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Cartridge Optical Analysis

The purpose of this article is to guide you through the optical analysis test report and provide a clear understanding of what you get for your investment. The report is divided into two main sections: stylus wear evaluation and geometrical analysis of the moving assembly.

The “moving assembly” refers to the diamond stylus and cantilever, as well as their alignment in relation to the cartridge body and/or the generating element. If you’re unfamiliar with these terms, don’t worry! Below is a quick summary of the key components of a phono cartridge and their respective angles to help you follow along.

Vertical Tracking Angle (VTA) –  refers to the angle between the line connecting the cantilever’s pivot point and the stylus contact surface, and the surface of the record.

Stylus Rake Angle (SRA) – is the angle formed between the stylus contact surface and the surface of the record.

Although these are technically different angles, they are interconnected once all parts of the moving assembly are glued together. Adjusting one inevitably influences the other by the same amount.

I prefer to use SRA as the target parameter because it is much easier for the end user to understand and measure. In contrast, determining the actual pivot point of the cantilever for VTA can be extremely challenging, sometimes even impossible, without disassembling the entire cartridge.

It’s also worth noting that these angles will change depending on the Vertical Tracking Force (VTF) your cartridge is set to and whether or not the stylus experiences dynamic loading. By dynamic loading, I mean whether the record is spinning and if there is a drag forces applied to the diamond tip.

Stylus wear evaluation

This topic is a very large can of worms, and I won’t delve into it here and now. You will just have to believe me when I say that there are no exact measurements universally agreed upon to determine how large a contact surface must be to declare it “worn out” for a specific stylus geometry. Instead, let’s focus on the main question: Is it still safe to operate this cartridge with a stylus that has this amount of wear?

In more technical terms, we can rephrase the question as:

  • How far are we from factory specs?
  • Can a contact surface of width X reproduce the highest frequencies near the inner grooves of a record?
  • How much clearance remains between the stylus and the bottom of the groove?

Let’s try to tackle them one by one.

  1. How far are we from factory specs?

Answering this seemingly simple question is far from straightforward. When you check your cartridge specifications, you’ll typically find the stylus described with its major and minor radii—for example, “Elliptical 8 µm x 18 µm (0.3 x 0.7 mil).” However, this does not represent the size of the actual contact surface! These radii refer to the curvature the stylus tip is polished to, which is a very different parameter.

To determine the actual contact surface, you must place the stylus on a real vinyl record, apply the specified tracking force, and then measure the resulting deformation of the groove wall under load. This introduces a level of complexity and variability that makes precise measurements challenging. Fortunately, historical literature provides some “ballpark” contact surface values for common stylus geometries under typical conditions:

Spherical (Conical)

  • Min: ~3.8 μm
  • Max: ~6 μm
Simplest shape with the largest contact area. Limited ability to trace high-frequency details, especially in inner grooves

Elliptical

  • Min: ~2.5 μm
  • Max: ~3.5 μm
Improved tracing ability compared to spherical styli. Smaller contact width allows better high-frequency reproduction.

Hyper Elliptical

  • Min: ~1.5 μm
  • Max: ~3 μm
    More refined than standard elliptical styli, offering better tracking and reduced distortion.

    Shibata

    • Min: ~1 μm
    • Max: ~3 μm
    Designed for quadraphonic records but also excellent for stereo. Tracks high frequencies with precision.

    Line Contact (Fine Line)

    • Min: ~1 μm
    • Max: ~2.5 μm
    Long contact patch reduces groove wear and enhances detail retrieval. Excellent for high-frequency reproduction and inner grooves.

    Micro Ridge (MicroLine)

    • Min: ~0.5 μm
    • Max: ~1.5 μm
    Advanced profile with extreme precision. Closely mimics the shape of the original cutting stylus, providing superior high-frequency performance.

    *All these values above is for minor radii or vertical groove contact length.

       2. Can a contact surface of width X reproduce the highest frequencies near the inner grooves of a record?

    At 20 kHz, the groove wavelength becomes very short, especially near the end of the record where the linear velocity of the groove is at its lowest.

    At 33 1/3 RPM, the linear velocity near the inner groove can drop to approximately 8 cm/s. For a 20 kHz signal, the wavelength in the groove is:

    Wavelength = Linear Velocity Frequency = 8 cm/s 20 , 000 Hz = 0.0004 cm (4 μm) . \text{Wavelength} = \frac{\text{Linear Velocity}}{\text{Frequency}} = \frac{8 \, \text{cm/s}}{20,000 \, \text{Hz}} = 0.0004 \, \text{cm} \, \text{(4 μm)}.

    To reproduce this high frequency without distortion, the stylus contact width must be significantly smaller than half the wavelength (to trace the groove modulations accurately). A typical guideline suggests a maximum contact width of 1-2 μm for optimal high-frequency reproduction.

    We can already see that basic spherical and simple elliptical styli have no chance of accurately reproducing a 20kHz sine wave near the end of the record with any kind of fidelity. Even when new! We can argue endlessly about whether it matters, but that’s a fact.

        3. How much clearance remains between the stylus and the bottom of the groove?

    This is probably the simplest question to answer, as there are strict specifications under which LPs are produced. All currently manufactured LPs adhere to the IEC98-1987 standard. As the date in the standard indicates, this is the latest iteration concerning 30 cm (12-inch) LP disks. It states that Bottom Radius (Maximum) is 8 μm and Groove Angle is 90° within ±5°.

    Putting these numbers on the drawing gives us a clearance of ~2 μm for the smallest conical stylus with a 13 μm radius. And what do you know? That same standard states: “The clearance between the stylus tip and the bottom of the groove shall be 0.002 mm minimum, and the stylus tip shall not touch the groove edges”. 

    In practice, no cutting shop runs their cutting lathe stylus so blunt; hence, the actual clearance is usually somewhat higher. Additionally, more advanced stylus shapes are designed with rounded tips to further increase this clearance. By how much?

    Here are two real photos of an LP groove with a micro-ridge stylus in place. With a new stylus, there is approximately 6 μm of clearance, while a stylus used on about 200 records shows around 4 μm. Why does this distance matter? Beyond the obvious risk of the stylus tip turning into a chisel and scraping the bottom of the groove, reduced clearance significantly increases surface noise. Thus, 2 μm is the absolute limit you want to avoid exceeding. 

    For simplicity sake, we will measure this clearance not from the bottom of the groove but from the point where the 90° groove walls ideally intersect. This sets an absolute limit of less than 5 μm.

    Stylus wear report

    Now that we have a general understanding of what we are measuring and the established limits, we can move on to reviewing a sample test report. In this case, it’s a B&O cartridge equipped with a Shibata stylus and Sapphire cantilever.

    We begin by identifying the dimensions of the contact surfaces, visible as wear patches. Here, we measured contact lines of 18 μm and 15 μm, which are x6 and x5 times larger than new. I think it goes without saying how problematic this is – it’s significantly worse than even the largest new conical stylus.

    While the increased vertical patch size helps maintain contact with the groove walls, the horizontal length of 18 μm limits the stylus’s ability to reproduce higher frequencies. In the inner grooves, the best-case scenario is a maximum of ~11 kHz.

    What’s even more concerning is the risk of damaging grooves with higher modulation and frequency content. The sharp leading edges of such a worn stylus could act as a cutting-tool, stripping away delicate high-frequency information from the record.

    Next, we switch to the front view to double-check the vertical contact lines and identify any differences. In this case, the imbalance is 5 μm, which suggests insufficient anti-skating force was applied. Remember, the skating force tends to push the stylus toward the inner groove (left channel). To counteract this, proper anti-skating adjustment is essential.

    Finally, we measure the distance to the ideal groove bottom, which is found to be 2 μm. Previously, we established that this distance should be no less than 5 μm. At 2 μm, the stylus is essentially bottoming out if the real groove radius is 8 μm.

    With this data in hand, all that’s left is to write a conclusion.

    And that is all the information you will receive if  you order a “Only wear analysis” from the Optical Analysis Service.

    Native SRA & VTA

    However, if you choose to order a full optical inspection, the report will also include the native SRA and VTA angles. By “native,” I mean the angles the cartridge exhibits when mounted on a headshell that is perfectly parallel to the record surface and loaded with its factory-specified vertical tracking force.

    For simpler stylus geometries like Conical or Elliptical, these angles are somewhat less critical to get perfect. However, for any line-contact stylus, getting them right is essential. The difference in sound quality – including spatial resolution, micro details, and even tonal balance – is enormous when SRA/VTA is properly set. It can feel like the difference between a $100 cartridge and a $1,000 one.

    We begin by determining the stylus contact surface angle. Typically, this angle is 90°, but the Shibata stylus is one of the geometries that includes an offset. In this case, the offset is -13°. Next, we measure the angle at which the stylus is glued to the cantilever, which, for this particular example, is 102°.

    Taking it a step further, we add the contact surface offset to the stylus-to-cantilever angle, resulting in a total Contact-to-Cantilever angle (TCC) of 115.12°.I would argue that getting this TCC angle right without high-resolution macro photography is nearly impossible – especially when determining the exact contact surface angle.

    Next, we proceed to mount the cartridge on the test turntable, apply the factory-specified VTF, and measure the static cantilever-to-record surface angle. Here, static refers to measurements taken on a stationary record.

    We observe this angle to be 23.6°. Using our calculated TCC of 115.12°, we subtract the cantilever-to-record surface angle (23.6°) to determine the static SRA, which results in 91.52°. The same process applies for determining the dynamic SRA, which is measured when the record is spinning, and the stylus is affected by the drag force.

    Finally, we calculate the difference between static and dynamic angles, which in this case is 1.07°. This difference allows us to measure static angles and simply subtract 1.07° later to estimate the dynamic SRA. This approach is particularly useful since photographing moving objects is significantly more challenging.

    Dealing with VTA is both simple and tricky at the same time. On one hand, we have a very simple measurement to make – draw a line from the cantilever pivot point to the stylus contact surface and measure the angle to the record surface. Here, the type of contact patch doesn’t matter. Simple, right? But where is that pivot point exactly?

    With simple MM cartridges, we can remove the replaceable stylus and inspect it easily. But what about all the other cartridges with closed systems? That’s the tricky part. Fortunately, I know the insides of these B&O cartridges pretty well by now, so this measurement is straightforward for me. We find VTA to be ~26° on average.

    Optimal VTA & SRA

    Again, with a headline like that, I’m risking opening another large can of worms. This is a very broad subject, and we’ll leave it for future articles. For now, we’ll just stick to the general wisdom of keeping the ideal VTA at <22° and the SRA around ~92°.

    Now we adjust the tonearm until we find a happy medium for SRA and VTA. I always prioritize SRA over VTA and aim to introduce a slight positive SRA of 1–2°. Over the years and with many LPs, I’ve found that this approach works really well for me. It may not suit all material or situations perfectly—but this is analog, and we are always aiming for “good enough.”

    In this particular case, we were nearly spot-on when the tonearm (and headshell) was parallel to the record surface, so the adjustments here are negligible.

    Above are pictures showing how cartridge should look when adjusted for optimal SRA and VTA. You might ask, “Why do I need them? They don’t tell me anything I don’t already know.” But they do!

    They serve as a target for you to replicate this optimal cartridge vertical alligment. It doesn’t matter what turntable you have or what tonearm is mounted – you just take your phone, snap a picture, and measure the cantilever angle relative to the record to match the ideal one. Neat, isn’t it?

    You can use any app on your phone, like Angle Meter, or free software on your PC, such as ToupView, to inspect the current angle after making the necessary adjustments. Once you match the angle to the one specified in the report, you can be confident that your SRA and VTA are at their optimal settings.

    To guide you in the right direction, there will be visual instructions on how to proceed with your tonearm’s vertical adjustment, starting from the point where it is parallel to the record surface. The adjustment is shown in degrees, but if you’d like to know the precise millimeter adjustment required for your tonearm, you’ll need to know its effective length. You can then use the calculator below to determine the exact value.

    As you can see, there are several ways to achieve the perfect VTA/SRA. You can set up using the cartridge cantilever as a reference or adjust based on your tonearm. However, at the end of the day, both methods will lead you to the same result.

    And for a final piece of advice on VTA/SRA – always trust your ears. Sometimes even a small adjustment of 2-3 mm can make a noticeable difference. Take time for a proper listening session and don’t be afraid to experiment and extract the very last drop of performance from your setup.

    Azimuth & Zenith Errors

    This is the last part of necessary measurements and adjustments. The azimuth angle refers to the error angle of the stylus relative to the grooves of the vinyl when looking from the front. Ideally, it should be perfectly perpendicular to the grooves, at which point we say the azimuth is 0°.

    The term zenith angle is less commonly used, but it describes the perpendicularity of the stylus contact surfaces relative to the cantilever and it should also be 0°. This is especially important for contact-line stylus shapes, as proper alignment ensures optimal tracking and sound reproduction. But as the saying goes, ‘a picture is worth a thousand words.’

    Unfortunately, finding a cartridge with both of these errors near 0° is a very rare and surprising occasion. This is understandable, as gluing (pressing) the stylus to the cantilever is an extremely delicate and precise job. Having done quite a few re-tippings myself, I know this firsthand.

    For this particular example, we find that the azimuth is at -2° and the zenith at -3.5°, which is fairly typical, and I would consider this a ‘good result.’

    We finish the optical inspection report with the necessary corrections and a visual guide on how to apply them. Again, you can use photographs tools like an Angle Meter or ToupView to make these adjustments.

    And that is all the information you will receive if  you order a “Wear and VTA+SRA analysis” from the Optical Analysis Service.

    Plus all the high-res photos that you can print latter and hang them on the wall near your LP setup. Neat!