Left Border Content - RF Cafe
Copyright: 1996 - 2024
BSEE - KB3UON
RF Cafe began life in 1996 as "RF Tools" in an AOL screen name web space totaling
2 MB. Its primary purpose was to provide me with ready access to commonly needed
formulas and reference material while performing my work as an RF system and circuit
design engineer. The World Wide Web (Internet) was largely an unknown entity at
the time and bandwidth was a scarce commodity. Dial-up modems blazed along at 14.4 kbps
while typing up your telephone line, and a nice lady's voice announced "You've Got
Mail" when a new message arrived...
All trademarks, copyrights, patents, and other rights of ownership to images
and text used on the RF Cafe website are hereby acknowledged.
My Hobby Website:
Sub-Header - RF Cafe
Inside the Hi-Fi Tone Arm
1960 Popular Electronics
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
eBay 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.
February 1960 Popular Electronics
Table of Contents
Wax nostalgic about and learn from the history of early electronics. See articles
published October 1954 - April 1985. All copyrights are hereby acknowledged.
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
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 inner-most
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
Decreasing Tracking Error. 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.
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.
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.
Free Movement. 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.
There are 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.
BALANCES WEIGHT OF ARM
(B) TENSION OF SPRING BALANCES
(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.
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
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 counter-weight and
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 Stromberg-Carlson
use a single needle-point suspension. In addition, several arms
employ viscous-damped bearings.
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 cartridge.
Viscous Damping. 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 London-Scott 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.
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.
Footer - RF Cafe
Right Border Content - RF Cafe