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Dead Wax Cafe

Understanding The Issues Behind Cartridge Alignment

By Gary Markowitz

Proper cartridge alignment is critical to the sound of your analog rig. Terms like SRA, azimuth and zenith are often thrown around in discussions of cartridge alignment. This article is intended to explain these parameters and others you need to set to properly align your cartridge. I will try to explain some of the methods commonly used to verify these settings, as well as some the common audible manifestations of each. The following topics are covered:>
Overhang
Zenith
Azimuth
Vertical Tracking Angle (VTA)
Stylus Rake Angle (SRA)
Tracking Force (VTF)
Anti-Skating Force (HTF)

Overhang

The first and most commonly known parameter involved in aligning your cartridge is known as setting the overhang. The technical term means the distance past the spindle that the stylus tip will reside with the arm rotated so that it's exactly over the spindle. Don't try this, though, as most arms aren't designed to rotate that far. There are a number of tools, mostly called cartridge alignment protractors, that aid in this.
Pivoted arms, due to their design, can only perfectly aligned at two points on the record. These are generally called null points. Linear tracking arms when properly adjusted, have an overhang of zero and will be properly aligned across the entire record's surface. Which two points you choose as null point will affect some of the distortions inherent in pivoted arms. The actual choice of the exact points requires that you estimate the distance from the spindle that your average record end. Fortunately, you don't really have to decide, as all of the cartridge alignment protractors are designed with pre-chosen null points. Among the 6 different tools I've used for this purpose, there hasn't been what I'd consider a significant audible when using any one than another. It seems far more critical that you align as precisely as possible using whichever tool you choose.

Zenith

The next parameter is called the zenith. This refers to the horizontal rotation of the stylus relative to the groove path. (See figure 1). Most cartridge alignment tools give you some method of determining zenith. Some align the sides of the cartridge body to a parallel lines, and others have a line traced which is meant to line up with the cantilever. The first method makes the assumption that you have straight parallel sided on your cartridge, and that the cantilever is perfectly parallel to those sides. Both methods assume that the long axis of the actual stylus is perfectly perpendicular to the cantilever. Look for an alignment tool that will allow you to check the setting relative to the cantilever, not the cartridge body. That method makes fewer assumptions, and will allow a more precise setting. To properly set zenith, you will generally have to rotate the cartridge body relative to the cartridge mounting holes in either the armtube or headshell.
Improperly set zenith will result in a small but possibly significant phase shift between channels. In a linear tracking arm, this error will be present equally across the entire record, and is therefore more critical to set precisely. Due to the nature of pivoted arms, there is going to be some error in zenith everywhere but the null points, and as such very small errors in zenith are less critical than on linear arms. You should still strive for as precise an alignment as possible whichever arm type you have. The most precise method of setting the zenith is to use a dual trace oscilloscope, using one trace for each channel on a mono track. When the zenith is just right, the two sine waves showing on the screen will be exactly in sync.

Figure 1
View of stylus in groove from top. The stylus footprint is in red, and the black line is an imaginary line along the long axis of the stylus. Ideally, this axis should be perfectly perpendicular to the groove walls at the point of contact. The arrows represent the plane in which zenith can be adjusted. This is usually done by twisting the cartridge body in the horizontal plane relative to the tonearm tube

Azimuth

The next parameter is called azimuth, which is the rotation from vertical of the stylus when the cartridge is viewed from the front. This parameter helps define proper channel balance and interchannel crosstalk. (See Figure 2) Basic approximations can be done by eye, however, not all tonearms will allow azimuth adjustments. If you're tonearm does not allow these adjustments, the use of a shim, either under the tonearm base or between the cartridge and arm are your onyl options. The method generally prefered is to shim the base of the tonearm, as shimming the cartridge may affect it's coupling to the headshell or arm, and additionally may have an undesireable affect on the effective mass of the cartridge/arm combination.
Azimuth is generally set during the alignment process. If you have chosen a cartridge alignment protractor that allows you to align the cantilever itself, that line can also be used to try to judge azimuth. The most accurate method is to use tracks from a test record that have modulation in only one channel. For example, you play the track that is right channel only, and measure the voltage level in the left channel. Then you play the left channel only track and measure the voltage level in the right channel. The goal is to adjust the azimuth so that the levels of crosstalk are equal in both channels. Poorly adjusted azimuth can result in shifting of the soundstage and less than precise imaging.

Figure 2
Front view of the cartridge. The gray line is an imaginary line from the tip of the stylus, up the cantilever. This line should be perfectly perpendicular to the record surface in a perfectly made cartridge.

Vertical Tracking Angle (VTA)

Perhaps the most misunderstood and misused term is VTA. Most people, dealers included, think that this term means the angle of the cantilever to the record surface. They’re only partially correct. It actually refers to the plane of motion (or more correctly arc) of the stylus in the vertical plane (see figure 3). The VTA is in and of itself not a critical factor, but is tied to the factor that is far more critical, the SRA, which will be discussed next. While industry standard for cutting head VTA is 20 degrees, +/- 5 degrees, each cartridge is designed to function at or close to a particular VTA, most commonly 20 degrees. This is because VTA and SRA are not separable, and as you'll see in the following section, it is very important with line contact and eliptical styli to match as closely as possible to the SRA of the cutting head.

Figure 3
Figure 1. Side view of cartridge and tonearm tube. Note the arc denotes the "VTA" or plane of vertical motion.>
 

Stylus Rake Angle (SRA)

SRA or Stylus Rake Angle, is a critical parameter that's associated with VTA, and due to the necessities of cartridge design, is firmly bound to the VTA. This is the angle of the stylus itself relative to a line perpendicular to the record. It is critical that the SRA match that of the cutting head as exactly as possible. Unless this happens, not all of the information available in the groove can be read. In a way it's analogous to a tape head alignment. Since there's no effective way to directly or indeirectly measure the actual SRA, as we can't know for sure what the cutting head's SRA was, it is necessary to determine the best setting by ear.
To get the absolute best sound from a record, it is necessary with line contact style and eliptical cartridges to adjust the SRA for each record to match the SRA of the cutting head. If this is done, most arms will require tracking force adjustments also (see the next section on VTF). Since most arms do not allow for easy and repeatable micro-adjustments of VTA/SRA, and most audiophiles want to enjoy music instead of adjusting equipment, a compromise position is usually chosen. Choosing this compromise usually involves setting SRA/VTA using an average thickness record and not worrying about it. Other methods might include using several different thickneses of records and finding the best compromise, or using platter mats of different thickness, or even using your favorite record label as the reference disc. All methods require some kind of tradeoffs.
The General effects of improperly set VTA/SRA will be in the frequency extremes. If the setting is too high (where the arm slopes down toward the cartridge), the bass region generally gets muddy and thick, and the cartridge may sound more bright and strident than it should. Setting it too low (where the arm slopes down from the cartridge) may result in overly taught bass, and a reduction in upper frequency information. When you get the VTA/SRA right, the sound seems to lock in and become very focused, and the music will just sound right. This has been reported by many listeners to happen over a very small range of adjustments. In other words, you may make adjustments over a relatively large range with little change in sound character, and then you'll hit a small range where minute changes are readily audible. My personal theory is that this happens when the SRA is almost precisely aligned to that of the cutting head, although there are other theories out there as to the lock in effect.

Figure 4
Figure 4. Side view of cantelever and stylus with imaginary line extending from stylus tip along it's vertical axis. Note the arrow denotes deviation from 0 degrees SRA.

Vertical Tracking Force (VTF)

The fifth parameter is the Vertical Tracking Force, or VTF. It's also commonly referred to as tracking force and tracking weight. It is defined as the weight measured at the stylus tip, usually expressed in grams. This parameter is more critical than most people might realize, and some cartridges are especially sensitive to this setting.
There are two main issues affected by the VTF. The first is the alignment of the magnetic motor system. There are three major methods of creating a magnetic phono cartridge: the moving coil, the moving magnet, and the moving iron. I'll explain each briefly before I go on.
The moving coil cartridge has two coils attached to the actual cantilever, usually at a 45 degree angle (to be perpendicular to the groove wall it is meant to reproduce). These coils, when moved within a fixed magnetic field, generate the voltage that represents the musical signal. The moving magnet cartrige has the magnets attached to the canelever. The coils are fixed, and moving the magnets relative to the coils generates the voltage in the coils required for the musical signal. The last method, moving iron, or sometimes called moving flux, uses a magnetic metal, usually iron, which is attached to the cantilever. In this type of cartridge, both the magnet and the coils are fixed in place, and voltage is generated in the coils by moving the iron which modulates the magnetic field picked up by the coil to generate the voltage.
As you can see from the descriptions above, each method relies on varying the magnetic field in some way to induce voltage in the coils. In each case, it is important to the proper operation of the cartridge that the magnetic generator be properly aligned. In the case of a moving coil cartridge, you want the coils centered in the magnetic field when there's not groove modulation. Likewise the magnets should be centered relative to the fixed coils in a moving magnet, and the iron centered in the magnetic field between the coil and magnet for moving iron cartridges.
VTF also affects the cartridge suspension. The cartridge suspension, like a car suspension, is designed to operate optimally within a certain "load". Overload your car, and when you go over a bump, you can bottom out the suspension. If it's not loaded enough, you get an overly bouncy ride. In a way, your cartridge suspension acts in the same way. Either way, don't go by tracking ability alone to set your tracking force. It is important to understand that the goal is to both align the magnetic generator and to ensure the proper operation of the suspension. Properly set VTF may not result in the absolute best tracking.

Anti-skating Force

The last parameter this article will discuss is anti-skate, or Horizontal Tracking Force (HTF). This is only necessary on pivoted arms, and is designed to counter the force drawing the arm towards the center of the record due to the offset of the cartridge relative to the armtube. This is most often set using a tracking test on a test record. While playing a mono track or series of mono tracks with increasing groove modulation, you listen for the effects of mistracking in either channel. Adjustments are then made to try to get both channels to mistrack at the same level, hopfully much higher than the peak level of any record in your collection.
If the anti-skating force is improperly set, it will cause some of the same effects as misadjusted VTF. The suspension of the cartridge, this time in the horizontal plane, will not be properly loaded. The magnetic generator will also be misaligned. Additionally, if left misadjusted for a long period of use, the canelever can become permanently skewed toward one side.
Finally, the last three parameters, SRA/VTA, tracking force, and anti-skate will each affect each other. Changes in SRA/VTA will change tracking force, and changes in tracking force will cause changes in SRA/VTA. And any time you adjust tracking force, you will need to verify anti-skating force.


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