I have been designing products for more then 30 years spending most of my time in the lighting industry and a few years in the suspension (for hard-disc drive) industry. I have had an intense interest in audio in general, but really got into vinyl as a young lad spending lawn mowing money on my first turntable, an AR-XA. From that point I was hooked. As time went forward, I began to merge my designing interest with the turntable geometry, as it really intrigued me. I knew that the LP master was cut on a linear arm so the 45/45 degree grooves were always perpendicular to the center point of the spindle, but here we are, running an arcing tone arm across the face of the record.
As I grew older, I realized that my concerns were shared by others as linear tracking tone arms soon appeared in the market place. But the cost of these arms was out of reach for many, and I always wondered about the new type of error these arms would encounter. The cutting arms on a mastering lathe must certainly cost thousands of dollars; I just didn't see how these new style arms could perform at a fraction of the cost.
Time passes by again and I discovered two fellows named Baerwald and Loefgren. Both of these gentlemen also realized the problem of running an arm that makes an arc thru a record that was cut straight on a lathe. Both came up with some pretty impressive formulas and ultimately settled on the what must have been the best geometry for the arm to produce the least amount of tracking error and tracking distortion. Though their numbers are slightly different for slightly different reasons, they both ended up pretty much in the same conclusion.
Several companies have developed alignment cards based on these two men's geometry and have created the 2 point alignment card based on the null points (where the stylus is in perfect alignment with the record groove). The user was to align a feature of the cartridge body, usually the front, with provided lines to position the stylus. The user then believed his cartridge was properly aligned for optimum performance.
Fiction: The realization.
One day I decided to start laying out the geometry from the known specifications for the perfect arm using my CAD software. Baerwald's numbers were 228.6mm overall length (stylus to tone arm pivot center), 18.173mm for the overhang (distance from stylus to platter spindle centerline) and 24.128 degree offset angle. This resulted in 2 null points occurring at 65.9mm and 120.8mm producing and average RMS distortion of .431%. Loefgren chose a different overhang of 18.690mm which changed the null points to 70.2mm and 116.6mm producing an average RMS distortion of .388%
Here is the fundamental problem. Unless your turntable/tone arm combination matches 228.6mm overall length with a 18.173mm overhang, you cannot achieve the null point position with the proper offset angle.
As examples of this, I laid out the parameters of the Empire 598 mk II turntable. I then set the positions of the Baerwald null point locations into the geometry. Yes, you can get the stylus to fit on both spots, but here is what happened. At the outer null point of 120.8mm, the measured offset angle was 24.248 degrees. Not the same as the original numbers, but I thought with the different spacing, the angle would change, hoping that the second null point angle would be the same. When I measured the second angle at the inner null point location at 65.9 degrees, the angle changed to 24.278 degrees! We have a problem and it's geometrical.
I then did the geometry for a Technics SL-1200 mk II. At the outer null point the measured offset angle was 22.509 degrees, but the inner null point was a whopping 21.302 degrees! That was even worse!
Baerwald?s Geometry
(Upper Dimension in English,
Lower Dimension in Millimeters)
This was proving my point that there really isn't such a thing as a universal alignment protractor as the geometry simply cannot support the theory. How did this happen? I believe that those involved didn't really understand the geometry at all. They simply new that there were null points and if they set the cartridge to those positions all would be good. This is simply a matter of not understanding geometry, and with my many years of experience using CAD, I knew that the answer had to be something different.
Empire 598 mk II Geometry
(Respecting Empire's Overhang)
While the Empire's angles are not that far off, the fact that they are different proves that the change in distance between the pivot and the spindle has an ever increasing effect on the null point locations, or that there was a lack of support for the currently accepted null point locations.
Technics SL-1200 mkII Geometry
(Respecting Technics Overhang)
By the time the Technics's geometry was laid out, the problem was more then obvious.
Now that we know we have a problem, what are we to do? Fear not, a solution is at hand. There are several null point calculators out there on the internet, and I?m sure if you ask your table's manufacturer, they will tell you the null points for their particular arm/table combination. If your one of the lucky ones that has an arm with an adjustable base, you can dial it right into the Baerwald's or Loefgren's numbers.
For the rest of us with fixed arms, there is only one thing to do. Find your null points for your arm geometry. To do this, there are 3 things to know about your particular set-up. First is the distance from the turntable spindle to the tone arm pivot center. Second, you need to know the overhang or distance from the stylus tip to the turntable spindle center. And last, you need to know the offset angle.
One of my favorite null point calculators is offered free by
www.enjoythemusic.com. This is a wonderful tool and I thank them for offering it to the vinyl community. This tool is Excel based and all you have to do is plug in the values for your table and it will give you a distortion curve and the location for your null points. You may try varying the overhang and offset angles in small steps to find your best location.
The results for the Empire turntable put the inner null point at 66.6 degrees, and the outer null point at 120.1 degrees for an average RMS distortion of .419% which is lower then Baerwald?s numbers.
The results for the Technics put the inner null point at 58.8 millimeters and the outer null point at 113.5 millimeters for an average RMS distortion of .536%, still in an acceptable range.
Of course, these numbers can be played with a little by varying the overhang if your arm allows it and by changing the offset angle. The object is a low number for average RMS distortion.
In conclusion, the purpose of setting the cartridge is two fold, first for correct tracking and sound and more importantly, to reduce record wear. The examples shown here prove that the standard 2 point alignment systems in use today are flawed, and may not give you the best results with your particular table/arm combination. Perhaps this explanation has shed new light onto your understanding of tone arm geometry and how critical all of the factors are governing proper stylus placement and angularity.
Happy Spinning!
