STP distance. Is it critical ?

Dear Nandric: I already email you an example of equations used to find out tonearm design geometry parameters where you can see/read that the input/variables for those equations are these " numbers ": effective length and inner-most and outer-most values.

The STP distance comes by: effective length minus overhang.

IMHO a tonearm designer define first if he want a 9" or 10" or 12" effective length tonearm ( he does not cares about where the stylus in the cartridge comes, it does not matters: the cartridge designers has almost no standards about. Every one is almost different where the stylus comes in the cartridge. ) and not the STP figure that is only a result after knowing the other parameters through the equations, in specific the overhang.
At least this is the way how we start our self tonearm design.

Maybe there are other tonearm designers that goes through STP parameter but I doubt. Like I posted maybe I can be wrong but this is MHO.

Regards and enjoy the music,
raul.

Raul, I'm not sure we're in complete agreement on all points.

I do agree that respecting the designer's geometry will result in the tracking they had in mind when creating a given arm. Most of the time, this is the geometry that will yield the lowest "worst case" tracking error.

The effective length, cartridge mounting angle and spindle-to-pivot distance are closely related. Designers must consider all of these variables for an arm. Changing the value of one changes the value of the others FOR A GIVEN GEOMETRY. (Not shouting, just emphasizing the point.) It's all about math / geometry.

For any given arm, when the cartridge mounting angle is fixed as designed and the effective length of the arm is as designed (this includes stylus overhang), there will be one position of spindle-to pivot length that will work for the designer's lowest "worst case" tracking error and null point across the swing of the stylus over the record surface. (sorry for the long sentence) Changing one parameter of the three will change the geometry. There will be different null points on the arc, the "worst case" tracking error may be higher.

To illustrate this, let's look at different arms from the same manufacturer. A VPI JMW-series 9" arm will have a designer's fixed effective length, spindle-to-pivot distance and cartridge mounting angle. Changing to a 12" wand changes EVERYTHING (for a given geometry). The designer's spindle-to-pivot distance and effective length increases, the cartridge mounting angle changes (decreases). The arc shallows across the record resulting in smaller "worst case" tracking errors FOR THE SAME GEOMETRY (fixed null points). The spindle-to-pivot distance is critical to the success of the geometry.

It is possible for a user to change the effective length of an arm by increasing the spindle-to-pivot distance and increasing both the overhang and slightly reducing the cartridge mounting angle. These variables are all closely related for a given geometry. Careful computation and positioning in this scenario could result in a smaller "worst case" tracking error (for a given geometry).

It's not difficult, it's not subjective, it's math (geometry).

HTH,

Bill

Dear Bill: Now that I read both of your posts I think we are talking almost the same.

All those different set of equations were made to optimize/put at minimum the tracking error somewhere but to bring at minimum the tracking error means that we have to choose in what part of the LP recorded surface grooves we want to do it because we can't do it all over the recording LP surface.
So always is a trade-off choice.

The interesting for me in this whole subject is that in almost any pivot tonearm we can change those equations parameters and with the same cartridge see what happen and what works better in the audio system.

Now, IMHO for we can heard/hear differences in quality performance due to this geometry changes it is a must that in each time we test it the cartridge/tonearm set up were made almost perfect.
The analog medium is so imperfect that in this subject we have to be " perfect " to achieve a valued information.

Regards and enjoy the music,
Raul.

Dear Bill, Not so sure if the math. is so sure (see fundations). Enyway the math is not subjective. But we all
are trying to get the stylus on those null points. But if
we use,say,SME V with holes and not slots how are we (pre)
supposed to get there without changing the SPD? Ie I assume that eff.lenght is pivot- stylus distance.

Regards,

Hi Raul,

I think the point you were trying to make earlier that a designer didn't need to consider the spindle-to-pivot distance was what caught my eye. If you intended to mean otherwise, then we are in agreement. All pivoted arms have some degree of tracking error. It's just the nature of the beast.

Wouldn't it be nice if all tonearms were linear tracking to match the cutting process?

Bill