The object is simple: Keep the stylus in alignment as the record groove radius decreases and the needle moves. But perfect aligmment of this delicate moving contact point isn't so simple. Some use straight arms, others use "S" curved and other shapes. These cause the needle movement to form an arc. So some use linear tracking mechanisms to avoid that.

What geometry is involved in getting the best alignment? What are the best shapes and why? What tables use(d) them?

Good question. I have had a hard time finding answers too. I'm interested in building my own arm and finding any information regarding how to get going and some pro's and con's to different designs, materials, mass and geometry is a tough one. If I find anything more than bits and pieces I'll let you know.
I just rebuilt a vintage Empire deck and it plays very well. It has a Grado cart and just the stock high mass arm. I was thinking of selling the car for a nice dynavector or sme arm. The busses run pretty regular around these parts, so what the hey.

Linear Trackers are the most accurate however they must be actively driven across the platter which causes another set of problems designing for the least applied bias as well as constant correction applied.

Bill mentions the Dynavector DV 5o5 I beleive? That arm uses some tricks to overcome inertia in both the verticle and horizontal planes. The object is to try and provide a large effective mass in the horizontal plane with good damping while effecting the lowest mass in the verticle plane. They have a great tutorial on their website.

The longer the distance from pivot point to stylus overhang the better in most gimbal or uni-pivot arms since that allows for the most accurate overhang adjustment and describes a flatter arc.

These are simple explanations for very complicated issues. Really asking which is the best geometry is like asking is a 2a3 better than a 300b? It depends on execution. More important than geometry is bearing integrity and resonance control. Unfortunately good bearing manufacture is expensive. In Gimbal arm bearings the inertia in both planes is the same but the fine line exists between accuracy and friction depending on the bearing tolerances.

Uni-pivots solve that problem but in the verticle plane they tend to misalign on large excursions as the stylus is thrust upward.

The most important aspect of TT is mating the right arm/table/cart. I have seen very high compliance carts. mounted on high mass arms all the time, not to mention misaligned carts. improperly loaded.
Before you go with that bargain cart. check with the manufacturer of the TT. Rule of thumb is gimball bearings on suspended TT and uni-pivots on solid plinths.

There is a guy in one of the clubs who built a tonearm out of a no.2 pencil. He built a scaffold out of aluminun and hung the arm from it with fishing wire. The cart. was glued to the erasure and he rigged up a rubber band as a anti-skate device. As the arm traced the grooves the rubber band wound up providing bias counterclockwise to the arc. I never heard it but it got good reviews. Mr. Martinelli, if you are serious about DIY tonearm try this site; Bernhard Kistners Audio Pages. He is building a Linear Tracker that looks real nice. J.R.

If you would like my opinion, I use my Linn with the Linn Ittock type LV111 simply because they were made to work together and while slightly bloated in the base and they run a little slow, they still play music better than most other tables within reason. If I were to look for something more modern I would check out the Audionote tables. Maybe we can get some input on those. I really liked the MM cart. they make and I think I am ready to change from MC.

I have a Grado and a Sure cart. So far I like the Grado on the Empire arm a little better. The Grado has better bass production and is more dynamic sounding. The Sure is more nuetral sounding and has a little more detail. Just some observations with a few 75.00 carts.

It's s lot of fun and brings the relaxation level of listening to music to another level.

Bill

Thanks I appreciate the info. I'm happy with the deck the way it is. The arm is rough looking from the years and I would like to upgrade its appearance. The high end arms are too pricy for me. I'm looking around for a sme 3009. It's just an ongoing project you see.

I agree with you on the Grado in that arm. I'm not sure how low the compliance for the Shure M97xe is. But, with the Grado I was getting some resonance showing in cone movement, that is not there when I use the Shure. I raised the vta a little and the Shure opened up and sounds very nice now.

I found good results tracking at 2.1 or 2.2 grams. The Grado was tracking well at 1.8, 1.9 The Shure needed a bit more. I'll have to give the Stanton a go in a few weeks.

Do you have a pointer on glueing the stylus in?

Bill

S-shaped arms are more accurate at the two null points of the arc the arm transcribes across the record surface, but it is less accurate at all other points. The straight arm is a little less spot on at the nulls but more closely algned throughout the rest of the arc.

Not quite true!

Let's say you've got two tonearms: one with an S-shaped armtube, and one with a straight armtube....

As long as both arms have the same effective length, both arms are mounted on the turntable with the same pivot to spindle distance, and both cartridges are aligned with the same headshell offset angle, then then two arms will trace the EXACT SAME path across the record surface! It can't be any other way!

Mike