Geometry: A Brief Guide to Using an Arc Protractor
Basic geometry for a pivoted tonearm involves setting the pivot to spindle distance, the effective length (for tonearms which allow the cartridge to be moved fore and aft in the headshell), and the offset angle. The term “overhang” is used to express how much longer the tonearm’s effective length is than the pivot to spindle distance. For example, a tonearm with an effective length of 250mm and a 233.5mm pivot to spindle specification has an overhang of 16.5mm.
The most accurate tool for setting up the basic geometry is an arc protractor. It magnifies small errors, which can be frustrating, but ultimately results in an audibly superior setup.
Important note: In the instructions below, we use terms like rotate the platter, rotate the protractor, move the arm, etc. All of these actions are performed with your arm cued up, so you do not damage your cantilever.
Note: Refer to the notes in the tracking force and VTA/SRA sections – to begin this process with at least an approximation of your final VTA/SRA setting as well as a tracking force in the midrange of the manufacturer’s recommendations. These settings can subtly affect the effective length measured by the protractor. If after fine tuning of VTA/SRA or tracking force, you find that you’ve made dramatic changes from your starting position, re-check the geometry. Similarly, if you’ve made significant changes to the effective length in this procedure (repositioning the cartridge fore/aft in the headshell), re-check your tracking force.
The process: rough setting the parameters
The purpose of these preliminary settings is to minimize the number of iterations you go through in order to achieve a precise adjustment. The better your starting point is, the faster your fine adjustments will be.
Rough Setting Pivot-Spindle Distance:
Most turntables do not allow you to adjust your pivot to spindle distance. Galibiers allow you to make fine, repeatable, vernier adjustments to the pivot-spindle. For other turntables, consult your owner’s manual. Also, know that many tonearms have (at a minimum) some intentional play built into their mounts to allow for fine correction of pivot-spindle mounting errors. Offset mounts like the Tri-Planar, the Durand Talea, the Schroeder Reference, and others provide some additional flexibility in this regard.
As far as a rough measurement of pivot to spindle distance, you’ll be much more accurate than you think you can be.
Some tonearms provide you with a very accurate method of setting your pivot-spindle distance. The Durand tonearms and the Kuzma 4Point are two of these and your starting point for the fine adjustment will be within approximately .010″.
Assuming that you do not have an arm with such a tool, measuring the pivot to spindle distance with a ruler will work. The trammel provided with the Feickert tool will get you closer. Your arc protractor will of course be used for your final setting.
When using a ruler, the key challenge is maintaining the ruler parallel to the record platter while looking down at the record spindle from a 90 degree angle. On Galibier turntables, the machining pattern on the record spindle (how it reflects light) will help you view the spindle from a 90 degree angle. Some people have adapted a small spool of thread (or a similar spacer), placing it over the record spindle to assist in keeping the ruler parallel to the platter while measuring. You will likely be able to measure to +/-1 1mm, which is fine for this step.
At the tonearm end, measure to the center of the tonearm bearing. On some arms, you’ll have to approximate this. Use whatever clues the arm gives you – machining pattern, bearing bolt, etc. Again, the closer you can measure, the fewer corrections you’ll have to make when working with the arc protractor. Remember! With offset tonearm mounts like the Schroeder Reference, Tri-planar, etc., you are measuring to the center of tonearm bearing and not the center of the mounting collar.
Rough Setting – Cartridge Position in the Headshell:
This step helps you to approximate the effective length. Again, the closer you get here, the less iteration you’ll go through with the arc protractor. When setting this initial position, try to approximate the intended offset angle as well as the fore and aft position of the cartridge. With most headshell designs and cartridges, this is merely setting the cartridge body parallel to the front of the headshell and the mounting screws in the center of the mounting slots.
Some tonearms provide you additional clues as far as the fore and aft location. If you measure the Tri-Planar tonearm, you’ll observe that it’s 250mm effective length results in the stylus being located in plum with the front of the headshell. So, if your cartridge manufacturer provides the offset distance between the stylus and the cartridge mounting screws (average is 9.25mm), you can position the cartridge screws this distance from the front of the front edge of the headshell.
With the Artisan arm (using a Galibier supplied protractor), start by positioning the cartridge bolts about 4mm in front of the center of the slots (longer effective length). This is due to the arm being specified with an alternate geometry. To maintain the specified pivot-spindle distance for people who have mounted their arm on a fixed mount turntable, the protractor was developed around the specified pivot-spindle distance (214mm), and the effective length was calculated accordingly (approx. 4mm longer in order to achieve Baerwaald geometry).
Familiarize yourself with the protractor:
At this point, we recommend placing the protractor on the platter and leaving the tonearm in the cued up position. Rotate the protractor to its approximate location, and verify by swinging the arm across the path of the arc (arm cued up). Adjust the position (rotation) of the protractor until you’re tracking the arc as closely as possible. You’re trying to:
Get a general sense of how small changes affect the relationship between the stylus path and the arc.
Position the arc at your best approximation of where it should be – where you are closest to having the path of the stylus to be concentric with the arc – either in front of, directly over, or behind the arc – it doesn’t matter at this point.
Take a few moments to play with this. Position the stylus on the arc at the record spindle side of the arc and observie how a small rotation of the protractor results in a much bigger difference at the lead-in groove. This understanding is key to reducing your frustration factor.
Once you have a general idea of how the protractor works, you’ll begin the serious work of achieving a correct alignment. Pivot-spindle, overhang, and offset angle are an iterative process:
p-s –> overhang –> offset angle –> p-s –> overhang –> offset angle –> p-s –> overhang –> offset angle.
It’s difficult to get p-s right on the first pass, and only when you get closer with the overhang adjustment will you be able to return to the p-s measurement to be exact. It will likely take you 2 or 3 iterations. Each one gets easier.
Set Pivot-Spindle Distance:
The greatest point of confusion comes determining what to do when you place the stylus down at one end of the arc and find it does not land on the arc at the other end.
Keep reminding yourself to first work to have the stylus trace a concentric arc – either ahead of or behind the arc by the same distance across the arc. If you try to land the stylus on the arc at one end or the other, you will likely get “lost”. Note – at this point, you will be very lucky if you are actually tracing the arc. If you get at all confused, return to your “familiarization” step – swinging the arm over the arc while the arm is cued up.
Once you are tracing a concentric arc to the protractor, your pivot-spindle distance is set.
Adjust Overhang (cartridge position in the headshell):
Throughout these adjustments, keep in mind that small changes at the spindle side of the arc (say 1mm) translate into HUGE changes at the outside/lead-in groove (10-15mm). This will puzzle you until you have that light bulb (aha!) moment. Return to the familiarization step when you get lost.
Start by positioning the protractor so the stylus lands on the line at the lead-in side of the arc.
Check how far off the stylus is at the record spindle end.
Let’s say that in “ii” above, the stylus lands .5mm in front of the arc (too long).
This is counterintuitive – rotate the arc to increase the error at the record spindle side of the arc.
Rotate the platter/arc counterclockwise so the stylus “moves” a slight bit further forward (increasing the error at the record spindle side)! By slight we mean perhaps .1mm. Remember, a .1mm error is magnified at the lead in groove end of the arc.
You should now find that when you swing the tonearm over to the lead-in groove side, that it’s also ahead of the arc. If you’re lucky, you will be the same distance (.6mm) in front of the arc.
Now you know you need to slide the cartridge back by .6mm to “meet” the line.
Repeat these steps until you trace the arc perfectly.
Proceed to setting the offset angle.
Set Offset angle:
Galibier protractors have only one landing point for setting the offset angle. Unlike a two-point (traditional) protractor, only one point is necessary with an arc protractor. The landing point has been positioned near the outer part of the platter because we’ve found it’s easier to site the cartridge at this location.
Remember that you need to keep the protractor positioned so you are tracking the arc when checking/setting this parameter.
After setting the offset angle, repeat setting/verifying the three parameters by returning to pivot-spindle section.
Note – Schroeder, SME Owners:
Arms like the Schroeder Reference series, the DPS, as well as the SME V have mounting holes rather than headshell slots. This means that the effective length cannot be fine-tuned.
This is inconsequential, except in the specification of an arc protractor.
Because there is no industry standard for the forward offset of the stylus from the cartridge mounting bolt holes, the effective length (bearing pivot to stylus distance) of these arms varies with the cartridge.
The impact of this is that an arc protractor for these arms needs to be fabricated for each cartridge.
In order to specify an effective length for your arc protractor, you need two pieces of information:
The design assumption the tonearm manufacturer made in order to specify their nominal effective length. Did they use 9.00mm? 9.25mm? 9.50mm? See below, for Schroeder example. For SME, you need to contact the manufacturer.
The stylus’ forward offset for your cartridge.
This drawing (from Dynavector’s website – opens in new window) shows an 8mm forward offset of the stylus, relative to the mounting bolt holes.
Example: Schroeder Reference/DPS – from the DPS manual:
“Pivot to spindle distance for this arm is 222mm nominally. It depends upon the distance between the cartridge mounting holes and the stylus (should be 9,25mm, varies between 7 and 13mm depending on manufacturer and cart).”
For Schroeder’s tonearms with a 222mm pivot to spindle dimension, the effective length for this cartridge with a 9.25mm offset is 239mm. If you own a Dynavector XV1-s (8.00 mm), then the “new” effective length this tonearm becomes 237.75mm (9.25mm – 8.00mm = 1.25mm which would be subtracted from 239). This would be the length you need to specify for an arc protractor.