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J. R. SANK - RCA, Camden, New Jersey (ein Artikel aus 1970)

A number of observations are reported on an improved disc reproducer consisting of a tone arm and cartridge assembled as a matched system. Interchangeable stylus assemblies with spherical or biradial tips were used with conventional test records. Since only the stylus assemblies were changed, the effects caused by changes in tip radii are readily seen.

The data touch on several problems involving biradial and spherical stylus relationships for which a theoretical model is incomplete, especially such effects of biradial styli as extremely high stress imposed on the record groove and the possibility of shorter record life.

Record wear data are shown for the biradial stylus which indicate that record life is about the same as with a spherical tip, when both are used in low-inertia assemblies.

The biradial stylus has become widely accepted for braodcast. professional audio, and high-fidelity applications. In 1968 more than half of the pickups sold in these markets were so equipped.

Unfortunately, it is very difficult for a user to make a choice of pickup and stylus, as very little scientifically measured data are available on pickups currently on the market.

The scanning electron microscope (bei uns REM genannt) photographs published by Woodward [1] show apparent record damage caused by a biradial stylus, but he mentions, that high-frequency distortion results favor the biradial tip. Since the distortion data is not presented, the evidence seems to weigh against the biradial, and in favor of the spherical tip stylus.

RCA has introduced a complete tone-arm- pickup-stylus system, known as the BDR-1 Broadcast Disc Reproducer. The BDR-1 has quick-change stylus assemblies with several tips having different radii, which makes it possible for the user, to select the stylus best suited to his application or personal preference.

The .0002 X .0007 in biradial stylus and the .0007 in spherical stylus assemblies have identical mechanical systems except for the stylus tip. This makes it possible to stud) the effects of changing only the stylus tip.

The RCA BDR-1 is shown in Fig. 1. The inertia (Trägheit) of the arm is much lower than that of most arms currently in use, as may be deduced abgeleitet) from the photograph. The low inertia is achieved by incorporating a lightweight moving-magnet cartridge as an integral part of the arm; thus the arm does not need to be built to a Gibraltar-like scale to accommodate a wide range of cartridge masses.

The arm tube (Tonarm-Röhre) is close to the record to reduce excitation (Erregung) of torsional resonance modes. The straight-line design eliminates a bend (Verdrehen) in the tube and achieves lateral balance without "outrigger" adjustable weights. Stylus force is obtained by a small offset in vertical balance.

The system is factory-adjusted and individual stylus assemblies are weighted as required for optimum force. Antiskating force is obtained by a linear spring and lever arm arrangement under the decorative cover plate, and is fixed. Precision instrument bearings (Kugellager) used on both axes have starting torque very much less than the torque generated by operating forces.

The data were measured using an RCA BA-36A equalizer/preamplifier modified for test purposes, plus various filters and processing networks, and automatically recorded. The curves are unretouched and show the performance that would actually be obtained by a user with a complete RCA disc reproducing system.

Pickup "X" is a laboratory reference moving-coil pickup with .0007 in spherical tip, that was manufactured in Denmark about 5 years ago, which has the lowest tip mass that was available previous to the BDR-1. It was mounted in a high-quality 12" arm of British manufacture which has several adjustments to optimize pickup performance.
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The tips were best seen visually through an 80x stereo microscope, and were sketched approximately to this scale. Figure 2 shows the various BDR-1 styli and the stylus of Pickup X.

The flats on the biradial tips are a normal result of fabrication and do not contact the groove. It is apparent (einleuchtend), that the BDR-1 .0002" X, the .0007" biradial and the .0007" spherical, and Pickup X styli have comparable masses and are good units for comparison.
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Figure 3 shows the performance of the .0002" x .0007" biradial stylus on the (CBS, Shure oder RCA ???) STR-100 record, with the measuring system flat for constant velocity (konstante Schnelle). An RC pickup load flattens the response to 20.000 Hz. The CBS records appear to have consistently less output on the left channel at high frequencies. No significant resonances are apparent.

Figure 4 shows the same stylus on the STR-120 record. At low frequencies the measuring system was flat for constant amplitude, while at high frequencies the measuring system was flat for constant velocity.

The capacitance load was omitted in order to reduce roll-off above 20,000 Hz. Thus a "sag and peak" characteristic are observed between 3.000 and 20.000 Hz. The lowfrequency resonance is high enough for good immunity from record warps and mechanical shock excitation, but low enough to permit good performance in the audible range. The apparent vinylite resonance is 20,000 to 22,000 Hz.
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Figures 5 and 6 show the same tests repeated with the .0007" stylus. Results are surprisingly similar to those for the biradial stylus.

This is not in agreement with theory, which reasons that the vinylite resonance should increase with a larger stylus radius:

The larger radius applies less indenting pressure on the groove wall, so that the compliance of the vinylite then appears to decrease, and if tip mass is unchanged, the resonance frequency increases.

Previous pickup observations have followed this theory. The best explanation of this phenomena was ottered by B. Jacobs [2], who stated, that the peak which is displayed on the curves results from a resonance of the mass of the stylus tip and the compliance of the stylus lever arm, in these particular stylus assemblies.

The vinylite resonance is about one octave higher but is not displayed due to the electrical system roll-off, as cartridge impedance is very high around 40,000 Hz.
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Figure 7 shows the performance of Pickup X on the STR-120 test record. It can be seen, that the apparent resonance is about 30,000 Hz. These data were recorded without capacitance load to maximize high frequency output. Figure 8 shows the performance of Pickup X on the STR-100 test record. An RC load was used to flatten the response. Under these test conditions, an apparent resonance of 15,000 Hz is observed.

The evidence tends to confirm Jacob's explanation. The 15,000 Hz and 30,000 Hz resonances of Pickup X would correspond to 22,000 Hz and approximately 40,000 Hz for the BDR-1.

The effective tip mass of the BDR-1 styli would then be lower than that of Pickup X, making the BDR-1 a very good pickup indeed!

The differences in response above the audio range are of course inconsequential to the user. However, the decreased mechanical impedance in the audio range is important and could make the BDR-1 biradial stylus as gentle to the grove as the spherical stylus of Pickup X.

An easily demonstrated advantage of biradial tips, even before the low-tip-mass .0002" x .0007" BDR-1 was introduced, was the reduced diameter loss compared to spherical tips. It is obvious (ersichtlich) that a loss of this kind, analogous to the scanning loss in an optical film reproducer, can be reduced by making the scanning slit (Spalt, Ritze) smaller. In this case, this corresponds to the stylus side radius.

Figure 9 shows the diameter loss of Pickup X on the STR-120 record. The 20,000 Hz loss is about 12dB. This is thought to be typical of high-quality .0007" spherical-tip pickups.

Figure 10 shows the loss of the BDR-l biradial stylus. It is much less severe (sehr viel weniger stark) and obviously due to the .0002" contact radius.

Figure 11 shows the loss of the spherical stylus. This accords with theory in that the spherical stylus has more loss than the biradial one; however, it has much less loss than Pickup X, and therefore agreement with theory is very poor in this respect.
(Die Theorie stimmt mit der Messung nicht überein.)

No suitable theoretical model has been found that would explain these data. Sufficient time was not available to investigate the diameter equalization presently used by record companies. It would be difficult to choose an optimum equalization from these data.
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Record life is important for high fidelity applications and some professional users, but perhaps less important to broadcasters.

In my experience, a record with the best of care (mit der größten Sorgfalt) picks up enough dirt (Staub) in 50 plays to cause "ticks" that would be objectionable (unzulässig) in professional use.

The simplest and most readily performed test was to make 50 plays of a frequency-response record with each pickup in question. The middle bands of the STR-120 test record were chosen to present average wavelengths to the stylus. The 50,000 Hz range would insure that some wear would be recorded on the very best pickup.

The .0002" x .0007" biradial was an obvious choice. Pickup X was chosen as the comparison spherical-tip pickup because of the interesting differences observed previously between it and the BDR-1 biradial. A second reason is that the .0007" spherical tip was not yet available for the BDR-1 when the tests were made.

Figure 12 shows normalized data for Pickup X, and Fig. 13 shows similar data for the BDR-1 biradial pickup.

It is evident that either pickup produces negligible wear in the audible range. They are both particularly good compared to the previously developed definition of "negligible wear", that is, less than 2 dB change up to 15,000 Hz in 20 plays.

The BDR-1 .0002" x .0007" and .0007" were compared to Pickup X, using individual BA-36A preamplifiers with optimum RC pickup loads for the tests.

A few listeners prefered the biradial, but most could not tell the difference between the three pickups. This is a good result, since Pickup X has for years sounded distinctly better than other pickups.

The preferences for the biradial were obtained using RCA Dynagroove records, for which the biradial theoretically has no advantage (Vorteil). A non-RCA record was found for which the distortion on trumpet passages was equal on all three pickups.

The large differences observed for diameter loss were not heard, and no reason for this is known.
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The .0002" x .0007" biradial stylus as used in the BDR-1 broadcast disc reproducer has no performance disadvantages (Nachteile) compared to the .0007" spherical-tip stylus. There are some small advantages to the biradial evident in these data, and others have found some more significant advantages.

The biradial stylus appears best for the critical user, but a choice of tip radii allows for record variations and personal preferences. These conclusions are of course not valid for biradial styli of greater effective mass than the type studied.

If the resonance theory is correct, the BDR-1 comes very close to the criteria proposed by Hunt [3] for 1.5 gram tracking. Further work is needed to positively identify the high-frequency resonances, and to explain the mysteries of diameter loss. A standard test for record life needs to be developed. Beyond this it appears that records exist that have distortion independent of the playback transducer, and the cause and cure remain to be determined.
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• 1. J. G. Woodward. "Record Groove Wear," Stereo Review (Oct. 1968).
• 2. B. Jacobs, private communication.
• 3. F. V. Hunt, "The Rational Design of Phonograph Pickup," ./. Audio Eng. Soc. 10, 274 (1962).

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J. R. Sank received the B.S. degree in Electrical Engineering from Drexel Institute of Technology in 1957. After graduation, he joined RCA in Camden. N. J.
As an electroacoustics engineer he has designed and developed various types of microphones, loudspeakers, and phonograph pickups for broadcast and professional audio applications. He has one U.S. patent filed in the microphone field.

Presented May 5. 1970 at the 38th Convention of the Audio Engineering Society, Los Angeles.