the day and night so you can hear the changes in resonances due to traffic and other noises. You’ll be amazed to discover the amount of noise that travels up through the floor and walls and through the air to rattle the tiny stylus as it tracks that speeding, twisting, music-packed groove. Then play some music with the volume turned up and listen for airborne vibration.

This airborne or acoustic vibration comes mainly from the system’s own speakers (acoustic feedback). It masks important low-level information and detail that gives the music much of its “realness.” That’s why the table should always be kept out of a direct line with the speakers. Lower-frequency drivers are omnidirectional, so placing the table right behind the speakers is not a good solution either.

When severe, acoustic feedback has been quaintly termed howl- around, because it can sound like a storm wind howling around the house. It is now largely a thing of the past, however, unknown today with any table of the quality of an AR ES-1 or better. As a quick check for howlaround, flip on your system, very gently place the tonearm down on the record without the platter turning, then very slowly turn the volume up all the way (and be ready to turn it down real fast if howlaround does occur). On a good table, you should get none. If you do get any, there’s something very wrong with the table.

Any comprehensive suspension that helps block mechanical and airborne resonances is better than none—it may be as basic as special compliant rubber feet, or as elaborate as a sprung subchassis that isolates the table from both external and internally generated sources of resonance.

Tables that have minimal or no built-in suspension of their own are more susceptible to resonances and vibrations, so the surface they sit on must be carefully chosen for absolute stability and protection against footfalls and other gross shocks. Whenever possible, it’s always best to use a specially designed support like the Arcici Iron Cloud, secured to a load-bearing (usually external) wall.

When compliant feet provide the sole suspension, then only the plinth is being isolated from mechanical and, to a more limited extent,’ acoustic vibrations. Motor vibrations are filtered from the platter by only a belt at most. This method is commonly used on cheap belt-drive tables and on almost all direct-drive tables. The standard mass-fi compliant feet of coil springs encased in rubber should be avoided, including the handsomely packaged after-market sets. They make lousy acoustic isolators—in fact, if you hit them and listen closely, you can easily hear the springs inside going pwang, pwang—BAD! Replace spring feet with Tiptoes or pucks of Sorbothane. Never use these spring-loaded feet in combination with a sprung subchassis because the carefully tuned frequency of the subchassis will be confounded.

A sprung subchassis is by far the most widely employed means on good tables of isolating the platter from vibrations. (This method is never found on mass-fi DD tables.) On good belt-drive tables, the platter and tonearm are on a floating subchassis mounted on specially tuned springs that isolate platter and stylus not only from vibrations caused by the table’s motor but also from the environment.

A way of roughly judging isolation effectiveness is to play a record at your highest NORMAL listening volume. Then, without changing the volume, turn off the motor and GENTLY place the stylus on the motionless record. Tip with your fingernail on the plinth and the support surface the table sits on. The quieter the thump you hear from the speakers, the better the table’s isolation from vibrations.

One hassle of a sprung subchassis is that it requires the table to be correctly “set up” by tuning the springs for both levelness and correct compliance or tension. The springs, drive belt, and tonearm cable are all that connect the platter and subchassis to the plinth (and environment), and consequently exert a major effect on the table’s sound. Non- sprung chassis are far easier to set up but, with very few exceptions (such as the WT and Elite Rock), a sprung subchassis design is the wisest way to go.

There are a number of different sprung-subchassis designs. The first rudimentary versions of this design were disasters because, while the platter was suspended, the arm remained rigidly fixed to the plinth! With the arm going one way, the platter another, severe distortion and even groove jumping resulted. This damaged a lot of records and styli, let alone paining the ear.

Most subchassis designs now derive directly from the milestone three-point system developed by Edgar Vilichur for Acoustic Research, circa 1960, and then refined in Britain by Linn and later by George Merrill in the United States. In this design, the subchassis is suspended from three or sometimes four springs secured to the plinth. Other de signs, like the VPI, support the subchassis from underneath on springs. This method of “support” rather than “hanging” can allow greater lateral or side-to-side movement, which must be corrected for. A spring, when compressed to support a weight, will tend to bend over sideways unless compressed very uniformly. A spring extended by a hanging weight has much better stability and naturally reacts up and down, rather than from side to side.

However, since good sound is always the result of tasteful, work able compromises, there’s no single “best way” to design a sprung subchassis. The success of any particular suspension seems to have more to do with the quality of construction, and the intelligence with which each ingredient is integrated into the overall table design, than with its ingenuity in the abstract.

For example, rather than springs, the Well Tempered Table uses special neoprene cushions specifically designed and tuned to filter out everything above 15 cycles. Bill Firebaugh of the Well Tempered Lab points out that if you tap the plinth of any sprung subchassis, you can hear the springs “ringing”—he believes (and he’s not alone in this) any spring suspension unavoidably imparts a “ring” to the music. Sal Demicco, for one, emphatically concurs—he’s a firm believer in unsprung tables like the Technics SP-10. Springs are- also inherently unstable, being subject to vertical and lateral movement, and this instability can permeate and distort the music—unless handled just right.

Peter Moncrieff reports that Japanese high-end table designs (which don’t see the light of day here) try to minimize the effects of external shock not through springs but through mass. The Japanese too believe springs will color the music by their inevitable movement. Moncrieff reports that the dedicated Japanese audiophile will cut a hole in the floor of the house, sink a huge concrete block into the ground below (without letting it contact the walls of the hole), and then mount the table on this concrete block.

This mass-loading approach to quarantining the record and cartridge from the environment exploits the fact that high mass acts like a sonic sponge to dampen and actually absorb resonances. It is used over here by, among others, Harry Weisfeld of VPI.

The highly respected English table, the Pink Triangle, employs the opposite philosophy. The designers have struggled over the years to come up with an incredibly lightweight but highly rigid design using Aerolam, an aluminum honeycomb-structured material. These proponents of low-mass design believe that the less mass is used, the less material there is to resonate, and also the less material there is to store resonant energy. Mass loading and low mass are two opposing but equally valid approaches to controlling resonances.

Another important technique for minimizing resonances is the closed- loop or mechanical-grounding-loop design, employed on all good tables with or without a suspension. In effect, it is also an extension of the mass-loading approach. Visualize a loop extending from the stylus up to the cantilever, into the cartridge, through the headshell and along the tonearm to the arm’s bearings, down the arm pillar into the suspension structure, then back up into the platter hearing, up the spindle shaft, into the platter, into the platter mat, and finally to the record, which contacts the stylus and closes the loop.

If all links in this loop are rigidly coupled, then the mass of the entire turntable system is available to each link. Each link of that loop then serves as a sink or “mechanical ground” to conduct and dissipate resonances into the total mass of the table before they reach the stylus and groove. Any flaws in this path can result in spurious additions or subtractions to the music from non-music-signal vibrations. It is therefore essential that the connections—between cartridge and headshell; head- shell and arm; arm, arm board, and suspension—all be solid and tight.

George Merrill takes the mechanical grounding loop one step further by eliminating some of the linkages altogether. He considers what he terms arm release energy to be such a major source of sonic problems that he designs his table to have no separate tonearm board. Instead, the tonearm is secured directly to the subchassis. (If you want to change arms, he’ll happily trade you the old subchassis for a replacement with the new armhole already precision-drilled.)

Equipment designers have to juggle many complicated choices and design considerations. The proof of success is not in the theory but in the excellence or mediocrity of the sound. Good sound is a result of refinements on an already good concept.