February 1960 Popular Electronics
[Table of Contents]People old and young enjoy waxing nostalgic about
and learning some of the history of early electronics. Popular Electronics was published from October 1954 through April 1985. All copyrights (if any) are hereby acknowledged.
Even in the year 2012,
there is a huge cadre of turntable aficionados out there whose players can only be taken from them by prying them
from their, cold, dead hands. Look at all the buying and selling of turntables that takes place on
if you don't believe me (the search I just did turned up more than 19,000 items). I remember in my U.S.
Air Force days, the only time ever lived in mass group quarters (no college dorms for me), guys were in heated
competition against each other over who could accumulate the most extensive and expensive hi-fi gear. A large
percentage in my barrackrs were mobile communications types, and they seemed to spend about as much time adjusting
their turntables, reel-to-reel tape decks, and receivers for ultimate performance. Entire beer and pizza parties
were centered around topics like balancing tone arms. As with most subjects where many "experts" debate, no two
could agree on the best method. Here's a little sage advice in case you're not a seasoned tone arm balancer.
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Inside the Hi-Fi Tone Arm
By Joseph Marshall
you think the tone arm of a high-fidelity system can't seriously affect a record's sound quality, you are wrong.
Actually, a poorly designed or improperly adjusted arm can do more damage to the sound quality of a record than
perhaps any other part of the system. And with the advent of stereo, the demands imposed upon the tone arm have
become much greater than they were in the old mono days. Tracking Error
. When a master
record is cut, the cutting head is moved across the record by a lathe screw and is always at a 90° angle to the
radius of the record. Ideally, in playing back the record, this relationship should be maintained. It would be
possible to do so if the pickup traveled along a radial track running from the center of the record to some point
outside the turntable.
But this solution would raise another serious problem: the stylus would have to drag the cartridge along some
kind of rod or rail. It is very difficult to design a method to accomplish this end without putting an excessive
load on the stylus. One or two acceptable radial arms have been produced, but most manufacturers find it more
satisfactory to pivot the arm outside the turntable.
Clearly, an arm that is pivoted outside the record cannot move along the straight line of the radius. It will
necessarily trace a curve. There may be one or two points at which the arm precisely reproduces the angle of the
cutting head, but at all other points it will depart from the original cutting angle to some extent. This angular
deviation is called "tracking error" and is illustrated in Fig. 1 on page 48. Tracking error can be considerable.
For example, if you were to mount a straight 8" arm so that the point of the stylus passed over the turntable
spindle, the error would vary from about 22° at the outermost groove to 7 1/2° on the innermost groove. The
resulting harmonic distortion would run well over 5% and (at 22° tracking error) the maximum stereo separation on
a stereo disc would be limited to only 30 db - not taking into consideration the separation losses in the
cartridge and amplifier. To meet high-fidelity standards, tracking error should be no more than 1.4° on the
innermost grooves and 4° on the outermost grooves. At first consideration it would seem impossible to keep
tracking error this low, but ways have been found to do it.
For one thing, the longer the arm, the
shallower the arc it will trace becomes and the closer it will come to the straight line of a radius. If we could
use an arm several feet long, the arc could be close enough to a straight line to provide a very small tracking
error. But an arm longer than 16 to 18 inches is usually out of the question, and even these lengths are longer
than is convenient for most home applications.
Fig. 1. Tracking error occurs in the area shaded with the diagonal lines.
. A more practical way of reducing tracking error takes into account the fact that records are
recorded only on the outer three or four inches, so there is no point in worrying about tracking the inner two
inches. If we mount the arm a little farther out, it will under hang the turntable spindle and the stylus will
come short of the center of the record. This will reduce tracking error, especially on the innermost grooves where
it is most serious. The effects of arm length and underhang on tracking error are indicated in Fig. 2.
Another way of improving tracking is shown in Fig. 3. The lower arc is the path of a straight 8" arm with no
underhang or overhang. The solid lines indicate the actual path of the arm and stylus, while the broken lines
indicate the path the arm would have to take to provide perfect tracking. Note that on the outermost groove the
tracking error is about 22°. Now suppose that we offset the head of the same arm by about 22°. The angle of the
arm at the point where the needle rides the outer groove would be exactly right. As the arm moved inward, however,
an error would begin to occur. At the halfway point, there would be an error of about 8° instead of the 14° we had
with the straight arm. Beyond this point, things would get worse and the error would be greater than before.
However, we can correct this situation by moving the arm closer to the turntable so that the arm overhangs the
spindle. This maneuver will have the effect of reducing the offset of the arm at the inner grooves while
maintaining it at the outer grooves. By choosing optimum values of offset angle and amount of overhang, we can
achieve a tremendous improvement in tracking. The upper arc in Fig. 3 is for an 8" arm with an offset of 28°.
There are now two points where there is no tracking error at all, and the error is 2° or under at all other
Offsetting a longer arm permits even better tracking. The longer arm requires less offset and a
smaller overhang. Practically all arms have an offset, although sometimes the offset is not obvious. Thus,
although the Pickering 190D and the Shure "Studio Dynetic" arms appear to be straight, they actually provide
offsets by the way the pickup is oriented in its mounting. By combining these various ways of improving tracking,
modern tone arms achieve tracking errors of 1° maximum and thus prevent degradation of fidelity and stereo
Fig. 2. Effects of various amounts of underhang and differing arm lengths on the amount of tracking error.
. Assuming the arm has low tracking error, its next problem is to move freely up and down
and from side to side. Unrestricted lateral movement is necessary because the stylus has to pull the arm across
the record. Vertical freedom is necessary because the arm must be able to track warped records.
two general approaches in designing a tone arm that will permit these two types of motion. One is to use the same
pivot point for both the vertical and the lateral movement. In this design, the entire arm moves up and down when
it tracks a warped record. If something weren't done to prevent it, this would place the entire weight of the arm
on the stylus.
(A) COUNTERWEIGHT BALANCES WEIGHT OF ARM
(B) TENSION OF SPRING BALANCES ARM
(C) COUNTERWEIGHT PROVIDES BALANCE AND SMALL SPRING ADJUSTS STYLUS PRESSURE
(D) SPRING PULLS UP
HINGED SECTION OF ARM TO BALANCE DOWNWARD PULL OF CARTRIDGE
Weight is nothing more than the force of
gravity working on the mass of an object. If the force of gravity is counteracted by another force, weight can be
reduced, or in effect, eliminated at a given point in space. One way to do this is to balance one weight with an
equal weight or counterweight. We can extend the arm on the other side of the pivot point, install a counterweight
on it, and adjust it to achieve a near balance.
Actually we do not want an exact balance. The stylus must
be held in place in the groove by a light pressure. So we adjust the counterweight to provide a stylus pressure of
from 1 to 8 grams. A mass so nearly balanced becomes very light and easy to lift - as those of us know who have
used a see-saw with someone else of nearly equal weight. The counterbalance approach to tone-arm design is shown
in Fig. 4 (A). The Grado, Pickering, and Rek-O-Kut arms are examples of this approach.
Another way of
applying a counteracting force is by means of a spring. See Fig. 4(B). Here the pull of the spring is adjusted
until it almost - but not quite - equals the weight of the arm. The difference provides the needed stylus
pressure. But the trouble with this arrangement is that a spring is not a linear source of force. It exerts less
force as it compresses and therefore causes the arm to become, in effect, heavier as it is raised. For this
reason, spring balance is employed only with arms in changers and manual players where the use of a counterweight
balance is not feasible because of various mechanical considerations.
However, several counterweight-balanced arms use a small spring for fine adjustments of stylus pressure.
See Fig. 4 (C). In this design, the arm is first balanced with the counterweight and then the spring tension is
increased to provide the proper stylus pressure. Note that the spring pulls the arm toward the record and thus
tends to hold the stylus in place. Such arms have a high degree of stability and will operate satisfactorily even
if the turntable is not perfectly level. Some of the newest arms - the ESL, the Audio Empire, and the Dynaco -
employ the combined counterweight and spring-balance system.
The second approach to achieving free tone -
arm movement is to use two pivot points: one at the mounting point for lateral movement, and one somewhere between
this point and the cartridge for vertical movement. This technique is shown in Fig. 4(D). Since only the end of
the arm moves up and down, the amount of mass we have to deal with is comparatively small, and a simple
spring-balance system can serve nicely. In most cases, little more than the weight of the cartridge has to be
moved. The Gray SAK-12 and Fairchild 280A arms are examples of this type of design. However, in the Shure "Studio
Dynetic" arm, a small counterweight on a threaded rod balances the weight of the cartridge; this arm achieves high
stability at stylus pressures as low as 1 gram.
Whatever pivot arrangement is used, friction in the pivots
should be as low as possible. Many types of bearings are used in tone arms, and while each has its special
advantages, it seems that, with proper design, various types can produce equivalent performance. The simple thrust
bearing is used by Audax, Fairchild, Grado, Gray (lateral), Shure, and Weathers. Ball bearings are used by Audio
Empire, ESL, Garrard, London-Scott, and Rek-O-Kut. The Dynaco features gimbal pivots in both bearings, and
Pickering and StrombergCarlson use a single needle-point suspension. In addition, several arms employ
Fig. 3. Comparison of the amount of tracking error from an 8" straight arm with
no overhang or underhang and
an 8" offset arm with overhang.
Fig. 4. Four ways to counteract the weight of the tone arm and
. At first glance, viscous damping seems to violate
the principle of keeping friction low. A viscous-damped arm opposes any rapid and sharp motion such as being
dropped onto the record acidentally. -But at the speed at which the arm customarily travels over a record, the
friction of a viscous-damped arm is insignificant.
Viscous damping has several advantages. First, it
produces a high degree of stability even at very low stylus pressures; the Weathers arm is stable with only one
gram of pressure. Secondly, it provides a high degree of damping against arm resonance, another serious problem in
arm design. Some degree of viscous damping is employed in one or both pivots of the Gray, Weathers,
Stromberg-Carlson, and LondonScott arms.
Resonance of the arm at any frequency within the audio range
must be avoided. The simplest way to minimize the possibility of resonance is to make the arm out of a material
which is not prone to vibration in the audio range - such as wood. Both Weathers and Grado make their arms out of
wood. When other materials are used, all the physical characteristics of the arm such as shape, cross section,
length, and weight - must be carefully coordinated to keep the arm from resonating above 20 cps. A counterweight
helps in solving this problem because its mass tends to damp resonance.
It is highly desirable, especially
with stereo records, to reduce stylus pressure to the least amount necessary for good tracking. One way of
achieving this reduction is to design the arm specifically for the cartridge and tailor the characteristics of the
arm to complement the cartridge. Examples of such integrated combinations are the Weathers, the Shure "Studio
Dynetic," the Dynaco, and the London-Scott.
Though modern tone arms differ in many respects in their
design approach to the problem of providing good performance, they have all been specifically developed to meet
the highly critical demands of today's records and provide excellent results when they are properly installed.