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In Reply to: VTA and bass posted by eliot on June 23, 2000 at 15:31:02:
The appended article on VTA answers your questions.
Peace at AA
The Importance of Vertical Tracking Angle: How To Get It Right by Ear Alone
[This article is slightly modified from one published in TAS June 1980, authored by David C. Shreve, a professor of mathematics at the University of Wisconsin and the father of the Shreve modification to the Ranco SL-8E arm.]
Incorrect vertical tracking angle (VTA) causes highs to be edgy or dull, the midrange harsh, or the bass thin or muddy. These problems can be ameliorated by correct pickup arm adjustment. Dramatic improvements can be made with stereo systems costing from a few hundred dollars to tens of thousands by adjusting the pickup to optimize the geometry of the stylus in the groove, giving improvements equivalent to spending two or three thou sand dollars more on equipment. The most important to audio quality of the various adjustments of the pickup is the VTA.
In January 1976, John Curl heard the effect of changing the height of the back of the arm to change the VIA by two minutes. In July 1976, Ralph Hodges heard these effects, and the next night he demonstrated them for Mitch Cotter, who previously had been skeptical that such small changes could be heard.
There are a few major problems with stylus-groove geometry. Consider a stylus with a tall contact patch in the wall of a groove, such as an elliptical, Shibata, or line-contact stylus. The long axis of this patch is a line which, when viewed from the side of the stylus, makes an angle with the vertical when the stylus is at rest in the groove. Call this angle the stylus rake angle (SRA). There is a similar angle for the cutter stylus when the master disc is cut, determined by the angle of the cutter head and the mounting of the stylus in the head. If the BRA of the playback stylus does not agree with that of the cutter stylus, then obviously the playback stylus will not "fit" properly in the groove modulations and will not be able to follow closely the path of the cutter stylus. This is true for both lateral and vertical motion.
If the SRA is correct but the VTA of the playback cartridge does not agree with the VTA of the cutter, then there still are problems. If the error is small, there is a form of frequency-intermodulation distortion. If the error in VTA is large, there are gross problems when the stylus tries to change direction sharply or make large excursions in following the groove modulations. A conical playback stylus has a round contact patch and there is no SRA, but there is still the problem of VTA. A cartridge with very high VTA cannot possibly trace the groove accurately; in any good arm, the symptoms of high VTA will be obvious to the listener. High VTA also leads to groove damage because the stylus chatters on sharp modulations.
Few cartridges are optimized when the tops of their bodies are parallel to the record surface. Most will require an arm pivot height adjustment or the body of the cartridge is lower than the front. It is possible to reduce the SRA or VTA of most cartridges by no more than two or three degrees. In some shims may be needed to lower the back by one or two degrees.
With some brands there are inadequate standards of quality control in mounting the stylus tip on the end of the cantilever, and in mounting the cantilever in the cartridge body. Variations can occur among various samples of the same cartridge model; consequently, when these samples are optimized for the correct VIA, each looks different, relative to the record surface. It is the proper task of the manufacturer to determine the relationship between SRA and VTA. and build his cartridge with both angles optimized. This situation is not helped by the international standard specification for VIA of 20 degrees Â± 5 degrees. This is no standard at all in view of the fact that a change in arm height corresponding to a change in SRA and VTA of one minute is easily audible under the proper conditions. The problem is complicated further by the popularity (at the time this article was written) of an 18 degree angle as the disc cutting head's VTA. Obviously the SRA of the cutter stylus is changed when its VTA is changed - a chaotic situation!
Sonic Symptoms of Incorrect VIA
The following will define the arm is low (or high) to indicate that the back of the cartridge is too low (or too high). The exact characteristics of improper adjustment depend on the particular cartridge, the stylus shape, and the associated equipment. Some of these symptoms may be present while others are missing. Some symptoms may be observed with mono records.
When the arm is too high, the sound lacks deep bass, the highs are edgy, and the midrange is harsh. The sound is bright, hard, and induces listener fatigue. The extreme highs may also be rolled off, depending on the program material. The arm is high if the sound is edge, steely, glassy, hard, bright, it the bass is thin or too tight, if the bass lacks dynamics, fullness, and strength, or if attacks are too sharp. Dynamic range is reduced as extremely low level details between musical notes are obscured by distortion, and peaks are attenuated. Fundamentals in the lower registers are rolled off and consequently there is not the proper weight in the lower midrange and bass of an orchestra, the proper tone of a single instrument, such as a piano, or of a voice. When the tone is correct, there may appear to be a slight loss of detail because of a masking effect in the ear, but standing in front of your tweeters will reassure you the detail is present.
When the arm is extremely low, the highs are rolled off and the lows are muddy. The deepest lows may be missing. If the arm is low, the bass lacks definition, attacks are dull, and upper harmonics are rolled off. Brass and strings sound dull and lifeless. Some bass instruments may sound as though the notes are drawn out for a longer time than is realistic. Dynamic range is reduced. A voice may be harsh with certain combinations of equipment. The midrange may lack definition.
What are the best clues? It is not wise to listen for only one or two things and try to get them right. Listen to the instruments or voices and try to get everything about them right. The most important part of the sound of an instrument or voice is its tone, not its attacks or inner detail. Listen for the proper relationship between the fundamental and the harmonics. Listen for increased dynamic range. The detail of the instruments or voices should be realistic, neither suppressed nor exaggerated.
Small changes in arm height are difficult to hear when SRA and VTA are far from optimum. When the angles are near optimum, extremely small changes are easily audible. With some arms it is possible to hear the effect of changing the height of the back of the arm by two thousandths of an inch. This corresponds to a change in SRA and VTA of about one minute.
If the pickup arm is rigid and has excellent pivots, there will be an obvious "best" adjustment. Very small adjustments centered about this "best" position may reveal intermodulation effects. (Damping of the arm may decrease the sensitivity to small changes near the optimum, but close listening will be rewarded.) Tone, dynamics, detail, attacks, air, ambience, and everything else will come together and appear more realistic at the correct adjustment. Surface noise should be unobtrusive, and tape hiss should be smooth.
Discs for Hearing VTA
Find a disc with clean plucked string bass. It should have deep, full, solid bass and clean attacks on the plucking. If the plucking is too sharp and the bass is not deep, the arm is high. If the plucking is dull and the bass is muddy, the arm is low. Experimenting with small changes while playing this cut can be edu cational and rewarding.
A disc with a highly dynamic piano sound is another good test. If the piano sounds hard or tinny, or if there is mistracking on attacks, the arm is set too high. If the piano sounds dull, the arm is set too low. When the arm is correctly adjusted, there is an obvious improvement in dynamic range; the notes sound louder, and quiet parts between the notes are quieter. Experiment with other records containing material which includes dynamic bass or midrange, preferably both, and also a good live recording of a voice. The applause in a live recording can be an excellent test.
Other adjustments of the pickup system have varying effects on the sound. The lateral tracking error should be small, of course. But an error of a few minutes of arc here is much less significant than the same error in SRA and VTA.
The azimuth of the cartridge must be as accurate as possible.
Overhang errors will have severe effects on overall distortion and on the bass, which may almost vanish.
Improperly balanced skating and antiskating forces will affect tracking ability, focusing, and bass. A good straight-line arm avoids these problems at the outset.
Design and construction of the pickup arm are also important to audio quality. The arm's pivots must have extremely low friction, but must not be loose. The arm itself, including the headshell, must be as rigid and non-resonant as possible. It may be possible to further deaden your arm by applying Duxseal to the headshell, if this does not increase the mass too much. Any arm resonance should be undamped to avoid constricted dynamic range, dull attacks, and tonal coloration.
A poor arm may influence the symptoms of improper adjustment, so that the result is moderately pleasant although inaccurate. Proper adjustment will still pay dividends although it will be more difficult to do it right.
ALIGNMENT PROCEDURE FOR MINIMUM LATERAL TRACKING ERROR ADJUSTMENT OF OFFSET PICKUP ARMS
By Mitchell A. Cotter Co., Inc. 35 Beachwood Avenue, ME. Vernon, NY 10553 PH; (914) 699-1874
Reference is made to the attached drawing (see the end) for definitions and to assure that the correct measurement is performed. The procedure described deals with the real situation presently found in the audio equipment available at all price levels at the time of this memorandum. None of the arms we have seen are of the correct offset angle design nor are the best instructions given for obtaining the optimum performance in each case, given the design deficiencies.
The purpose of this memo is to permit each case to be fully optimized in spite of the apparent deficiencies of the designs. In order to accomplish this it is necessary to twist the pickup in the mounting, whether a demountable head shell or a fixed head arm. It is also necessary to be able to set the pickup to the optimum overhang in order for the approach to work. So, determine just how this will be possible in each case before starting the procedure.
Many arms permit the pickup to slide back and forth in the shell. and the arm mounting base in the case of the separate arm/TT systems ran also be varied in position. The basic parameter that determines the setup conditions is the "effective arm length". Consult Fig. 1. Note that this is the length of the arc that the stylus swings on as the arm moves across the record. Ibis length should be measured carefully with a precision rule as an error of less than 0. 02 inch should be obtained. We recommend that two narrow flexible machinists scales be used for this. The Starrett 6" type C30SR and the 12" C316R are quite good for this use. The ruling is fine enough to serve being in .01 and .1 inch graduations.
Step 1:Mount the pickup not fully tightened and measure effective arm length. (See fig. 1).
Step 2:Consult the graph (Fig. 4) to find the optimum overhang for that length arm. Measure the overhang by swinging the arm over the center pin and using the small rule, find the overhang that exists in the starting setup. Some arms are easily raised to make it easy to do this without a large error from having to lift the arm too far from the position in which it will track on the record.
It is convenient to use the distance from the edge of the center -pin since the diameter of 0.282 to 0.284 is the common range of use. This allows the arm to be lifted just enough to clear the pin and makes it possible to position the rule so that it is easily read from the side. It is simple to deduct the 0. 142" from the value to be used and read it as the stylus just crosses over the rule in line with the arm pivot point at the center pin. It will, in most cases, be necessary to go back and forth several times to obtain the correct settings since the effective length may be changed by the adjustment.
Note from fig. 4 that the optimum overhang changes very little as the arm length alters but the offset angle changes rapidly. The perfection of all the following efforts and the final result depend upon getting the overhang correct for the actual working arm length. Do not at this point, be concerned with the offset angle but try to get the overhang correct and to measure the effective arm length accurately.
Step 3:Construct and retain for future use a 5 X 7 file card ruled as shown in fig. 2. The hole for the center pin can be made by starting it with a sharp pencil and pressing it over the center pin. The radial line and the two perpendicular lines should line up with the center pin center. These two record groove radii are the positions that have zero tracking error laterally for all arm lengths.
The zero error points are derived from the inner and outer diameter dimensions of the record, thus are independent of the arm size. Using this fact it is possible to correctly adjust the offset angle of the previously set up arm so that the optimum and minimum tracking error distortion will be achieved. The arm can be positioned as though to play a record by the cueing lever if possible - just over the outer (4. 76,) mark and by sighting the pickup to get it (by twisting) to lie on a line that is perpendicular to the radial line intersecting the stylus at the zero error points. After the pickup is set to the outer zero, try it at the inner point (2.60").
With some pickups that have a flat front face, it is possible to sight and to test with the small rule to see that the face lines up parallel to the radial line. One such pickup is the Denon 103. It is easy to observe the position of the center pin using the front face of this pickup as a "gun-sight", noting that the sight line should be to a point just post the center pin's center by the same distance that the face is in front of the stylus. A little experience with this approach will readily develop the ability to set up quickly to the optimum.
With more complicated - shaped pickups, it may be desirable to use a small mirror, such as a ladies purse mirror, with two lines scribed on it, at right angles to each other. Place the mirror under the pickup facing up and use it to align the pickup stylus so that it is perpendicular to one of the lines placed on the radial line from the turntable center pin in the position of the outer TE null.
Avoid allowing the stylus to touch the surface of the card to prevent bending it if the table is rotated backwards during the handling. When correctly set up the actual error angle is greatest at the outer record diameter and each of the other two maximum errors (in angle) are progressively less. This is because the distortion that is produced by the error increases as the groove speed decreases towards the inner grooves. It is directly proportional to the angle divided by the groove speed. Thus the degrees error per inch of groove radius is equal at the three maximum error points and is also minimum in magnitude for the available arm length.
Step 4:To set up the skating force compensation to the ideal value, it is only necessary to play a relatively low level record passage and to raise and lower the stylus from the grooves while watching closely to see that the stylus remains centered in the same position in the pickup when in the playing position as in the raised position. The adjustment is important because of the free compliance of the stylus and the fact that it is of only a few millimeters length allow it to move a very considerable angle from any steady skating force that is allowed or from too much compensation. Such an error causes the alignment to err and defeats the adjustment (as well as the effects of the unbalanced force on the two groove walls).
This procedure corrects the lateral tracking error only. It is necessary that the vertical angle of the movement of the stylus cantilever also can be considered since the vertical angle errors cause exactly the same type of distortion. Such distortion, will in general be the same across the entire range of playing diameters.
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Topic - VTA and bass - eliot 15:31:02 06/23/00 (4)Follow Ups
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