How Phono Cartridges Work (Mar. 1982)

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By Peter Milton

The arguments over the relative merits of moving-coil cartridges versus those with fixed coils has been going on, I suspect, since Alan Blumlein developed the moving coil principle in the 1920s. There has always been a mystique associated with the delicate art of making a moving-coil cartridge, and the higher cost of production has normally led to the conclusion that the quality of the product is higher.

This may have been true when the audiophile had a choice limited to the standard groove-gougers and a custom-made product from a hobbyist-turned-manufacturer, but times have changed; high price and high precision are no longer the sole domain of the moving coil. But legends persist, and most of the moving coil's glamour remains. There are those whose flat statements lead me to believe that they would prefer a $100 moving coil to a $500 moving magnet on principle, but it is a rare listener who can hear the difference consistently under controlled conditions. It is time, perhaps, to re-examine our Articles of Faith and try to see what the basis is of the fabled moving-coil sound.

During my research I spoke to several respected phono cartridge engineers about the practices and politics of pick up design, attended long listening sessions, and discussed the problem with others, like me, whose reviewing activities have forced them to pronounce, in a few hours, on the patient results of years of skilled research. (The words I quote are as accurate and as near to context as possible, but the opinions are mine.) I am often asked, usually at a party, when my mind is occupied with higher things, "Which is better, a moving-coil or a magnetic phono cartridge?" This is typical of the woolly thinking which tends to surround the topic. I am asked to generalize about a product which has brilliant examples of both types, which is bad enough, but evidently my questioner seems to regard the principle of moving-coil cartridges as something other than magnetic. There is a parallel question regarding speakers which implies that electrostatic speakers are nondynamic. Sometimes, by defining the question clearly, we are well on the way to an answer. In any case, moving coils belong to the general class of magnetic cartridges.

Three things are required: A magnet to provide a suitable magnetic field, a system of coils in which an electrical signal can be induced, and finally a stylus to translate the undulations of the groove walls into movement. The way these items are tied together determines the type of cartridge.

The moving-magnet design forms a very large subclass, and its simplicity has made it very popular. In essence, a tiny magnet is attached to the end of the cantilever which is pivoted in a ring of rubber or plastic fitted around the cantilever near the magnet. Usually a re straining or locating wire holds the cantilever assembly in place and defines the location of the pivot point. A good example of this type of construction is found in Shure's V15 Type IV, shown in Fig. 1, and Fig. 2A is a simplified diagram of the system. A soft iron core, passing through the coils, is shaped to form an air gap which intercepts the flux from the magnet.

The early moving magnets, such as those introduced by Elac, were relatively massive affairs, with magnets nearly a quarter of an inch long. The sensitivity was adequate, but the high-frequency tracking performance was very poor by today's standards. Research into new materials for magnets and into stylus de sign has led to a progressive reduction in tip mass and magnet size. The shape has also changed from the square section bar magnet to the minute disc used by Technics and to the, dual magnet system of Audio-Technica (Fig. 3). The idea behind the variations on the theme is to reduce the moment of inertia of the moving parts, and hence the mass reflected at the stylus tip, to an absolute minimum.

The approach used by Audio-Technica is to cause the magnet associated with the unused channel to rotate about its own axis. This minimizes the effective mass of the unused channel, and since a bar magnet which rotates about its axis produces no effective flux change, the separation between channels can be improved.

The broadest group of magnetic cartridges is the moving-iron type, which can be subdivided into two categories, variable reluctance and induced magnet.

The variable-reluctance types place the magnet in the iron-core system of the pickup coils; thus there is always a magnetic flux across the air gap. The stylus bar terminates in a hollow iron tube which moves inside the magnetic gap.

As the iron moves from the central position, the magnetic reluctance in the gap changes and the flux linking the coils is changed (Fig. 4).

The straightforward variable-reluctance principle carries with it the disadvantage that the reluctance in the magnetic gap varies with the square of the gap spacing. Although the system is virtually a push-pull arrangement, very careful design is required to minimize second harmonic distortion. The challenge has been met by what could be called a series of Elegant Variations, one of the most successful being Ortofon's Variable Magnetic Shunt (Fig. 5). In this design a ring magnet surrounds the iron armature at the end of the cantilever.

The magnetic field does not go across the gap as in the normal variable-reluctance type, but runs fore and aft, parallel to the axis of the structure, in a doughnut shape surrounding the magnet. The iron armature is therefore parallel to the lines of magnetic flux. As the armature moves closer to the inner surface of the mag net, a progressively greater proportion of the lines of magnetic flux are shunted, which changes the flux linking the coils and produces an output.


Fig. 1--Cut-away view of the V-15 Type IV cartridge, courtesy Shure Bros.


Fig. 2A --Simplified drawing of a moving-magnet type phono cartridge. (After Ortofon's "Everything You Need to Know About Cartridges.")


Fig. 2B--Simplified drawing of an induced-magnet cartridge. (After Ortofon.)


Fig. 2C --Simplified drawing of a moving-coil cartridge. (After Ortofon.)


Fig. 3A--Audio-Technica's dual magnet system shown in a moving-magnet cartridge, AT155LC. Photo: Susanne Buckler


Fig 3B --Audio-Technica 's dual magnet stylus assembly.


Fig. 4 --Simplified drawing of a variable-reluctance cartridge. (After Osawa's "A Consumer Guide to Phono Cartridges.")


Fig. 5 --Simplified drawing of Ortofon's Variable Magnetic Shunt principle. (After Ortofon.)

The induced magnet is an ingenious method of obtaining the linearity of the moving magnet and, at the same time, dispensing with its mass. A hollow iron armature is attached to the end of the cantilever and an external magnet induces a magnetic field into it. The field moves with the cantilever and, in turn, induces an e.m.f. into the coils wound around the soft iron poles. The advantage here is that the magnetic circuit operates effectively at zero flux for maxi mum linearity.

A popular example of the induced-magnet design is illustrated in Fig. 6, showing Empire's construction. The importance of zero flux in the magnetic cores is emphasized in this design, which has an exclusive method for eliminating the residual flux, as does the Sonus.

Moving-coil cartridges are almost as diverse in construction as fixed-coil cartridges. In all cases the coils are rigidly attached to the stylus bar, which usually makes it necessary to return the complete cartridge to the manufacturer when the tip needs replacement. Two examples will serve to illustrate the construction of a moving-coil cartridge. Figure 7 shows the Ortofon MC-20. The cantilever projects through one pole of the magnetic structure so that the lines of flux are parallel to it. The pivot point is just behind the pickup coils, which are wound on a flat square former. Movement in one channel causes one pair of coils to rock, which allows the flux to cut the turns and produce an output. The coils associated with the other channel twist around their central axis, which produces no output.

Technics uses an entirely different form of construction. The coils operate in completely separate magnetic gaps and are attached to a driving yoke so that the front view has a strong resemblance to Mickey Mouse. The magnetic field is radial, but operates over half the area of the coil so that the output is not cancelled. The inoperative channel twists around its diameter so that there is no output.

Each of the designs described has been the basis of a successful and highly respected line of cartridges. However, we have not touched on the other important features --damping, stylus construction and material, tip shape and mounting --because the point at issue is the relative merits of the moving coil versus The Rest. But let us redefine the question: Which fixed-coil is better than which moving-coil cartridge and under what circumstances? If we do not consider this, then the inescapable conclusion is that one or the other type is intrinsically better. The question was asked in a different way of James Kogan, President of Shure Brothers: Why does he favor the moving-magnet design? His reply brought in another factor. "We have thought about the moving-magnet and moving-coil approaches very carefully; we are quite capable of making a superb moving-coil cartridge. There are trade offs to be made in both designs. We have evaluated most of the cartridges of the opposition, and we feel that the moving-magnet cartridge gives a better balance between price and performance." He then added the intriguing comment, "Of course, that does not rule out a moving coil in the future." The real progress, he felt, will be in refinements of technology rather than in a particular method of generating the output. The trade-off known as cost effectiveness is also a factor which we should consider and if this is to be the criterion, then the traditional moving coil is inferior.

If the moving coil were better technically, then we could smugly explain the presence of the fixed-magnet types on the grounds of cost alone. However, brilliant and marginal performers are found in both categories, so the cost criterion is unacceptable. But, this does give rise to a new form of the same question.

During the November 1981 AES Convention in New York, I asked the cartridge designers: "If you were starting from scratch with an unlimited budget, what form would your cartridge take?" No clear-cut preferences were expressed. The area of cartridge design is so confined with patents that it is most likely that the designer would tend to work along lines already established by his own company.

Are there no inventors out there? Well, there are, but the commercial world is competitive and nobody likes to tip his hand.

By what criteria do we judge the car ridges? Ortofon's Frits Nygaard rates he moving coil superior on the grounds of lower moving mass and improved phase response. The moving coil also has a better rise-time and its lower source impedance makes it less susceptible to amplifier interface problems.

This prompts a few specification problems. One important factor is tip mass, which is normally automatically assumed to be lower in a moving coil by reason of the ultra-miniature construction. Lower moving mass can only be taken to mean the effective mass as "seen" at the stylus tip, which is not necessarily the mass of the actual tip.

The effective mass comprises the total mass of the moving parts --tip, cantilever, and generator reflected through the lever action of the stylus arm and pivot. We are dealing with moments of inertia which can be distributed at the discretion of the designer.

This is not necessarily borne out by the published literature, since for the Ortofon MC-20 we find 0.5 mg effective tip mass and for their LM-30 cartridges, the effective tip mass is 0.35. The contradictory state of affairs is underscored by the 0.33 mg of the Shure V15 Type IV compared to, say, the 0.3 mg of the Technics EPS-305MC. To be fair to both camps, the 0.6 mg of Empire's 600LAC should be mentioned.

It is clear that effective tip mass alone does not account for the differences be tween the two methods of construction.

Identical frequency responses can be obtained with a wide range of tip mass, since it is possible to juggle the effective stiffness of the record surface by changing the contact area.

However, under normal circum stances, the difference between a moving-coil and a fixed-coil cartridge can be heard. On occasion one is substituted for the other in order to achieve an acceptable sound in a hostile listening environment. For instance, when faced with a "hot" hotel room, no time to modify the acoustics, and a preamp with an inflexible high-frequency roll-off, a smart demonstrator will substitute a moving magnet for a moving coil. Whether you favor the "airy" ambience of a moving coil or the less spectacular moving mag net, the fact remains that there are differences. In my book, if two units are "high fidelity," they should sound the same. If there are differences, one or both must be inaccurate. "Fidelity" is faithfulness; you either keep the faith or you don't.

Unfortunately, "high" fidelity implies different heights of excellence.



Fig. 6 --Exploded drawing of an Empire cartridge, which uses the induced magnet principle. (Courtesy Empire.)

If cartridges having the same tip mass, or nearly so, can have different tonality, then we are left with phase response and frequency response.

Arnold Schwartz, President of Micro-Acoustics, in the March 1981 issue of Audio, indicated one possible cause of the differences in sound when discussing the cartridge output network. In his laboratories, he demonstrates the experimental setup which simulates the effect of the pickup coils, the amplifier input resistance, and the cable capacitance on the electrical frequency response of the cartridge. These elements form a low-pass filter for each type of cartridge.

The higher relative resistance for the moving coil, coupled with its lower source impedance, renders it less susceptible to the effects of cable capacitance and gives it a more gentle roll-off than the fixed-coil type. Consequently, in order to extend the overall frequency response to beyond audibility, the mechanical system must compensate for the fall in electrical output. This can only be achieved by allowing the stylus/ record system to resonate, thus producing a compensating peak. In the hypothetical cases illustrated, if the stylus resonance is assumed to be 20 kHz, then the mechanical Q of the moving-magnet phono cartridge would have to be 2, and that of the moving coil a little over 1.

Unfortunately it is difficult to achieve exact compensation in a magnetic cartridge, and the resulting curve often has a 1 to 2 dB dip between 5 and 15 kHz.

The gentler moving-coil characteristic usually causes a slight but smooth rise in the high-frequency range --minor but significant differences.

The bandwidth limitations imposed by the electrical network also affect the transient response, as measured by the rise-time, significantly. If the rise-time is to be measured in terms of the reproduction of the leading edge of a square wave, then bandwidth limitations which suppress the upper harmonics will also slow the response of the cartridge. The solution to the problem of obtaining flat response from a phono cartridge, at least from the point of view of the generating mechanism, is to make the electrical bandwidth greater than the mechanical so that the stylus assembly is freed from its compensating function.

The ultimate test of a phono cartridge is the listening test. It is almost impossible to control all factors in a listening test, but interesting results can be obtained if a large panel of listeners under takes a series of blind tests and the responses subjected to statistical analysis.

Dr. Floyd Toole of the National Research Council in Canada conducted large scale tests in Ottawa during 1980, first of all with nine cartridges and 16 listeners, and then three cartridges, selected from the first batch, with 13 listeners.

The listeners were placed in the optimum stereo seats, not more than three at a time, and were cautioned against moving, since some of the differences would be subtle. They were also cautioned about the possibility of nonverbal communication ("body language") influencing the opinion of the group. The three final cartridges selected were the Ortofon MC-30, Denon DL 103D and the Shure V15 Type IV, with the tests divided into two sections --equalized and non-equalized.

Differences were noted during the tests with the non-equalized cartridges.

The Denon was found to be brighter than the Ortofon, and the Ortofon seemed to sound similar to the Shure cartridge. In most of the cases the excess of high frequencies was criticized, although there were two listeners who consistently preferred the extra highs of the moving coil. The effects were notice able only with selected good records, during certain passages and with experienced listeners, but even then, the differences were not particularly different statistically.

During the second part of the test, the Shure was equalized using a Technics 9010 parametric equalizer so that the response was within 0.2 dB of the Ortofon. Again, the results were close, with the interesting result that the moving magnet gained a slight edge over the moving coils, not so much by increasing its score on the evaluation sheet, but by causing the marks given to the moving coils to drop slightly.

It is very tempting to generalize from a test of this nature. One listener was able to pick out the moving-coil cartridge consistently and expressed a clear preference for it.

The closeness of the results surprised several listeners, particularly the moving-coil aficionados who were embarrassed to find that they had given their votes to the moving magnet! But are these results so surprising? Many critics hold the opinion that two amplifiers having the same measured performance will have an identical sound when used under identical conditions.

Any listening differences which can be proved to exist would then be the result of some parameter as yet unmeasured.

Why should this not also apply to phonograph cartridges? Mr. Shuichi Obata, General Manager of the Engineering Department for Technics, says of his philosophy of cartridge design: "The recording engineer and the performers have worked very hard to get the sound of their music into the grooves of the phonograph record. I think that it is the duty and the courtesy of the cartridge designer to extract all of this information as accurately as possible. The cartridge should have no tonality of its own. Regarding the choice between moving coil and moving iron, both choices are equally valid in the hands of a competent engineer and will likely be influenced by the historical preferences of his company. Two cartridges having identical measured performance in all respects, even though the principle is different, will have the same tonality. I think that it is more important to concentrate on the stylus assembly and the distribution of mass.

"Up to now there have been too many other factors interfering with our judgment of the moving-coil principle compared with the moving magnet. We have done a lot of research into this and this is one of the results."


Fig. 7--Ortofon MC-20 MkII.


Fig. 8 --Technics EPC-1 00 Mk3.

He then produced the latest Technics moving-coil cartridge, the EPC-305 Mk2. The tapered boron cantilever used in this design has the same 0.098-mg effective tip mass as their previous top-of-the-line cartridge, the EPC-100C Mk3. The frequency response is 10 Hz to 100 kHz, wider if anything than the EPC-100C Mk3. The performance is backed by a new "amorphous" transformer, rated to 300 kHz. In effect, the two cartridges have the same stylus system and identical frequency response for at least an octave above the audio band, which should isolate the operating principle.

A very interesting hybrid, approach has been taken by the sister firms, Stanton and Pickering, in their XLZ/7500S, 98OLZS, and 981 LZS cartridges. In basic design these are moving-magnet types, but they have a very low inductance, 1 mH; low d.c. resistance, 3 ohms, and low dynamic tip mass, 0.2 mg. Their output is also low, .at 0.06 mV/cm/S, so they require some sort of step-up device. However, these cartridges appear to offer all the advantages' of a moving-magnet cartridge without the disadvantages of an MC type.

The moving-coil controversy is far from dead. As cartridges get better and closer in measured results, the tonality differences become vanishingly small.

Moving magnets have become so advanced that at the high end, the majority of listeners would find no real difference and the decision of one type over the other would be based on economics.

We must not, however, ignore the lone golden-eared individuals who can hear the differences. We could have an other TIM situation on our hands.

It is usually the prodding of some lone, serious, experienced listener that spurs the research and development of hi-fi equipment to greater refinement. At the moment, we may admit the differences. We must now find out if they are due to the construction of the cartridge, and indeed, if the "acceptable" sound is indeed accurate. Even our lone and consistent moving-coil fan admitted that his preferences were towards a slightly "hot" high end.

A grace note to our interview, Shuichi Obata said that even the latest and best records are not perfect and that considerable work is needed to perfect them.

"Ask for better records," he said, "as well as better cartridges." The differences are becoming so small that, from my vantage point on the sidelines, the whole issue is becoming a non-controversy. But my guess is that there will still be arguments about it when the last cartridge rolls off the line and into history.

(Adapted from Audio magazine, Mar. 1982)

Also see:

Phase Testing in Phono Cartridges (Mar. 1983)

Birth of a Spec? PHONO CARTRIDGE NOISE (March 1977)

Understanding Phono Cartridges (Mar. 1979)

The Phono Cartridge Electrical Output Network (March 1981)

More Than One Vertical Tracking Angle (March 1981)

Construct a Magnetic Cartridge Preamp (Jun. 1974)





 

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