High Fidelity Phonograph Cartridge - Technical Seminar
This is a copy of a 1978 technical seminar on phonograph cartridges.
Table of Contents
Introduction - "Integrated System Design: A New Concept in Phonograph Cartridges" by J. H. Kogen
The Stylus Tip and Record Groove - "The Stylus Tip and Record Groove-The First Link in the Playback Chain" by B. W. Jakobs and S. A. Mastricola
Getting the Signal From Tip-To-Terminals by F. J. Karlov
Design Considerations of the V15 Type IV Phonograph Cartridge by L. R. Happ
The Hyperelliptical Tip by B. W. Jakobs and S. A. Mastricola
Phono Arm Damping Revisited by C. R. Anderson
Charges on The Record - "Charges on the Record--A Study of Static Electricity on Phonograph Records" by C. R. Anderson
Questions and Answers
J. H. Kogen
One of the most refreshing aspects of research in high fidelity reproduction is the continuing opportunity to discover problems and provide solutions. The search for perfection provides an unending challenge to the scientific researcher to discover the sources of imperfection. Once the cause has been discovered, the engineer is then challenged to find a solution.
This paper will tell the story of a series of research programs and the identification and evaluation of several problems associated with phonograph reproduction. We will also outline the approach to a satisfactory solution of those problems.
Before starting a detailed discussion, we should explain the title of the article. In performing our research the sources of several playback imperfections were discovered. Solutions to each of those imperfections were proposed, either as independent accessories or as appendages to be attached to the tone arm. While such solutions can be useful in helping correct individual deficiencies, a far better approach is to provide a single design which solves all the identified problems in a mutually compatible, rather than independent, manner. The single design integrates the solution to several problems into one system. An integrated solution can have the advantage of both simplicity and compactness. More importantly, an integrated design requires that the solution be worked out and tested as a system, which can be measured and optimized in the laboratory. Consideration and resolution of all of the problems within a unified design offers significant potential for a better solution.
The development of the Shure V15 series of phonograph cartridges provides a useful background setting for the research and product design concepts which will be described later in this paper. It is particularly interesting to note the progress of our objectives over the years and the changes in cartridge design needed to satisfy those objectives.
The original V15 introduced in 1964, featured the biradial elliptical tip and emphasized optimum vertical tracking angle. At that time there was considerable emphasis and discussion in the technical journals and within the professional societies on the subjects of tracking and tracing distortion. Reduction of tracing distortion required small playing radii. The smallest practical spherical tip was .5 mil radius. Introduction of the elliptical tip allowed reduction of the playing radius .2 to .3 mils. A reduction of distortion caused by vertical tracking angle error vas accomplished by providing a playing angle equal to the cutting angle found on commercial discs.
The V15 had a response peak of about 5 dB at 15,000 Hz. Trackability at 400 Hz was on the order of 15 centimeters per second and 10,000 Hz trackability was about 10 centimeters per second, both at 3/4 grams tracking force. Emphasis was placed on playing at low tracking force, 3/4 to 1-1/2 grams while maintaining excellent separation between channels throughout the audio spectrum.
The V15 Type II Cartridge introduced in 1967 resulted from a detailed study of the problem of mistracking. In trying to sort out the relative significance of various forms of distortion, it was discovered that mistracking was a far more significant cause of poor sound reproduction than the tracing and tracking (as related to vertical tracking angle) distortion which had been emphasized earlier. Thus, the prevention of mistracking loomed as a much more urgent requirement than any other problem at that time.
One obvious solution to the tracking problem was to increase the tracking force. Extensive life testing showed that praying at low tracking force has significant advantages in extending the life of both the stylus and the record, provided that no mistracking occurs. The problem that required resolution then, as it does now, is that of offering sufficient trackability throughout the frequency spectrum found on the phonograph record at low tracking force, preferably below 1-1/2 grams.
Through the use of extensive analog computer studies, it was found that a phonograph cartridge design suitable for tracking most of the high level modulation found on phonograph records of that day was feasible. The solution required a stylus much smaller and lighter than those in current use. A design was worked out using a beryllium stiffening rod with a very thin wall aluminum shank and the V15 Type II was the result.
Resolution of the problem of tracking most phonograph records made it possible to recognize a number of sources of playback imperfection which had previously been overshadowed by the more serious mistracking problem. One of the remaining problems was that of nonflat frequency response. Another was a series of difficulties caused by warped records. An extensive study of record warps resulted in an explanation of the need for optimizing cartridge compliance. The studies of record warp as well as additional evaluation of then current recordings reemphasized the need for improving trackability across the total frequency spectrum found on records.
The result of that research was the introduction of the V15 Type III in 1973. That cartridge has a flat frequency response, plays at a tracking force of 3/4 to 1-1/4 grams, employs an optimized stylus compliance, and continues other advantages of the earlier V15 series.
Even before the introduction of the V15 Type III, research was continuing into additional problems of disc reproduction. .We will briefly describe some of those problems in this section of the present article. Subsequent articles will describe in greater detail the results of the research and the product development which resolved those problems.
Stylus/Tone Arm Resonance
The paper by Happ and Karlov provides measured data of the amplitude and frequency of record warps. That paper also describes a method of selecting cartridge compliance to minimize the difficulties caused by record warp with conventional tone arm/cartridge systems. While the Happ/Karlov analysis suggests a means of optimizing conventional systems, it does not offer an ideal solution--that is, one that completely eliminates the effect of low-frequency stylus/tone arm resonance. That resonance causes at least three significant problems in high-fidelity reproduction.
One major problem caused by the excitation of the stylus/tone arm resonance is that of mistracking. Assume for example, that a tone arm has a low-frequency resonance of 5 Hz, a frequency below the range of human hearing. If that resonance should be excited by a warped record, for example, the arm will move vertically as shown in Figure 1. The motion effectively increases and decreases the tracking force in an oscillating fashion at the rate of 5 cycles per second. At the points of reduced tracking force, mistracking is more likely to occur than would be the case if no resonance existed. At points of increased tracking force, record and tip wear are accelerated.
Figure 2 shows a second and concurrent difficulty that results from excitation of the stylus/tone arm resonance. Here we see the movement of the stylus along the length of the record groove. This results in a frequency modulation or wow of the program material which in this example would be at the rate of 5 Hz. Wow produced in this fashion can have a serious effect on sound quality.
A third problem which can result from the low-frequency stylus/tone arm resonance is the production of high-amplitude, low-frequency signals which can overload amplifiers. These signals may also produce low-frequency/high-amplitude motions in loudspeakers which will result in doppler distortion and possibly overload distortion in those speakers.
While we have discussed the effect of low-frequency stylus/tone arm resonance as a result of excitation from record warp, it has also been recognized that there are several other major causes of excitation of resonance. Included among the causes are structure-borne noise, positive acoustical/mechanical feedback and transient mechanical excitation from record modulation .
The measurement of electrostatic charges on records indicates several significant effects. These include the generation of electrical noise, the attraction of dust to the record, and an anticipated increase in record wear because of the higher tracking force that results from the attraction between cartridge and record.
Trackability and Low-Tracking Force
Studies indicate that trackability still loons as one of the major factors in phonograph reproduction. A modern cartridge must in any case avoid severe mistracking. Beyond that it must be recognized that there are many gradations iun the ability to track. We no longer think of tracking as a go/no-go phenomenon, but more in terms of how well that capability is achieved.
One might think of tracking at one extreme in terms of a very heavy, high mass stylus playing at a high tracking force. Trackability would in some way be achieved, but at the expense of significant distortion, serious record damage and stylus tip wear. Tracking at a lower force is essential to prevent such damage. Tracking properly is a further requirement; careful listening will indicate that some cartridges may seem to track high-amplitude passages on records, but when compared to a cartridge that tracks properly, a distinct difference in sound quality can be heard. The information developed in earlier studies is unchanged with regard to the need for trackability within the audio spectrum, with the most significant tracking problems still existing in the 1,000 to 15,000 Hz range.
Improving trackability is a complex matter which we have approached from both a theoretical and practical standpoint. Extensive tests, measurements, and analyses of the properties of materials have been made integrated with the search for the material shapes that lead to optimum performance. Our earlier theoretical studies assumed lumped parameters. Recent studies include distributed parameters and a detailed mathematical model that requires a large digital computer to perform the analysis. New designs are evaluated using the computer and are later perfected in the laboratory. One aspect of the analysis and development has been described by A. Groh of Shure in the AES paper, "The Dynamic Vibration Absorber Principle Applied to a High Quality Phonograph Pickup."
Detailed results of the research into the problems just outlined are described in two papers by R. Anderson entitled, "Phono Arm Damping Revisited," and "Charges on the Record--A Study of Static Electricity on Phonograph Records," and the paper, "The Stylus Tip and Record Groove--The First Link in the playback Chain," by B, Jakobs and S. Mastricola. The product design which resulted from those investigations is described in the paper, "Design Considerations of the V15 Type IV Phonograph Cartridge," by L. Happ.
Engineering Design Concepts
It was quite clear after reviewing the results of much research that the problems being investigated could not be solved with the conventional phono cartridge design. The solution required the addition of several devices which in the first consideration were thought of as appendages or accessory devices. Further study indicated that the design could be accomplished much more elegantly, and in so doing, the solution to one problem could be used to enhance the solution to another. This approach provided the addition of needed features to the phonograph cartridge and allowed us to integrate those features into the overall design. As a result we have decided to call this new concept in phonograph cartridges, an Integrated System Design.
Obviously, the perfection of an Integrated System Design is much more complex than the solution of each aspect considered by itself. In addition to resolving the individual problems, one is faced with the task of optimizing the combination in order to provide an ideal solution to all of the problems simultaneously.
Subsequent papers will provide details on the design which has evolved. The improvements that have been achieved are:
- Tone arm resonance is controlled to levels at which its effects are insignificant.
- Low-frequency damping is obtained without in any way deteriorating stylus performance. The need to compromise stylus dynamics in order to achieve low-frequency damping is eliminated.
- The design of the stylus system to provide optimum high-frequency damping is achieved without compromising low-frequency performance. The result is increased trackability and control across the entire audio spectrum.
- Changes in tracking force caused by electrostatic attraction between cartridge and disc are eliminated.
- Dust accumulation is reduced and dust removal is enhanced by elimination of electrostatic charges on the record surface.
- Noise is reduced by removal of both electrostatic charges and dust.
As stated earlier all of these features are provided in a mutually compatible manner. The design has been worked out with great care in the laboratory, and the evaluation of performance has been performed on large quantities of production cartridges. Optimization of the system has not been consigned haphazardly to the user; it has been accomplished by skilled engineers with highly sophisticated instruments capable of evaluating the performance in both an objective and subjective manner.
B. W. Jakobs and S. A. Mastricola
Function and Objective
Have you ever asked yourself "What is the function of that tiny pointed object at the end of the stylus cantilever?" Although the answer seems obvious, it is one of the basic questions that phonograph cartridge engineers must answer when designing that object, the stylus tip, if they are to optimize the performance of the entire phono cartridge.
There are other basic questions too. What constraints limit the design? How does this part interrelate with the other parts of the assembly? What aspects of performance does the stylus tip influence, and to what extent? Although these may appear to be obvious questions with simple answers, we have all too often seen evidence that these questions have been overlooked, or worse, that the answers have been simplistic, incomplete, and misleading. Let's examine the questions verve posed in general terms.
What, then, is the function of the stylus tip? Principally, the tip must provide the physical interface between the recorded format and the playback system (Figure 1). It must accurately transmit the information stored in the record groove to the moving assembly of the phonograph cartridge.
Why is it important that the tip perform its function well? Because any failure of the tip to do so is not correctable by the other components of the system. Any error introduced by the tip is transmitted to the electrical generating components of the stylus, and wi11 be carried on through the playback system. Conversely, the tip does not compensate or correct for errors introduced by other components of the cartridge. If the stylus assembly, excluding the tip, is of such a design that it cannot cope with a given signal, it may not allow the tip to do its intended job; i.e., the stylus will mistrack. The tip can do nothing to prevent it. Even if the tip is able to read the recorded information perfectly, it cannot correct distortion that is added by other parts in the system. An additional, very important consideration is that the stylus, while performing its function, must not cause abnormal noise. Record and tip wear must be kept to a minimum.
The function of the tip, as stated, leaves a reasonable degree of freedom in design; but, as in any design problem, there are constraints that must be considered--constraints that tend to restrict our design options. The record format is established and cannot be altered to suit our needs or make the problem easier to solve every time a new phono cartridge is designed.
Before we can begin to analyze tip designs, we must have knowledge of the record.
First, there are geometric constraints. A modern stereo "micro-groove" record has a typical cross section as follows (Figure 2):
- Approximately 76μ (.003") wide at the record surface.
- Approximately 5.1μ (.0002") bottom radius (maximum).
- 90º included angle.
But the groove is not a constant shape (unless it is unmodulated; i.e., silent). At any location along the groove, the cross section can be either wider or narrower (Figures 3 and 4). Recording industry standard stipulate that the width should not become narrower than 25.4μ (.001") at the record surface. The tip, therefore, should be designed to accommodate this worst case; i.e., it should be 25.4μ (.001" maximum) wide at the contact points with the groove. Tips that do not meet this requirement can cause serious playback problems; for example, playing the top corners of the groove, resulting in increased noise and distortion (Figure 2). The bottom of the groove is not pointed as is often depicted. It is, in fact, rounded. Again, the recording industry has restricted this radius to 5.1μ (.0002") maximum, but necessary precautions must be taken to prevent the tip from contacting the bottom of the groove. Unwanted noise will result if adequate clearance to the bottom of the groove is not maintained.
For other than spherical tips, another geometric constraint is that the entire contact area must not be tilted forward or backward with respect to the modulation (Figure 5). If a significant misalignment exists, one end of the contact region may reach the leading edge of the modulation before the other end. The result is as if the tip side radius has effectively increased, thus causing higher tracing distortion. Tips with long contact areas are more sensitive to this constraint because they cannot tolerate as much angular misalignment on a given signal as tips with shorter contact areas.
The stylus assembly must be able to accommodate a moderate amount of dust and lint. We could argue that the user has the responsibility of keeping his records and stylus clean, but realistically, we must acknowledge a potential problem and deal with it. A thread of lint trapped around the tip can gather small particles and lint like a broom. Before long, a ball of lint and dust surrounds the tip between the record and the cantilever (Figure 6). This, in itself, does not affect playback directly because the dust is not in the groove. But if the dust ball becomes large, it can lift the stylus upward and away from the groove walls. The result, as might be expected, is mistracking. By maintaining a sufficient distance between the cantilever and the record, we can minimize this problem. The dust ball then must become so large in order to disrupt tracking that it often falls off the tip before it becomes troublesome.
- The record material also imposes constraints. The tip should slide along the material without modifying, damaging, or destroying the signal; a formidable task when the material is as soft as a polyvinyl chloride record. Likewise, the tip should not be modified by the record. Thus, the tip must be designed to minimize wear on the record as well as on itself. These constraints, geometrical and material, apply to all possible tip designs. A new design does not imply that these constraints have changed or no longer apply.
Finally, we have a fabrication constraint. We must be able to manufacture the tip precisely, consistently, in reasonable quantities, and at a cost that represents top value and yields the best possible performance.
- First, there are geometric constraints. A modern stereo "micro-groove" record has a typical cross section as follows (Figure 2):
Currently Available Tip Geometries
Given the objective and constraints, what is the ideal tip design?
One approach is to copy the geometry that made the groove (Figure 7). But is a tip that is shaped exactly like the recording stylus the optimum geometry? We would like a contact radius as small as the edge of the recording stylus to accurately trace the modulation, but we don't want to damage the material, which a playback stylus of that shape would, do. After all, the functions of the recording stylus and the playback stylus are not the same. The former cuts the material to make a groove while the latter must repeatedly reproduce the path created by that groove without modifying the path. Thus, it does not necessarily follow that the ideal playback stylus has a shape identical to the recording stylus. In addition, we must be prepared to accommodate angular tolerance buildup on the order of 2º - 3º from both the tip and the groove and still achieve proper playback. This can be derived more effectively by a shape other than that of the recording stylus, which would not be tolerant of even small angular variations.
Other possible approaches to tip design are (Figure 8):
- Spherical: circular cross section.
Biradial: oval cross section.
The front profiles of an 18μ (.0007") radius spherical tip and a biradial tip of equal major radius are the same; both fulfill the requirements we have set forth. The advantage of the biradial is its smaller tracing radius yielding significantly lower tracing distortion, particularly on shorter wavelength modulation of high frequencies at inner record radii. Even in this regard, the biradial is not ideal. By virtue of the technique used to generate this shape, the contact radius is not constant along the contacting regions, but it increases slightly from the points of contact upward (Figure 9).
"Long Contact" Tips:
This name is assigned to a class of tips which have an elongated contact region. This type of geometry evolved in the development of the CD-4, four-channel system for the purpose of retrieving the carrier signal on "Quadradisc" records. This classification now includes Shibata, Pramanik, Quadrahedral, Hyperbolic, etc. (Figures 10 - 13).
The primary difference between these and the biradial tip is the front profile, but differences in this view of the tip imply nothing about the contact radius unless we know the details of how the particular tip was made. Thus, it is incorrect to assume that "long contact" necessarily means smaller tracing radii. In fact, all of the "long contact" tips named have approximately the same average tracing radii as the typical biradial--7.6μ to 8.9μ (.0003" to .00035").
In this discussion, we have concentrated on the contact area geometry, not the overall shape of the tip. Although the contact area geometry may be related to the overall shape of the stone, one should not assume that two tips which have the same overall appearance, such as two intersecting facets (Figure 14), will have the same geometry in the contact area. Furthermore, just because two facets may make a good tip, four facets do not necessarily make a better tip. If anything, additional facets only provide more opportunities for errors and tolerance accumulation as operations are added without assurance of an improved geometry or better control of key dimensions.
- One approach is to copy the geometry that made the groove (Figure 7). But is a tip that is shaped exactly like the recording stylus the optimum geometry? We would like a contact radius as small as the edge of the recording stylus to accurately trace the modulation, but we don't want to damage the material, which a playback stylus of that shape would, do. After all, the functions of the recording stylus and the playback stylus are not the same. The former cuts the material to make a groove while the latter must repeatedly reproduce the path created by that groove without modifying the path. Thus, it does not necessarily follow that the ideal playback stylus has a shape identical to the recording stylus. In addition, we must be prepared to accommodate angular tolerance buildup on the order of 2º - 3º from both the tip and the groove and still achieve proper playback. This can be derived more effectively by a shape other than that of the recording stylus, which would not be tolerant of even small angular variations.
Testing the Geometry
Thus far we have discussed considerations of tip design primarily from a geometrical standpoint. But how do we know which approach will provide the best performance? What evidence do we have regarding factors such as distortion and wear? Merely stating theories based solely on geometric considerations is inadequate. We must make performance measurements under carefully controlled conditions using many styli with various tip shapes. Unfortunately, the results are not allways as conclusive as we would like. Let's look at some of the difficulties encountered when we try to evaluate the tip by different measurement techniques.
A basic difficulty arises whenever these kinds of measurements are employed to evaluate a particular aspect of the phono playback system. Any distortion measurement includes all of the factors that may create distortion, not just the single parameter which is of interest to us (Figure 15). A single measurement provides no information as to the principal sources of distortion or their relative contributions. For example, the signal source, the record, has its own level of cut-in distortion. How does one separate that distortion from that created by the pickup? A single measurement under a given set of conditions is really not of much value unless it can be compared to another measurement made under identical conditions with only one parameter, such as tip shape, changed. If we want to determine how changing one parameter affects a given measurement, the only way is empirically, by changing only the parameter of interest and comparing the measurements. The absolute numbers are not as important as the difference between the numbers, the relative values. To do this with the stylus tip is difficult because we have no easy way of changing only the tip in a stylus with assurance that we have not also modified the other parts of the stylus. The best we can do is to take measurements on a large quantity of styli built as much alike as possible, some with one kind of tip and others with another kind. The proof of a difference between two kinds of tips is evidenced by different trends in the data for each group. A large number of styli must be measured so that methods of statistical analysis are applicable.
Tracing distortion is a general term used to describe distortion that arises because the tip and recording stylus do not have exactly the same shape. There is no method of measuring tracing distortion directly; but intermodulation and harmonic distortion can be measured. However, these measurements include distortion from all sources, such as tracing, indentation, distortion in the record, and many other factors in the playback process. If one kind of tip traces a signal more accurately than another kind, the measured distortion of the more accurate kind will be lower on the average.
As an example, we have tested spherical, biradial, and hyperbolic tips for second harmonic distortion in a laboratory prototype cartridge:
Conditions: Signal: 8 kHz Velocity: 5 cm/sec peak Record Radius: 5 inches Tracking Force: 1 .25 grams Measurement: Average 2nd harmonic distortion for both channels of several styli. Results: Spherical; 6.4% Biradial: 4.0% Hyperbolic: 4.0% Conclusions: Long contact tips with the same tracing radius as biradial tips will have the same tracing distortion on the average. This result corresponds to the theory of tracing distortion.
Another aspect of performance is surface noise. This noise is highly dependent on the particular record, master, stamper, material, and pressing techniques used to make the record and the number of times the record has been played. In the cartridge the mechanical impedance of the stylus assembly, the amount of indentation, the tracing radius of the tip, and its contact area are all possible factors affecting surface noise.
In studies of tip geometries, we were concerned specifically with the continuous "hiss" type of noise that is generated by the tip sliding along the groove walls. In these tests the wideband noise level of styli with biradial and hyperbolic tips was measured (Figure 16). Silent groove (unmodulated) records pressed from the same master and stamper were played, and a new record was used with each stylus. The same band on a record vas played in each test. Care was taken to assure that the frequency response was well matched among all styli, and the noise level of the measuring equipment was checked to be certain it was suitably low. These factors and others such as the tone arm, skating compensation, and tracking force were carefully controlled throughout the experiments. The following describes an example of these tests:
Conditions: Tone Arm: SME 3009 II Cartridge: Shure V15 Type III Tips: biradial and hyperbolic Tracking Force: 1.5 grams Record: silent groove Record Radius: 3.0" - 3.3" Results: Noise levels referenced to the output level of a 1 kHz signal at 5 cm/sec peak velocity are as follows: Biradial: -45 dB to -47 dB Hyperbolic: -45 dB to -47 dB Conclusions: These long contact tips had neither a tendency to increase nor decrease the level of surface noise compared to biradial tips.
Significantly lower surface noise on the same record under the same set of conditions may seen like a good thing but, in reality, may be an indication of greater indentation and possibly greater record wear. We theorize that the tip cannot differentiate between intended modulation and unintended variances on the surface of the groove wall. It tries to play both; but as the indentation increases, the tip smoothes or even "smears" the surface of the groove wall. By decreasing or eliminating small irregularities in the groove wall, the tip seemingly causes a reduction in noise. At greater indentation, however, this can cause premature breakdown of the groove wall surface.
Wear is another means by which we evaluate tip geometries. In the context of the tip-groove relationship, we may define wear as any modification to either the tip or the record groove as a result of their sliding contact with each other. Such modifications usually occur gradually as a result of repeated contact between the two surfaces. For convenience, we will divide this topic into two parts: wear on the tip and wear on the record. However, in reality, we know that wear is a process acting mutually on both parts and that it begins with the very first play and continues with every play thereafter.
All tips wear and eventually must be replaced. This is neither surprising nor unusual, but rather the expected result when two surfaces slide against each other. Various processes are used to describe wear such as adhesion, abrasion, pitting, etc. One process may dominate the manner in which parts wear, or a combination of processes may contribute to the wear simultaneously. In the particular case of wear on the tip, one theory points to abrasion and heat as the most likely candidates. Even on seemingly very clean records, minute particles in the groove present a gritty environment to the tip. Additionally, small, sharp grains of diamond may wear away from the tip and become deposited in the groove. The record, in effect, becomes a grinding wheel. In time, large flat regions are ground on the tip. Heat is probably formed by high tip velocities on the groove wall. This phenomenon is analogous to striking a match.
We have conducted many tests to help us estimate tip life and evaluate the effect of various parameters on tip wear. As always, the test conditions must be carefully controlled. The size of the tip, trackability or mechanical impedance of the stylus, the record material and recorded content, tracking force, skating compensation, the number of records to be played, the manner in which the records and tips are cleaned, and how frequently they are cleaned are only some of the parameters that must be considered when doing wear tests. The criteria for evaluating wear must also be selected. At what point can we say that a tip has reached the end of its life? The answer to the question will be different depending on the criteria chosen. Furthermore, any given criteria for determining tip life are not applicable to all playback situations and all listeners.
In spite of these difficulties, the tests we have conducted over the years have yielded some important results. For example, our data shows that diamond. has a significantly longer life than sapphire (Figure 17). Another test revealed the increased abrasive action of playing a stylus continuously on the same record as opposed to a limited number of plays (20) on several records. Tracking force is also a major factor of tip life, regardless of the shape (Figure 18). The results of several tests show a trend toward a faster rate of wear as tracking force increases, particularly above 1 1/2 grams tracking force.
Included among our tip wear tests are studies of the relative tip life of long contact tips versus conventional biradial tips. These tests were conducted on commercial changers of the same model, and each stylus played its own record continuously, tracking at 2.0 grams force. Photographs of the contact regions at 600x magnification were used to evaluate the rate of wear. A comparison of styli with long contact tips and styli with biradial tips revealed no significant difference in the rate of wear between the two groups as a whole, although there were differences between individual tips.
As indicated previously, wear occurs on the record as well as the tip, but the mechanism by which the record groove is worn is by no means less complex than the process of tip wear. In the case of record wear, several processes may be at work simultaneously. Hard foreign particles such as dust, grit, or grains of diamond from the tip may become trapped between the rubbing surfaces causing abrasive wear characterized by gouging or scratching. Another mechanism is called adhesion. It is this process that is used to explain friction. Friction, it is theorized, is due largely to the force required to shear localized welded junctions created by intense pressure at the infinitesimal individual points of contact throughout the tip-groove junction. We know friction is present because a skating force is generated during playback. Since the record material is not nearly as strong as the tip, these junctions are broken by tearing the vinyl rather than the diamond. Wear resulting from adhesion is often characterized by scuffing and scoring. Pitting is another form of wear. It usually results from repetitive subsurface stresses that exceed the endurance of the material. Our tests have revealed evidence of this type of wear in as few as ten plays on poor quality records, or as many as several hundred plays on good quality vinyl.
Having established that record damage can take various forms, we must determine which types of wear are most objectionable, under what conditions they occur, what the symptoms of the various forms of wear are, and how certain factors like tracking force or tip geometry influence wear. Our study of this complex subject has continued for many years. Space does not permit a detailed discussion of all the investigations we have conducted, but a review of some examples of tests and the results will provide some insight to record wear.
Our tests have shown that inadequate trackability, that is to say high mechanical stylus impedance, is by far the largest contributor to premature record wear or damage. In no way can a tip, no matter how well it is designed, prevent this type of irreparable damage. Examination of properly tracked grooves at 300 times magnification and greater has revealed that some modification to the groove is always visually detectable, regardless of the cartridge or the tip. A slight depression or shallow "trough" resulting from permanent indentation of the groove wall takes form in the first few plays. For most recorded signals, that amount of indentation does not significantly modify the reproduced signal. Assuming proper tracking and good record material, the "trough" is seemingly little changed after 50 or even 100 plays. In fact, some small amount of indentation is desirable to smooth roughness or irregularities in the surface of the groove wall, thereby keeping the surface noise low. In designing a tip, however, we would like to know how different tip geometries affect the shape of the record modulation with increasing numbers of plays. To do this, we have made many distortion tests. Following are some typical test results:
Test 1: 2nd and 3rd harmonic distortion versus number of plays Cartridge: V15 Type III Tips: biradial and hyperbolic Signal: CBS STR-100, bands 3A and 3B, 1 kHz Tracking Force: 0.75 gram Number of Plays: 100 Results: 2nd harmonic with biradial: no significant change on either channel. 2nd harmonic with hyperbolic: no significant change on either channel. 3rd harmonic with biradial: decreases about 33% of original value. 3rd harmonic with hyperbolic: decreases about 40% of original value. Test 2: Same as Test 1 but at a tracking force of 1.5 grams. Results: 2nd harmonic with biradial: increases by an average of 20% from original value. 2nd harmonic with hyperbolic: decreases by an average of 40% from original value. 3rd harmonic with biradial: decreases by about 33% from original value. 3rd harmonic with hyperbolic: decreases by about 33% from original value. Test 3: Same as Test 1 except at 6 kHz. Tips: biradial and hyperbolic Signal: CBS STR-100, bands 3A and 3B, 6 kHz Tracking Force: 0.75 gram Results: 2nd harmonic with biradial: increases an average of 48% from original value. 2nd harmonic with hyperbolic: increases an average of 40% from original value. 3rd harmonic with biradial: increases an average of 40% from original value. 3rd harmonic with hyperbolic: decreases an average of 66% from original value.
Additional Notes: The changes in distortion values for styli from both groups exhibit decreases as well as increases. Decreases as great as 30% and increases of over 100% were measured.
Test 4: Same as Test 3 but at a tracking force of 1.5 grams. Results: 2nd harmonic with biradial: average of both channels increases about 66% from original values. 2nd harmonic with hyperbolic: average of both channels increases about 25% from original values. 3rd harmonic with biradial: both channels increase an average of 45%. 3rd harmonic with hyperbolic: both channels decrease an average of 30%.
Additional Notes: Average initial 3rd harmonic distortion was higher with hyperbolic tips than with biradials, but final average 3rd harmonic distortion is about 25% lower with hyperbolic tips. Conclusions: The results were highly variable among the tips with distortion levels sometimes increasing and sometimes decreasing. A decrease in distortion may be as much an indication of groove wear as an increase. The tip may hare modified the groove such that the distortion generated by record wear partially canceled distortion from other sources. No change in distortion indicates the least amount of record wear. 0n an average, the tests indicate a trend favoring the long contact tip, however, the benefit is highly dependent on the particular signal and the tracking force.
We have presented these examples to emphasize the variability of data and difficulty of obtaining conclusive results. Conclusions as to the quantitative effects on wear must be drawn with great care.
- Tip Wear
- Distortion Measurements
In this paper we have discussed several important aspects of tip design: function; design constraints; various geometries and performance tests including distortion, noise, and wear. Let's review these briefly.
- The tip has the function of providing a physical interface between the record and pickup. It must accurately translate the physical, stationary signal stored on the record into a dynamic signal that can be transmitted through the playback system. However, it cannot offset other deficiencies such as nonlinearities in the system and mistracking.
- All tips are subject to the same basic constraints. They must conform to the geometric constraints imposed by the record groove. The design must consider the material limitations of the tip and record as they pertain to wear. Problems of fabrication may also impose constraints on the design.
Carefully controlled tests are necessary to evaluate the tip performance. These tests must not be limited to one kind of measurement, but must include distortion, noise, and wear tests to fully evaluate performance. When conducting these tests, one must be conscious of the complexities that are involved and remember that results of a particular test may not apply in all situations. The number of styli tested must be large enough to yield statistically meaningful results.
Our tests have shown that biradial and long contact tips with the same tracing radius yield the same average distortion as expected from theory. These geometries will reproduce the same amount of surface noise provided that they have adequate clearance to the bottom of the groove. The results of wear tests have shown longer tip life at tracking forces below 1 1/2 grams. Record wear tests have revealed the importance of proper tracking and low mechanical stylus impedance to prevent groove damage. In other record wear tests, the results indicate some advantage to long contact tips, but the advantage is dependent on the signal and tracking force, However, low mechanical impedance; i.e., high trackability and low tracking force (below 1 1/2 grams) is by far more beneficial in achieving long record and tip life.
It should now be evident that there is no black magic in the design of a tip. The best approach is practical and straightforward with extensive tests that attempt to confirm or refute theories and evaluate tip performance.
Frank J. Karlov
In the previous presentation, the important role of the diamond tip was explored. The problems associated with the need for the tip geometry to keep from deteriorating the record groove surface, while accurately "reading" the information contained in the groove, was discussed in detail. In this presentation, we will look at the roles of the remaining elements of the phono cartridge and their ability to accurately and carefully translate the notion imparted to the diamond tip into an electrical signal suitable for, and worthy of, further processing by the phono preamplifier.
The Stylus Assembly
Before the motion of the tip can be converted to an electrical signal, that motion must be transmitted to the moving elements of the transducer. Accurate transmission of this notion is the primary function of the moving mechanical system, which is a vital part of every phono cartridge. Shown in Figure 1 is the stylus assembly of the V15 Type III. It is a simple appearing assembly made up of very few parts: the diamond tip, the shank tube, the dynamic control lever, the transducer element, the elastomer bearing, the locator, and the support wire. It would seem that its operation is quite obvious--that the record groove modulations move the tip, and the structure merely rotates as a lever about the pivot provided by the bearing, while the support wire has the dual role of providing additional support to the stylus in preventing collapse during extended play and locating the fore-and-aft position of the pivot. This is looking at the system from a static point of view. However, when one considers that this structure must perform dynamically over several decades of frequency from below audibility to beyond audibility, the understanding of its performance becomes less obvious. By far, most of the engineering effort expended in improving phono cartridge performance is in optimizing the stylus assembly for its many requirements in playback. Also the advances made over the years in phonograph reproduction, that took us from the days when 20 to 30 grams was the typical tracking force to today's 1-gram requirement, have been primarily the direct result of advances in the understanding and implementation of stylus designs.
Some of the significant parameters the engineer must deal with in optimizing the moving system are: the compliance and damping properties of the elastomer bearing and its geometry, the strength and mass-stiffness distributions of the shank and (in this example ) the beryllium dynamic control lever, and, of course, the geometry of the diamond tip. Also a compatible interface with the stationary portion of the transducer must be achieved. All of these factors and others must be considered, refined, and then combined into an integrated design that ultimately determines every performance specification of the cartridge. Intimately dependent upon the dynamic excellence of the moving system are the cartridge frequency response, signal level, interchannel crosstalk, the various forms of distortion, and the vital requirement for the tip to remain in contact with both groove walls--trackability.
Perspectives of Trackability
There seems to be a tendency for physical parameters, which are meaningful to the cartridge designer, to be equated to performance specifications. The equation is often a poor one. For example, there still exists the notion that compliance is a true measure of tracking ability. Not only is compliance not a constant value for a given cartridge design, it is but one of the main ingredients that determine low-frequency trackability, and too much compliance can actually diminish trackability at high frequencies. Perhaps the confusion is the result of a semantic problem. Trackability could well have been defined as the property of the stylus which allows it to "comply" with the urgings of the record groove. But compliance is a very precisely defined scientific concept.
Another area of misunderstanding is with regard to the idea that reducing the mass of the stylus guarantees an increase in high-frequency trackability. One of the main factors which contributes to high-frequency trackability is not the mass (or weight) of the stylus, but its equivalent or effective mass, which is the inertial effect of the stylus system referred to the tip and which is actually "felt" by the record groove. The mass distribution of the stylus is a much larger factor in determining trackability than its actual mass. A mass element located near the pivot end of the stylus may have negligible effect on trackability, while that same mass element near the tip end could virtually destroy trackability. In analyzing today's top-of-the-line cartridges, one finds, for example, that while cartridge "A" has almost twice the stylus mass of cartridge "B", it may well have about half the effective mass of "B".
While using simplified terms to describe performance expectations can be convenient, it can often be deceptive. The only way to provide meaningful trackability information to the audiophile is by reporting actual measurements of trackability using calibrated test records. Other methods can only be misleading.
Another area, where there seems to be a common misunderstanding is in the real need for "super" trackability. While everyone agrees that trackability is important, many feel that only a few of the more highly modulated records require it. This is probably because most mistracking occurs when playing transient signals and is of such short duration that it is not perceived and identified as mistracking. No "thud," "clunk," "crunch," or "sizzle," some terms often used in an attempt to describe the sound of mistracking, may be audible. The sound quality, however, may be destroyed as well as the record groove. Figure 2 shows an example of what happens to the groove when the signal is not tracked properly. One photo shows the condition of the groove after being played 50 times with a stylus that tracked the signal. The other photo shows the same portion of an identical record played with a stylus that mistracked one time. Both records were played by current "top" cartridges at 1 gram tracking force. Note the modification of the groove resulting from one playing while mistracking, and the insignificant evidence that the tracked groove has been played at all. One may say that the primary function of a phono pickup is to "dig out" the information from the groove. Here we see that, indeed, there has been some digging, but the information has not been retrieved nor can it ever be from this abused groove.
The Vibration Absorber
One of the compromises in designing the moving system of a phono cartridge has always been the conflicting requirements in achieving high trackability simultaneously in the low, mid, and high frequency regions. The demands upon the elastomer bearing are different for each part of the total spectrum. In the low-frequency region (below approximately 1,000 Hz) where recorded amplitudes are of main concern, the stiffness of the bearing limits trackability and, therefore, it should be highly complaint. To track all audio frequencies below about 4,000 Hz, the damping properties of the bearing should be low, because of high recorded velocities. Above 4,000 Hz, the ideal bearing would have a large amount of damping to enhance trackability as the stylus resonance frequency is approached. Additional bearing stiffness may also be desirable to raise the stylus resonance beyond the audio limit of 20 kHz. Also to be considered is the need to limit compliance for sub-audible or warp performance. It is apparent that because of these conflicting requirements the bearing cannot, be ideal over the entire spectrum.
During the development phases of our M24H Cartridge designed for CD-4 quad playback as well as conventional stereo, there were additional requirements on the bearing to provide frequency response, trackability, and channel separation control through the carrier frequencies. It was at that time that the principle of the dynamic vibration absorber was first applied to the phono cartridge system.
Figure 3 shows the initial embodiment of this approach. Note that a mass is attached to the end of the transducer magnet through a block of elastomer material, which has both the properties of compliance and damping. Above some preselected frequency, where additional damping is most beneficial, the mass element is such that its inertia prevents its significant motion. The elastomer is vigorously exercised and its damping properties utilized. This effectively provides the additional damping only at the high frequencies where needed and not at low and mid frequencies where it is not desired.
Further exploration of the dynamic vibration absorber principle showed that a simpler yet more functional arrangement was possible. Figure 4 shows a structure which eliminates the lumped mass and takes advantage of the mass inherent in the elastomer itself. This distributed parameter structure allowed us to achieve much enhanced control of the initially conflicting compliance and damping requirements over the frequency spectrum. Its incorporation into the M24H design resulted in outstanding carrier signal retrieval without record destruction while also performing as an excellent stereo cartridge.
The principle of the dynamic vibration absorber provides the phono cartridge designer with a new tool to deal with some of the conflicting requirements that can restrict the attainment of optimum performance. The excellence of the VI5 Type IV stylus design is the result of using the dynamic vibration absorber optimized for stereo performance.
So far we have been mainly concerned with the moving system of the phono cartridge--the vital portion of the cartridge responsible for insuring that every motion imparted to the tip by the record groove wall is accurately reproduced by the moving element of the transducer, in our example, the moving magnet. In Figure 4, note that the magnet is positioned within a pole piece structure, which is that portion of the stationary elements of the transducer charged with sensing the motion of the magnet and transporting the flux changes to and through the coils.
Figure 5 shows the actual pole piece assembly as it has been incorporated in both the V15 Type III and the M24H. Note in the upper view that the structure for each channel consists of a stack of fine laminations passing through its pickup coil. The lower view shows two such stacks and coils nested to form a square tunnel which can independently sense both the left and right channel motions of the magnet. This pole piece assembly is encapsulated within the body of the cartridge to insure the integrity of its geometry as well as immobilizing the fine coil wires and leads, thus preventing their breakage. This basic structure has proved to be successful in providing consistent flat frequency response, exceptional channel separation, and higher signal level capability because of its inherent efficiency. The V15 Type IV Cartridge design takes advantage of the performance benefits afforded by this unique magnetic structure.
We have looked at the three vital parts of the phono cartridge; the diamond tip, the mechanical stylus system, and the transducer system. The performance of the phono cartridge depends not only on the excellence of each of these elements, but also on the extent to which these elements are integrated into a unified design that retrieves all the music and leaves the record worthy to be played another day.
L. R. Happ
In March of 1973, before an audience similar to the one today, the V15 Type III was introduced. At that time, we presented a study made by Shure engineers that surveyed the range of frequencies and velocities found in the grooves of commercial phonograph records. The data from that study was compiled and graphed as a distribution of measured points shown on the right in Figure 1. The area these points cover represents the total spectrum of signals that challenge the phono cartridge tracking ability. The "hottest" signals are located along the upper edge, particularly at the high frequency end of the distribution, and represent the toughest signals for the cartridge to track.
Also at the Vl5 Type III Seminar, a new definition of trackability was introduced. “Trackability" was extended to read “The ability of the stylus to maintain contact with the record groove ... across the frequency spectrum found on records." This broader definition includes the requirement of tracking CD-4, four-channel records. In addition to record modulation, many other signals are present on records in the form of unwanted disturbances. Our study of record warp characteristics shows that the distribution of these disturbance signals extends into the subaudible region. This is shown shaded in Figure 1 at the left side of the illustration.
By including warp signals, we can refer not only to the trackability demands of the pickup, but also to that of the tone arm and cartridge “system.”
The V15 Type III in an SME arm has a system trackability curve as shown in Figure 2. At frequencies where the trackability curve passes beneath the top limit of the signal distribution, the potential for mistracking exists. On records which contain these peak signals, the VI5 Type III, which possesses the highest high-frequency trackability of any present-day phono cartridge, should be played at its maximum tracking force to ensure adequate tracking. The remainder of the system tracking curve shows a tracking margin, illustrated by the clearance between the system tracking curve and the shaded signal regions (representing the range of observed record velocities at any frequency). Having a large trackability margin over part of the frequency spectrum does not mean that the tracking requirement has been satisfied. The trackability margin should ideally extend across the entire frequency spectrum. In addition, increasing the trackability margin beyond the minimum is always beneficial; it means reduced groove indentation and less signal distortion, less record and tip wear, and reduced surface noise buildup.
The dip in the trackability graph is due to a resonance between the tone arm effective mass and cartridge compliance. Because of the relatively undamped resonance (typical of present-day cartridge/tone arm combinations), the graph shows reduced tracking margins in the warp signal region.
Thus, the system tracking curve suggests two regions where trackability improvements would be desirable. The first is in the mid- and high-frequency regions, and the second is in the cartridge/tone arm resonance region where only a limited margin exists.
Our goal in the VÌ5 Type IV was to achieve improvements in these regions without trading off any of the excellence achieved by the V15 Type III. We did not want to give up any low-frequency tracking to gain an increase in high-frequency tracking or an increase in the warp signal rejection, nor did we want to trade off a flat frequency response for the sake of improved high-frequency tracking. It is only through this approach that a real improvement in trackability can be achieved, not just a rearrangement within the present constraints.
Significant improvements in tracking ability were achieved as illustrated in Figure 3 (Note: trackability is plotted on a log scale).
Improvements were realized throughout the entire signal range, not just in one frequency region. The greatest improvements were achieved in the high-frequency audio range and in the subaudible warp signal region. In the low audio frequencies, below approximately 100 Hz, the performance is similar to the V15 Type III and, as the figure indicates, adequate for commercial records. The dramatic improvement in the warp signal region indicates a new, significant margin of protection against warp signals. The significance of this will be explained in an ensuing paper by R. Anderson titled, “Phono Arm Damping Revisited."
We will now break the total improvement up into its various parts and examine some of the design aspects of the cartridge.
New Shank and Magnet Assembly
In a moving magnet phono cartridge the shank and magnet assembly is the heart of the transducer, and the design must evolve through a careful optimization process. It is necessary to have objective performance criteria by which quality can be measured. These performance criteria include low equivalent mass, high resonance frequency and low resonance Q, high resistance to bending and fracture, and the proper geometrical consideration. Since many of these items lead to contradictory requirements, any one of these features cannot be maximized independent of the others, or a less than optimum design would result. All must be considered and evaluated in order of importance. The engineer must evaluate each of these technical design factors and weight them in concert with the needs of the audiophile.
At Shure, for the reasons previously stated, mechanical impedance (trackability), frequency response, and geometry are rated high in importance. In the design of the V15 Type IV shank, a study was made that mapped various aspects of the performance criteria. Through new computer techniques, many different shanks could be compared with respect to equivalent mass and impedance, flatness of response, stiffness, and physical geometry. Based on these results, many prototypes were constructed, measured, and subjected to extensive listening tests. The final design is called the "telescopic" shank and is shown in Figure 4.
The telescopic shank employs a precision outer reinforcing tube in intimate contact with the shank. To achieve this critically tight assembly, new processes and highly specialized tooling were developed. The magnet is of a new, high-energy material and is reduced in size and mass. By comparison, the V15 Type III uses a slightly larger diameter shank and an internal solid beryllium rod reinforcement. The net effect of these changes over the V15 Type III was to reduce the overall mass and equivalent mass of the stylus while maintaining the same overall geometry and bending strength with respect to the record input. In terms of the performance criteria, the contribution of the new shaft is to improve the high-frequency trackability, maintain the shank resonance beyond the audible frequency range, and improve the control of the resonance by reducing the mass the bearing must control.
New Computer Model for Shank Evaluation
Although a new shank design is relatively easy to illustrate, the development of the shank is not an easy process. The description of one aspect of the project, our new computer model, which was used in the development of the V15 Type IV, will give some insight into the extensive engineering efforts behind such a development. The computer model is not an electrical-mechanical analog as discussed in past literature. The model is a mathematical derivation of the dynamic system shon in Figure 5.
The model allows for mass and stiffness distributions that vary over the length of the shank; in fact, up to four separate shanks can be pieced together to form a composite system. The supports, shown in the diagram as simple springs, represent record and bearing impedances which are each complex parameters determined from theory and empirical data. When given a sinusoidal input from the record, the model can describe the stress or strain characteristics at any point within the system. Figure 6 illustrates the type of information the program can produce.
The figure shows the centerline of a simple tubular shank at an instant of time during vibration at its resonance frequency, This example of shank and bearing is uncontrolled due to the high mass of the shank and low damping in the bearing supports. Notice the transducing element has vibrated away from its intended pivot position. Figure 7 illustrates positions of the same stylus system at sequential instants of time. The series represents 1/4 of a full cycle at the resonance frequency. Note the mistracking of the input sine wave, the shank flexing, and, again, magnet motion away from the pivot point. Continuing in this type of analysis, using similar methods, we have created an animated dynamic representation of a stylus system.
The following filmstrip shows an example of this computer-drawn animation. In the example shown in the filmstrip, the shank has been made lighter and the bearing damping improved compared to the previous example (Figure 7). The frequency sweeps from 1 kHz to 50 kHz in approximately 3 kHz increments. Although this analysis shows the pivot position under control, the various resonances in the system are still uncontrolled and indicate probable mistracking, distortion, and groove damage. The V15 Type IV shank and bearing system employs significantly more damping and less mass than the example used in the film, and thus is very much under control compared to the filmed demonstration.
During the development of the V15 Type IV, over 50 different theoretical assemblies were simulated on the computer, and many were selected and built as experimental prototypes. Of these, the telescopic stylus shank was chosen as best suited and well-matched for the bearing and vibration absorber assembly. A paper describing the computer program will be presented in the near future.
New Bearing and Dynamic Vibration Absorber
Up to now, we have concentrated on the development of the stylus shank with references only to the bearing and damping system. However, both must be carefully integrated if an optimum design is to result. In addition to the constraints imposed by the stylus shank material, it is the bearing or properties of elastomers in general that impose many of the constraints when we strive to maximize trackability over the entire signal spectrum. For example, if we were to use a single type of material and reduce the hardness grade from high to low, we would observe that improvements in trackability at the low-frequency end generally are offset by reductions in the high-frequency tracking. This may produce no change in total trackability, only a redistribution of tracking performance. Therefore, in our design, if we are to make significant gains it Ís necessary to examine the very nature of these bearing material constraints to see how fixed they really are.
An investigation of material was undertaken to study the relationship of stiffness and damping of various elastomers over the subaudio and audio frequency spectrum. Figure 8 illustrates the measurement technique used in this investigation. Using what is referred to as a mechanical impedance transducer (Figure 8b), it is possible to discriminate between the forces within the material which are primarily "springlike” and those that most resemble a "dashpot" in character (Figure 8a). A small cube of each material was loaded into the transducer and measured under the same stress/strain conditions as would be expected in the phono cartridge application. By measuring each material over a wide range of frequencies, it is possible to generate the overall impedance characteristics of the material (Figure 8c). By further processing this information, we can obtain the dynamic stiffness and dynamic resistance characteristics of the materials (figure 8d). Thus, we can determine both the degree to which elastic materials become stiffer while damping diminishes with frequency, and an indication of which property of the material, its spring or damping nature, is most influenced by the stylus motion at each frequency.
Ideal material qualities can also be defined. For example, the material should be stiff in the subaudible region, compliant in the low- and mid-frequency region, and then stiff again in the very high-frequency region--not an easy requirement to satisfy! However, we found that by a special compounding process, these characteristic curves can also be blended together to produce a variety of qualities; and thus we were able to more closely approach the ideal.
If we take the overall material impedance characteristic, a combination of both material qualities, we have a parameter that relates to system trackability. Looking at just this function, we can compare the V15 Type III and V15 Type IV bearing material characteristics in the subaudible to mid-frequency range (Figure 9). This graph shows the V15 Type IV bearing to be "stiffer" in the very low and subaudible frequencies, yet it is more "compliant" in the mid and high frequencies. This is a desirable characteristic since it maintains proper compliance for optimum arm resonance frequency and provides for improved mid-frequency trackability difference of the V15 Type III and V15 Type IV shown in Figure 3.
Also, it should be stated that the final bearing material chosen is formed from a blend of materials chemically similar to the V15 Type III material. Therefore, it maintains the same stable non-aging properties as the V15 Type III material, is highly resistant to chemicals, and is even less affected by temperature.
In addition to the improved bearing material, the stylus system uses the dynamic vibration absorber principle. This is an effective technique for controlling the stylus shank resonance, thus improving the tracking without a significant compromise in other performance areas. An Audio Engineering Society paper on the design and performance of a phono stylus using the vibration absorber principle is included in the seminar notes.
The construction of the stylus and bearing system is shown in Figure 10. The bearing and vibration absorber assembly are designed to complement each other, The mechanical resonance can be well controlled by the vibration absorber. With the bearing relieved of this function, we were able to use a bearing material that will improve the tracking in the other frequency regions. In the past, both of the functions were performed by a single part; now each function can be independently optimized.
Other Performance Specifications--Response, Loading, and Crosstalk
At this point, I would like to describe some of the other performance specifications of the cartridge. Frequency response is composed of the sum of the mechanical and electrical responses. However, the mechanical response of the V15 Type IV is flatter than that of the VI5 Type III. Thus, less electrical compensation is needed for flat response at the terminals.
The curve shown (Figure 11) is the typical response of the V15 Type IV. Also shown are the tolerance limits that every V15 Type IV will meet. Each cartridge is individually tested to ensure that this specification is met; in fact, our inspection standards are even tighter than the published limits.
The proper loading to achieve this response is 47 kilohms, 250 pF. This is a change from previous Shure stereo cartridges since the recommended capacitance has been reduced. This value of capacitance is more typical of that available on present-day turntables and tone arm wiring. We feel the consumer need not sorry about the exact capacitance value since a range from 150 pF to 350 pF will keep the typical response within the above published specifications. Therefore, in the vast majority of applications, flat response can be obtained without, any adjustments by the consumer.
Each cartridge is also checked to assure adequate channel separation. AIl cartridges meet a minimum of 25 dB channel separation at 1 kHz and a minimum of 1.5 dB at 10 kHz.
External Design Aspects
Figure 12 is a photograph of the V15 Type IV Phonograph Cartridge.
The external design philosophy was to keep the cartridge functional, compact, low in mass, and easy to handle. The mounting technique for the majority of changers and arms will be with two nylon screws through the headshell into a nut plate. This provides a firm convenient mounting means. The entire stylus is removable from the cartridge body. A large surface area on the stylus grip provides a means of holding when removing or installing the stylus. A cue mark is provided on the front of the grip for ease of setdown. The stylus guard is located inside the grip and R. Anderson will explain its unique features later in the presentation.
Summary and Conclusion
Throughout the presentation, we have stressed the importance of carefully integrating all the design aspects to achieve an optimum performance and have presented some of the tools and processes used in achieving this performance. In addition to subjective listening tests, we have used objective, scientific methods to weigh the alternatives in order to arrive at the optimum design. Regarding the compromise between a heavy strong shank and super-lightweight performance, we have been able to reduce the overall mass and equivalent mass without sacrificing shank strength. This resulted in the high-frequency tracking improvements shown in the trackability graph (Figure 3). In addition, we have shown how our investigation into the properties of elastic materials has yielded a better relationship between the subaudible vs. audible signal characteristics. Thus, we have realized improved mid-frequency tracking without changes in the cartridge/arm resonance frequency.
B. W. Jakobs and S. A. Mastricola
Earlier we described the function and constraints related to tip design in general terms and presented some results of tests conducted on this subject. Now we will examine a specific design, the “Hyperelliptical" tip, for a specific application, namely the V15 Type IV Stereo Phonograph Cartridge.
The basic aim in designing this tip was that of lowering distortion without sacrificing either tip or record life. This involved the careful analysis of the stylus performance criteria, principally trackability or mechanical impedance. Compatibility was achieved by carefully optimizing all important parameters.
The requirements of a tip for the Vl5 Type IV were considered in the examination of dozens of tips of many geometries. A variety of requirements for performance, including low distortion, noise, and wear, resulted in the design of the hyperelliptical tip. Let’s now examine this tip in detail to see how it fulfills the specifications set forth.
We know of nothing that is harder and more wear-resistant than natural diamond. The hyperelliptical tips, like other Shure tips, are made from gem-quality, natural diamonds; i.e., they must be of high purity and be free of inclusions or crystalline defects. This will ensure that they will survive the high stresses applied during the manufacturing operations and, more importantly, yield a relatively low wear rate during playback.
Stylus Tip Body
To be sure that the tip does not contribute more than a small percentage to the total effective mass of the stylus assembly, it should be made from a small diameter nude stone. This requires that significant manufacturing difficulties must be overcome in order to arrive at the correct contact geometry on a very small stone.
The major part of the cylindrical body undergoes a "roughing" operation (Figure 1), and for a very important reason: the stone must be affixed to the stylus shank. The rough surface assures a strong bond; a bond that is tested three ways. One of these is a push-out test in which the tip must not be dislodged when a force of 3/4 pounds is applied. In another test, the cartridge is mounted in a tone arm and loaded with a tracking force sufficient to collapse the stylus. The stylus is then scraped back and forth across the record 100 tines. In the third test the tone arm is dropped onto the record 100 times from a height of about 3".
It is noteworthy that too often diamond tips are assessed simply by taking a moderately enlarged photo, usually 50 or 100 to 1. Invariably, the more brilliant or glassy-looking the stone appears, the more well polished it may seem, The appearance of the tip under these conditions, by no means, guarantees a properly polished tip with correct geometry in the tip-groove contact region (Figure 1). In the most important contact area, the examination and measurements require high-quality instrumentation and microscopes (not necessarily scanning electron microscopes). In addition, technicians must be trained to evaluate diamond tips. It takes considerable insight into the areas of diamond manufacturing, mounting, and the actual application in order to assess diamond tips properly, even with suitable instrumentation.
Shure diamond tips are designed to have the correct geometric properties, are reliably bonded, are properly polished in the contact area as necessary to fulfill their intended function, and are rigorously evaluated.
The tip must be accurately and securely mounted. Angular tolerances are permissible only to the extent that they do not cause deterioration in performance.
The basic frontal contour is a hyperbola described by (Figure 2 and 3). It is generated by a patented manufacturing process involving intersecting cones. The frontal contour is approximately equal to 38µ (.0015") radius. The curvature has been carefully optimized and allows a certain degree of freedom for angular tolerances without degradation of performance.
- A minimum clearance of 5.1µ (.0002") to the bottom of the groove is required. The tip must never “ride” in the bottom of the groove since this will result in additional objectionable noise.
The elongated contact area permits a narrower, but longer, “footprint” in the groove wall, thereby reducing distortion (Figure 4).
- The tracing radius is smaller for lower tracing distortion (Figure 4).
It has an elliptical cross section (Figure 4 and 5) that assures uniformity along the contact length.
The tracing radius is smaller throughout, not just at the theoretical point of contact. By virtue of a patented manufacturing technique, the contact radius remains uniform along the entire contact length, an improvement over spherical, biradial, and some of the conventional long contact varieties. The desired geometry is a natural outcome of the manufacturing process. Notice that, even though the tracing radius is smaller, the contact area is not; it is just shaped differently. Therefore, we have the advantage of more accurate tracing without decreasing the contact area. This is an important point in that the contact area is related to the amount of indentation and stresses within the groove wall, which probably influence record wear.
- The basic frontal contour is a hyperbola described by (Figure 2 and 3). It is generated by a patented manufacturing process involving intersecting cones. The frontal contour is approximately equal to 38µ (.0015") radius. The curvature has been carefully optimized and allows a certain degree of freedom for angular tolerances without degradation of performance.
The design just described satisfies several functional and practical manufacturing requirements. However, the tip must be tested in the cartridge itself to determine whether it performs dynamically. Extensive testing is done to check the design and performance under dynamic conditions. This type of testing includes distortion ¡measurements, noise and wear measurements, and visual observations of tips and records in conjunction with controlled tests.
The dimensions of the hyperelliptical tip theoretically indicate that it will trace the signal more accurately than its predecessors. Tests performed verified this as follows (Figure 6):
2nd Harmonic Distortion
Conditions: Cartridge: V15 Type IV Record: CBS STR-100, bands 3A and 3B Signal: 8 kHz, @ 5 cm/sec peak velocity Tracking Force: 1.25 grams Results: Average values of both channels for several cartridges are as follows (Figure 6): a. Spherical (.6 mil): 6.4% b. Biradial (.3 x.7 mil): 4.0% c. Hyperbolic : 4.0% d. Hyperelliptical: 2.5%
Conditions: Cartridge: V15 Type IV Record: TTF-103, band 6 Signal: 1 kHz/1.5 kHz @ 25 cm/sec peak velocity Tracking force: 1.25 grams Results: Average values of both channels for several cartridges are as follows (Figure 6): a. Spherical (.6 mil): 2.4% b. Biradial ( .3 x .7 mil): 1.8% c. Hyperbolic: 1.8% d. Hyperelliptical: 1.4%
In all of the preceding measurements, great care vas taken to assure that the test conditions remained the same for all styli. Tracking force and skating force were set for each stylus. A new record was used for every stylus. The measurements were repeated to demonstrate that the results were repeatable and consistent. All of these precautions must be taken to obtain meaningful data that is repeatable. Even with these precautions, we still occasionally find individual styli that do not fit the pattern. We must remember that distortion measurements are measuring more than just the distortion caused by the tip. Therefore, the need to test many units is necessary in order to arrive at valid conclusions.
- 2nd Harmonic Distortion
Tests have been conducted to determine the effect of the hyperelliptical tip on surface noise.
Conditions: Cartridge: V15 Type IV Record: 33 1/3 rpm, record radius of 3”, unmodulated (silent) grooves Tracking Force: 1.0 gram Filter: high pass from 500 Hz Equalization: frequency response on each stylus equalized to be like all other styli within 1 dB Results: Average wide band noise output on initial play of both channels for several cartridges relative to the 1 kHz lever @ 5 cm/sec peak velocity: a. Biradial (.3 x .7 mil): -46 3/4 dB b. Hyperelliptical: -47 1/2 dB
The above difference is not significant, and shows that the hyperelliptical generated as little surface noise as the conventional biradial tip.
Comparative tests have been conducted to evaluate tip and record wear. In one test V15 Type IV styli with biradial, hyperbolic, and hyperelliptical tips were played continuously on the Shure TTR-110 records at 1.2, grams tracking force. All cartridges were mounted in the same model record changer. Tips and records were cleaned regularly. Based on photographs of the tips, no significant difference in the rate of tip wear was observed between biradial and hyperelliptical tips. The hyperbolic tips exhibited slightly less wear.
Record wear tests were conducted as well. Second harmonic distortion was measured after 100 plays as outlined in the previous paper (The Tip and the Record--The First Link in the playback Chain). The results showed no significant difference among biradial, hyperbolic, and hyperelliptical tips.
- Distortion Measurements