|
Audio Asylum Thread Printer Get a view of an entire thread on one page |
For Sale Ads |
118.148.238.39
In Reply to: RE: Wally Tractor Universal... is it worth it? posted by bcowen on September 28, 2016 at 05:41:44
"Linear offset? I don't even know what that is."
It is the basis of tonearm design and alignment.
Imagine a right-angled triangle with the hypotenuse defined by the pivot/stylus distance and the pivot centre and stylus tip being 2 vertices. Then the opposite side is the linear offset (opposite the offset angle).
The linear offset is a constant which can be associated with a desired set of null points and is independent of the tonearm length. For example, if you wish to have 66.1 and 120.9mm then the linear offset will be 93.52mm. If you wish to implement the same null points as a Technics SL1200 (at 58.8 and 113.5), the linear offset is 86.16mm.
Whether you have a 12" arm or 9" or 7" arm (but who would want one that short!), the linear offset can be used to define the exact position of the stylus and the offset angle unique to the arm being aligned. Hence any pivot/spindle distance can be accomodated with a unique solution for overhang and offset whilst still achieving the desired null points.
"However, the whole point of using the protractor is to set the overhang so the stylus tracks the arc "perfectly" at which point the pivot-to-stylus distance will also be perfect."
Yes, that is an obvious statement. However, your argument is predicated on an assumption that the pivot/spindle distance is known "exactly" and that the printer was correctly scaled in both axes to give "perfect" dimensions, the spindle hole on the protractor was a perfect snug fit around the spindle with no slack and doesn't shift during use; as you swing the protractor back and forth to set overhang you risk enlarging the spindle hole.
Do you see where I'm going with this?
There is no such thing as an "exact" measurement. Everything has a tolerance and all you have done is introduce an additional uncertainty as you attempt to find a solution that minimises the deviation from the arc.
"I have absolutely no problem getting the stylus to track the arc from end to end with a high degree of accuracy."
Good for you! It is highly unlikely that you hit the arc perfectly (which as mentioned before assumes the pivot/spindle distance was perfect as well) except by sheer luck. It is very easy to prove this to yourself with a compass and drawing two circles on a line representing the effective length; the first being the correct radius for the arc and the second such that the second circle radius hits the end of the line representing the stylus tip but with a shifted "pivot" position on the line. Obviously the larger the radii the smaller the deviation, but if you do small circles to magnify arc differences, you will have a better understanding of the errors.
Now the REAL question is whether any of this really matters? Probably not....
:)
Regards Anthony
"Beauty is Truth, Truth Beauty.." Keats
Follow Ups:
"Do you see where I'm going with this?"
No, not really. Appears that sight-lining of the pivot is no longer important or a source of error, so let's create some new issues maybe? The protractors I've used were made using polycarbonate or some other hard plastic as the substrate, so enlarging the spindle hole would be a herculean task using only the near-frictionless rotation on the spindle. The pivot-to-spindle distance is specified by the tonearm manufacturer, or can be easily and accurately calculated from the arm's effective length. And it can be verified easily and accurately at the point of use with a steel ruler with metric gradations. You make it sound like these parameters are either impossible to achieve or require a team of NASA scientists to get right.
"It is highly unlikely that you hit the arc perfectly (which as mentioned before assumes the pivot/spindle distance was perfect as well) except by sheer luck."
Seriously? What's highly unlikely here is that you've ever used an arc protractor.
If you don't like the arc-type protractor, that's fine. There are a number of different protractors that enable proper cartridge alignment, and the arc-type is only one. Personal preference also comes into play, as some will find the arc easy to use and some won't. I simply take issue with ascribing faults to it that either don't exist or aren't true.
"The pivot-to-spindle distance is specified by the tonearm manufacturer, or can be easily and accurately calculated from the arm's effective length. "
Could you remind me again how far off you said your Nottingham arm was?? 6mm wasn't it?!
I see where I'm going wrong.... use the specifications.....calculate from arm's effective length....
LOL!!
Regards Anthony
"Beauty is Truth, Truth Beauty.." Keats
Yes, the Nottingham was 6mm off when it arrived from the factory. Perhaps someone wasn't paying attention when it was assembled, perhaps (and more probable) was that it moved during shipment. So what? I reset the pivot-to-spindle at the specified 210mm and went on with life. What this has to do with the use of an arc protractor escapes me -- the alignment would have been off with any protractor if the p-to-s distance hadn't been corrected.
I'm done here, but feel free to change the argument a few more times and entertain yourself.
I've measured Nottinghams way off, and some other, more well known, significantly more expensive arms too. One such famous arm had varying armtube lengths, in error of up to 7.5mm, yet the alignment system keyed to the headshell and was one of the major "claims to fame" of this brand! Another brand at the premium price point had error of 9mm when using their own mounting locator, with up to 9mm differences from arm to arm due to assembly variations regarding the 3 hole flange mounting pattern's orientation to the tonearm, which is offset from the mounting pattern (not on the same center). This flange is drilled from templates, "transfer drilling", not to dimension. For that you pay $5k or more. It's hard to know who to trust in these matters unless you make your own templates and check pivot to spindle distance on the turntable yourself. For proper dimensions and offsets you can definitely trust the numbers on John Ellison's site.
This is precisely why the Linear Offset jigs are easier to use for those situations to get the "correct" nulls unless the error is so large that you run out of travel in the headshell slots.
The point bcowen has difficulty in grasping is that an arc protractor requires the EXACT matching parameters used for the arc otherwise there will always be an error tracing the arc.
As I mentioned sometime back, the real question is if small errors really matter (ie are audible), but if one were to determine which tool was "more accurate", then I'd go with the Linear Offset jigs all day long unless I knew for certain that the pivot was "correctly" located.
Regards Anthony
"Beauty is Truth, Truth Beauty.." Keats
I think I can measure pivot-to-spindle distance fairly accurately. I measure from the center of the top of the spindle to the center of the pivot point on top of the tonearm. Next, I use a box and some playing cards to elevate a ruler at the spindle so it lays across the top of the tonearm and appears parallel with the platter surface. Then, I measure the distance above the top of the spindle to the level ruler. This gives me two sides of a right triangle, the hypotenuse and the short side opposite the angle at the tonearm pivot. The other leg of the right triangle is the pivot-to-spindle distance.
For example, I measured the distance from the center of the spindle to the center of the pivot on my Technics SL-1200 to be 217.5-mm. Next, when I placed a level ruler from the top of the tonearm pivot to the spindle, it was 31-mm above the top of the spindle. Using Pythagorean's theorem, the pivot-to-spindle distance would be the square root of (217.5 2 - 31 2 ) = 215.279-mm. Technics specifies the pivot-to-spindle distance to be 215-mm.
If you are careful, I'm pretty sure you can achieve an accuracy to within +/-0.5-mm. If so, that is plenty accurate enough to construct an accurate arc protractor.
What do you think?
Thanks,
John Elison
Hi John
I'm not disputing that an arc protractor is a useful tool that is far better than a simple 2-point protractor. Nor do I dispute its potential accuracy (nor your calculations which I'm assuming you were describing in detail for the benefit of the wider audience... LOL )!
Also, I agree that in practical terms, obsessive levels of accuracy are probably unnecessary. Therefore putting to one side whether accurate alignment really matters and what is an acceptable tolerance range...
All I was suggesting was that in the absence of any solid data for an arm or confidence in the manufacturing tolerances, that a Linear Offset jig will enable a faster and potentially more accurate alignment(allowing for the same error due to parallax which can't really be quantified).
The key reason being that users need not care about the design parameters or small mounting errors (within the headshell slot adjustment range) of the arm.
The arc protractor requires a degree of care in use - for example one should start with the outer null to minimise the overhang uncertainty since the deviation from the arc near the spindle will be magnified given the greater radius from the spindle. This process adds greater uncertainty in both overhang and offset in the freedom of rotation off the spindle-pivot reference line based on an individual assessment of "good enough" agreement with tracing the arc particularly when considering that any error in the pivot-spindle distance between the protractor and the actual arm position will mean that the arc will never be perfectly traced under magnification.
On the other hand, the Linear Offset jig holds the reference line (for the better designs!) and therefore defines a unique and correct arc position for the arm at the null point irrespective of the mounting position and therefore reducing the "back and forth" step of tracing the arc at the outer and inner nulls and eliminating the issue of protractor/arm mismatch.
The point being that the OP was prepared to spend significant $ on a Wally Tractor and since the same amount of $ (actually less) could purchase a ProJect Align It I thought it worth providing as an option. The one limitation of this jig is that they don't provide suitable grid lines, but that was an easy modification and I was able to customise my grid and have exactly the linear offset that I wanted.
Just for the record, I use a combination of arc protractor and jig. When I was a schoolboy, the first tool I made to try and understand why our Dual 505 Mk3 sounded so terrible after I changed the cartridge to a DL110 from the supplied ULM165 (similar to OM10) using the supplied gauge was an arc protractor. The only data provided was the effective length and the offset. I couldn't understand how the manufacturer would allow such a wide range for the stylus position in the gauge so I constructed the protractor to work out what the preferred position in the gauge should be and followed a similar method to you to establish the spindle/pivot distance.
Suffice it to say that at that age I didn't know anything about Stevenson, Baerwald etc or the preferred null points, nor did I know about the impact of changing the cartridge height from the standard OM10 on VTA....
Regards Anthony
"Beauty is Truth, Truth Beauty.." Keats
> The arc protractor requires a degree of care in use - for example one should start with the outer null to minimize the overhang
> uncertainty since the deviation from the arc near the spindle will be magnified given the greater radius from the spindle.I think what you mean is that one should first align the arc protractor so that the stylus rests on the arc at its outermost position near the edge of the LP. Then, move the arm to its innermost position near the spindle and observe whether the stylus still rests on the arc. If it overhangs the arc, move the cartridge backward in the headshell slots until it rests on the arc and start the alignment sequence over again. It will converge very quickly to the point where the stylus follows the arc through its entire range of movement. When that happens, you can adjust offset using either of the two null-point grids. I always choose the inner null-point grid where alignment is more critical in eliminating inner groove distortion. Of course, unlike your linear offset protractor, all arc protractors have two alignment null-point grids so you can double check your alignment.
> Just for the record, I use a combination of arc protractor and jig.
It is imperative to do this if you are concerned about ultimate accuracy because there is no way to double check the accuracy of your linear offset protractor since it has one alignment grid only. Using an arc protractor with two alignment grids, there is no need for a second protractor to check accuracy. The arc protractor contains two alignment grids specifically for that purpose.
Most everything in this hobby is based on personal preference. There is no question that your linear offset jig allows you to achieve the initial steps of alignment very quickly. However, as you yourself admit, you really need another two-point protractor to satisfy your desire for accuracy. I don't need a second protractor because my arc protractor has two alignment grids specifically for double checking accuracy.
> The point being that the OP was prepared to spend significant $ on a Wally Tractor and since the same amount
> of $ (actually less) could purchase a ProJect Align It I thought it worth providing as an option.While this is true, you really need a second protractor to confirm the accuracy of your linear offset protractor whereas you need nothing else when using an arc protractor. Furthermore, Ken Willis makes excellent arc protractors that cost considerably less than $150. Or, if you have the ability, you can make your own arc protractors for free using Conrad Hoffman's Custom arc template generator for phono cartridge alignment. Of course, it really boils down to personal preference.
Best regards,
John Elison
Edits: 10/05/16
I always wonder why posters' use vague terms like "famous or well known" manufacture and just not name the brand??
Post a Followup:
FAQ |
Post a Message! |
Forgot Password? |
|
||||||||||||||
|
This post is made possible by the generous support of people like you and our sponsors: