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The following is a method for setting cartridge alignment using a two point protractor.This method is applicable to tone-arms with slots for positioning the cartridge fixing bolts. For tone-arms with adjustable bases, and fixed head-shells (such as SME arms), a different procedure is required. This article is an update of the instructions I posted last year on the same subject. The old instructions were a little loose in terminology and contained some errors. In particular, I have changed the procedure in step 6 slightly. I hope it is more clear now. The old article can be found at:
http://www.AudioAsylum.com/audio/vinyl/messages/38880.html
Preamble.
A two point protractor is a device which fits over the spindle; two grids are marked on the surface of the protractor, and these grids are positioned at different radii from the spindle. The distances from the centres of the grids, to the spindle are referred to as the null radii. When alignment is achieved, using a two point protractor, the tracking error will be zero at both null radii. A two point protractor will, in principle, give optimum alignment, as it sets the cartridge so that the tracking error of the stylus is zero at two points directly. There are many other protractors and jigs which use indirect method to set alignment. Examples include, the MoFi Geodisk, or proprietary jigs which are attached to the head-shell, and require alignment of the stylus relative to some reference point in the jig. For those who favour the indirect methods (which are often much faster than a two point protractor), they should check at least once, that their technique is correct using a two point protractor.
This note, does not make any recommendation on the optimum null radii for minimising tracking distortion. There is a lot of data on this subject, however it is a separate issue. The user can select the null radii of their choice. In particular, the user may choose null radii which are different to those selected by the tone-arm designer. In this case, the result, after alignment, may be that the cartridge is skewed in the head-shell. Alternatively the user may wish to set the preferred alignment of the tone-arm designer, in that case it is necessary to find out the chosen null radii of the tone-arm, and then to get a two point protractor with grids at these distances.
A commonly chosen pair of null radii are 66.00 mm and 120.89 mm. These values are based on minimising the maximum distortion between the IEC standard groove radii of 60.325mm and 146.05mm. Regardless of the choice of null radii, the alignment method is the same.
As an example, I have used the numbers 66.0 mm and 120.9 mm for the distances of the grids from the record spindle in the text which follows.
Definitions
Effective length is the distance from the centre of the tone-arm pivot to the stylus tip.
Offset angle is the angle between a projection of the stylus cantilever on the record surface, and a line from the stylus tip to the centre of the arm pivot.
Tracking error, is the angle between a projection of the stylus cantilever on the record surface and a tangent to the record groove at that point on the record surface.
For more info on terms and definitions,see http://www.AudioAsylum.com/audio/vinyl/messages/37478.html
Method.
1 Initially set the cartridge, so that its fixing bolts are roughly in the centre of the head-shell slots.
2 Place the protractor over the platter spindle.
3 Position the stylus on the outer alignment point (at the centre of the outer grid at 120.9mm).
4 Adjust the angle of the cartridge in the head-shell until the cartridge is squared up on the grid. The cantilever should be parallel to the centre line on the grid.
5 Move the tone-arm so that the stylus is at the centre of the inner alignment point (at the centre of the inner grid at 66.0 mm). It is necessary to adjust the angle of the protractor on the turntable to do this. If the cartridge is squared up with the grid, then the alignment is complete. However if the cartridge is not squared up with the inner grid, then determine which direction the cartridge should be rotated (when viewed from above) in order to achieve alignment on the inner grid. Do not make any adjustment at this point. Case 1: if correct alignment requires an anti-clockwise rotation of the cartridge, then the effective length is too short. Case 2: if correct alignment requires a clockwise rotation of the cartridge, then the effective length is too long.
6 Case 1:
Increase the effective length of the arm, by sliding the cartridge forwards in the head-shell. The correct effective length can be set by sliding the cartridge forwards by twice the apparent correction necessary to align the cartridge on the inner grid. For example, if sliding the cartridge forward by 1mm in the head-shell, will give alignment on the inner grid, at this stage, then the correct adjustment is to slide the cartridge forwards by 2mm. If possible do not adjust the angle of the cartridge in the head-shell while adjusting the effective length. After completing this step, the tracking error should be about the same as before, but with the cartridge skewed in the opposite direction.
Case 2:
Decrease the effective length of the arm, by sliding the cartridge backwards in the head-shell. The correct effective length can be set by sliding the cartridge backwards by twice the apparent correction required to align the cartridge on the inner grid. For example if sliding the cartridge back by 1mm, gives alignment on the inner grid, then the correct adjustment, at this stage, is to slide the cartridge backwards by 2mm. If possible do not adjust the angle of the cartridge in the head-shell while adjusting the effective length. After completing this step, the tracking error should be about the same as before, but with the cartridge skewed in the opposite direction.
7 Move the tone-arm back and reposition the protractor, so that the stylus falls on the outer alignment grid. Adjust the angle of the cartridge again so that the cartridge is squared up on the grid.
8 Repeat steps 5, 6 and 7 until alignment is achieved at both points.
Notes
The above method should achieve alignment in a few iterations of steps 5,6,7, and 8. When the procedure is complete, the cartridge and cantilever should be squared up with the grid, when the stylus is positioned at either alignment point.
This method of setting offset angle at the outer point, and effective length at the inner point is best, as errors in effective length are more apparent at the inner grid.
It is best to disengage the drive belt, and if possible to lock the suspension of floating sub-chassis turntables.
With a conventional two point protractor, it is always necessary to adjust the position of the protractor on the platter when switching from the outer grid to the inner grid.
If the cartridge cannot be aligned on both grids, due to insufficient movement in the head-shell, then the best compromise is to slide the cartridge as far as possible in the right direction, and set the angle on the inner grid. In this case reduced tracking distortion will occur at the inner grooves, in exchange for increased tracking distortion at the outer grooves. It is the opinion of the author, that increased tracking distortion at the outer grooves is a better compromise, as other distortions are more significant at the inner grooves.
After completing a successful alignment once, the previous alignment can be used to set the effective length when changing to a new cartridge. With the old cartridge still in place, position the protractor on the platter and line the stylus tip up at the centre of the outer grid. Then clamp the platter and protractor in place. Set the new cartridge so that it is aligned with the either grid; alignment at the other grid is guaranteed.
Brian Kearns.
Follow Ups:
I have twoOne copies the geometry of the Mo Fi Geo;
The other copies the trammel in a track arragement.
All numbers were obtained from the patents.
Both use sight lines and one point (point the sight line at the arm pivot point, align the cart).You will need Microsoft Publisher to view and print them, and you will have to make the spindle hole (crosshairs are provided).
Your printer needs to be up to snuff; I'm using a fairly budgety Lexmark, but it's 1200 dpi, and the MSoft Publisher's "hairline"s come out really fine.Incidentally, these use the same pair of null points (!)
but one uses the inner and one the outer.You will not doubt the geometry involved, because after you get the cart right with one (assuming your arm/arm placement will let you), switch templates and it will already be right. Geometry is cool after all.
I used to use a two point and rotate, tweaking the arm length, going back and forth. But the arm length, overhang, and offset angle are all derivations of each other (check out the patents). Once you know what null points you want (assuming Baerwald for these templates), the sight line rotates based on your spindle-to pivot length, automatically giving you the correct overhang and offset.
I haven't touched a 2 point since I made these.
I will try to get these uploaded to my site and post the address later.
I know there are a lot of free templates floating around, but I really am happy with/proud of these.
Mike
Thanks, Brian, also for clearing up the terminology. You're right: the tracking error (cartridge axis/groove tangent) is the difference between the lateral tracking angle (effective length/groove tangent) and the offset angle (effective length/cartridge axis). I've been thinking of it as the tracking angle.I had some difficulty following you when you say that you can set cartridge angle at the inner point when the headshell slots aren't long enough. How can you know that the protractor is correctly rotated relative to the arm pivot when you don't get it right at both points and grids?
Thanks again!
"I had some difficulty following you when you say that you can set cartridge angle at the inner point when the headshell slots aren't long enough. How can you know that the protractor is correctly rotated relative to the arm pivot when you don't get it right at both points and grids?"
You are correct, in that the protractor will be at the wrong angle, and this will result in the wrong effective length. But if you set the offset angle at the inner grid, then you can be sure that the tracking error is zero at that point. The effect of setting up a cartridge this way, will be to change the location of the outer null point.For example, the slots in many arms don't go far enough forward to set alignment for null radii at 66.0mm and 120.9 mm. In that case you have to opt for a compromise. If you extend the cartridge fully forwards, and align on the inner grid, then the inner null radius will be at 66.0mm, and the outer null radius will be at some value less than 120.9 mm. This will give a slightly higher distortion at the edge of the record, but a lower distortion between the null radii and at the inner groove radius.
If you experiment with the values in John Elison's Spreadsheet, (available free at http://www.enjoythemusic.com) you can see the effect of this kind of change on tracking distortion.
Best regards,
Brian
.
Don't most (all?) protractors, like my Cart-a-lign, allow you first to set the overhang by positioning the protractor over the spindle in a particular way? That is, at your Step 2.Once you set the overhang, you use the alignment point just to set the angle. If you've done it right it will be correct for the other alignmnt point. That checks your work. Or you need to make what should be only a small adjustment.
This simplifies the procedure considerably. If your protractor doesn't let you set the overhang first with some sort of precision I think you should get another.
Of course, as you note at the end, if you've already determined the overhang, you can use it for your next cartridge.
To set overhang directly, the stylus needs to be placed on a the line through centre of the arm pivot and the platter spindle. Many arms do not allow enough movement to do this.Moreover, for an arm with head-shell slots, to set overhang directly, you need to know the mounting distance (ie distance from platter spindle to centre of arm pivot). This should be measured, and the overhang calculated using the formula below. The manufacturers data is often (usually) unreliable.
Overhang = Sqrt(Lm^2 + N1*N2) - Lm ( ^2 means squared, * means multiplied by)
N1, N2 = inner and outer null radii respectively
Lm = mounting distance.
But you are right, if it is possible to set overhang firstly, it makes things easier, as there is no need to go back and forth between alignment points.
Best regards,
Brian
Brian
I think what you mean to say is that to set the overhang _according to its definition_ you need to place the stylus on the other side of the spindle.As I wrote earlier, it is set _directly_ by many devices, including the $15 dollar one referred to, without needing to place the stylus in any unusual position. Then, as you correctly note, it makes things easier and you don't have to iterate ad nauseum.
One of the reasons for bothering to enter this thread as I did was because it seems to me that all of this unnecessary and time consuming fiddling one has to do to use the so called "two point" method is the kind of thing that makes us look a bit strange and keeps people away from vinyl.
Sorry I misunderstood, I thought you were referring to another kind of tool, that has concentric circles marked on it, and fits over the spindle. That kind of tool can be used to set overhang directly.The tool you mention is based on the Geodisk. The Geodisk sets the arm effective length by using the arm pivot as a reference point. In my opinion that is an indirect method, as it relies on some preliminary adjustment. We could argue over semantics, but I think I made it clear enough in the original post what I meant by setting alignment directly. The merits of the Geodisk versus two point tools has been discussed before, check the archives. A full description of the Geodisk can be found in US patent # 4,326,283.
"One of the reasons for bothering to enter this thread as I did….."
Do me a favour: the next time you are loath to enter a thread I have started, then follow your instinct, and don't bother.
"the kind of thing that makes us look a bit strange and keeps people away from vinyl…………"
That is your hang-up, not mine; speak for yourself.
My first post was an attempt to explain clearly how to set up a cartridge using a two point alignment tool. If you have something to add on the subject then good for you, but if you want to make flippant comments, then why don't you find some other internet site to surf.
Best regards,
Brian
In keeping with the thread the "indirect" methods of setting overhang and angle like those jigs included with the Graham, Dynavector and some other arms allow for much easier and far more accurate setting of the cartridge alignment. They are in fact more direct and accurate if the instructions are followed which also set the pivot to spindle distance prior to directly setting overhang.
Sounds like a good idea, particularly the jig to set the correct mounting distance. It is a pity every arm manufacturer doesn't do the same.Best regards,
Brian
If you want to use your obvious geometrical skills and teach people to set their alignment with one hand tied behind their back, I guess that's OK, even if I don't understand why.I never thought that adjusting cartridge alignment is an end in itself, only a means to better performance. Different strokes, I guess.
What I mean by "direct" is that you set it once, and it's done! Not an iterative process that can go on and on.
The toolS I mentioned are based on geometry, as is the Geodisk I'm sure.
I'm sorry if you are offended; no offense was intended.
I entered the thread to demonstrate that there is a fast and easy way to do it as opposed to a slow and difficult one. New people discover this site all the time so it might be useful, I thought, that people should know there is an easier way so as not to be discouraged from aligning properly.
I shall stay on this site, thank you, and since each author is identified in the listing, please feel free not to read my posts.
And best regards to yourself,
Mel
Brian's piece covered two-point protractors. As he said, there are a few protractors, like Geo-disc, the Soundtractor, and the one you refer to, which do it in other ways. The one you refer to in your other posting (which certainly sounds like a good design) has two null points in addition to the sight line, which is a rare asset. The procedure is speeded up, while you still can check your alignment using the same protractor. It becomes a two-point protractor and a "sighting" protractor at the same time. I have a cardboard protractor called Polaris Plus which aspires to the same thing, but for some mysterious reason, the sight line is incorrectly positioned relative to the two null points (or the outer point is incorrectly positioned relative to the inner point and the line). Those who have (or make on the computer) a cardboard or paper two-point protractor might be able to draw a sight line themselves at the linear offset. The line should be placed (accurately, of course) at a distance from the stylus mark which equals the average of the two null points, e.g. 93.4 mm from the stylus mark in the case of points of 66 and 120.9 mm, and perpendicular to the axis from the spindle to the stylus mark (i.e. parallel to the gridlines). If two spindle holes and one grid are used, the line will even be valid for measurement at both points. I don't know the story behind the Cart-A-Lign line, which I think goes from the spindle to the arm pivot, but I suppose it's all right!The two-point protractor is more common and used by many, and will continue to be used by many. When the cartridge is actually aligned to both points and grids accurately, the cartridge is necessarily correctly aligned. By using both points and grids, the protractor achieves the same thing as other protractors achieve by other means. An advantage is that you are not dependent on having something to sight at in a precise way (so overhang and offset angle are instead set "directly", as Brian calls it), but the clear disadvantage is that it's *@&X! fiddly and can take a lot of time and patience. Brian attemps to help people by a method he believes can make this protractor a bit more effective.
The ultimate in convenience seems to be the arc-type protractor, such as the Wallytractor. Its only disadvantage is that it's made for one specific effective length.
> > I have a cardboard protractor called Polaris Plus which aspires to the same thing, but for some mysterious reason, the sight line is incorrectly positioned relative to the two null points. < <You'd think if the one thing you did was make cardboard protractors printed with critically positioned lines, you'd at least make sure you put the lines in the right place. I'm shocked! Have you called Martin Bastin about this? His name and phone number are printed right on the protractor.
Is his placement of the sight line at least consistent with a logical set of null points (that you disagree with), or did he just plain screw up?
My sight line is 92.0 mm away from the stylus mark when using Hole A. Are you saying I should just draw a new line parallel to the old one and make it 93.4 mm from the stylus mark?
Thank you, Helge!
Hi,First off, the 93.4 mm was tied to the example I used with null points of 66 and 120.9 mm; the Polaris Plus uses different null points, so the sight line should not be placed at 93.4.
Rulers differ slightly, but by comparing all of my rulers and measuring as carefully as I'm able to, I've come to 91.9 mm for the sight line, 63.9 mm for the inner point, and 120.5 mm for the outer. These figures must still be taken as approximations, as the measurements are not exactly done with laser or something. Also, the spindle holes may have been cut very slightly differently from sample to sample.
The instructions say that you can verify your alignment by using the outer point (i.e. spindle hole C) "if you wish". Every time I do this, the stylus will not hit the centre of the point, but is slightly too far forward. Maybe that's okay in practice if you know about it beforehand!
As far as I can see (and Brian Kearns believes the same thing), the sight line should be placed smack dab in the middle between the two spindle holes, which you can easily see that it isn't; it's closer to A than to C. A line like this is located at the so-called linear offset, which is the product of the effective length of the arm (no matter what this length is) and the sine of the offset angle (whatever that trigonometric stuff means). It also equals the average of the two null points.
With an inner point of 63.9 and a line on 91.9, the outer null point should be expected to be located at 119.9 mm. With an outer point of 120.5 and a line on 91.9, the inner point should be expected to be located at 61.9. With null points of 63.9 and 120.5, the line should be expected to be placed at 92.2. This assumes that the line is to be placed at the average of the two points (you just isolate X in the equations by multiplying and subtracting).
There are in principle three options:
Align as per the instructions, and accept that the real outer null point is at 119.9 (I have done this so far).
Align at the outer point instead of the inner, and accept that the true inner point is at 61.9.
Draw a new line (completely parallel) and use both points as they are.I'm posting from home, and need to look at some Excel stuff on my workplace machine..., but the first option seems to be the best of these. As Martin Bastin says, the inner point is the most critical (because of the notorious inner-groove distortion), so I wouldn't use alternative two, although this is what Geo-Disc does. Also, an inner point of 61.9 seems to be farther inwards that I would prefer. And in particular, the null points would be way too widely spaced. The latter point is possibly valid for the third option too. Bastin does write that the outer point is less accurate, but I don't know if he is referring to the same thing as we do. If so, why did he place the outer null point where he did?
An inner point of about 63.9 is not "incorrect" anyway; it gives you a good performance on the inner grooves, but, relative to the 66/120.9-alignment, at the cost of more distortion elsewhere. It's a kind of compromise between the Rega-style alignment and the 66/120.9-alignment. I will see what the outer point is doing to this, what it adds to the distortion pattern.
I have a (new, cleaned) record with a more sibilant vocalist than usual, and the sibilance grows during the middle portion of the record, and may seem to reach a high somewhere around the middle peak distortion point, while I think it's better at the very end (about 65 mm). I'm thinking that the balance could be too much in favour of the innermost grooves, but that's speculation for now.
I haven't called Martin Bastin.
The Polaris Plus is really excellent in many ways: The stylus mark is a small circle, not a black dot or a cross. There are plenty of gridlines, including crosslines for aligning the front or back of the cartridge. There is a line for aligning the cantilever, and a nice touch is that this line is positioned slightly to the side of the circle; in this way, you can view the line behind *and along* the cantilever. The infamous sight line is fairly thick, just right for the pillar of my Nottingham Interspace arm. It's also long.
OK, Helge, I'm with the program now. The only thing I didn't follow was:"With an outer point of 120.5 and a line on 91.9, the inner point should be expected to be located at 61.9"
I assume it's just a typo, but "61.9" should be "63.3," right?
In any case, I see now that this is just as much art as science, and that the null points you choose can be tweaked according to where you want the most (and least) distortion. (It is odd though that the Polaris gauge wasn't made internally consistent by drawing the sight line right in the middle of Holes A and C.)
,k
Now I have used the spreadsheet... Following is a table for distortion based on a calculator made by Brian Kearns. See previous message.First is distortion at 62 mm (arbitrarily chosen to represent the inner grooves), then maximum distortion between the null points, then distortion at the outermost groove (146 mm).
1. Hole A + old sight line: 0.19, 0.66, 0.66
2. Holes A + B + new line: 0.19, 0.67, 0.64
3. Hole B + old line: 0.01, 0.74, 0.66
4. "Standard" 66 + 120.9 mm: 0.4, 0.61, 0.62
5. Hi-Fi News: 0.29, 0.63, 0.65
6. Enjoy the Music: 0.89, 0.6, 0.45Following the instructions (no. 1) will yield a slightly lower distortion between the null points than drawing a new sight line right between the spindle holes (no. 2). On the other hand, distortion in the outer groove (146 mm) will be slightly higher. Many records start around 145 mm, and the middle grooves are more important than the outer grooves (due to more compressed undulations on the groove walls), so I'd say that following the instructions is the best. This
alignment is also okay, I guess, although the outer null point should ideally be slightly further inwards, at least at 119.6 or 119.7 mm. Option no. 3 (using the sight line with hole C instead of hole A) gives a far too high distortion on the middle grooves.It can also be seen that the alignment with null points of 66 and 120.9 gives a higher distortion on the inner grooves, but lower on the middle grooves. I have also joined the values for the Hi-Fi News protractor (not at all bad, IMO) and the Enjoy the Music-protractor (favours the outer grooves!).
Mel, life would be easier if all protractors allowed you to set overhang first independent of angle but most don´t. The Wallytractor does and apparently the one you have most most others don´t.And yes you are right overhang is everything and the classic two point protractor does not make it an easy task.
For the grand sum of $15 you can currently buy a device that not only lets you set the overhang first, but provides a mirrored surface so that you can line up the cantilever in the preferred manner.Look at http://www.turntablebasics.com/
(a site with which I have no connection-this device has been favorably reviewed by an inmate-do a search)Like many such devices including the late lamented Cart-a-lign, lining it up so that it points to the tonearm pivot can be difficult at first. I have always managed by shining a strong penlight at the pivot from over the TT spindle. If the pivot is surrounded by a cylinder, that will provide a thin vertical bright line which makes it easier to aim toward by lining up the "line up" line on the protractor and the pivot.
Alternatively you can glue a thin piece of wood (like 1/8" ply), plastic or cardboard stock cut long an narrow so that it reaches (or almost reaches the pivot) and extend the "line up" line almost to the pivot.
All of this is easier to do than to read about, and makes setting up the overhang almost instantaneous and adjusting the cartridge tracing angle a breeze.
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