STP distance. Is it critical ?


Is the Spindle To Pivot distance critical with a slotted headshell.

I am talking a difference of +/- 1 or 2 mm.

For sake of discussion, i have a Graham Phantom II and a Mint LP.
Will i still be able to track the entire arc if the stp distance is off a bit?

The specs are:
STP distance is 217.5mm
Effective length is 235mm
smoffatt
Dear Bpolleti: You can make any " move/movement " you want with a pivot tonearm and always will have worst results that if you respect the different geometry set up equations where what you can change are the most outer/inner groove distances, the tonearm effective length must be the same.

If you move the pivot to spindle distance at random you have almost " nothing " but higher distortions..

A pivot tonearm always was and is designed with a precise effective lenght in mind/target not pivot to spindle distance, at least I don't know any pivot tonearm designer that his main tonearm target was the pivot to TT spindle distance but I can be wrong.

regards and enjoy the music,
raul.
Dear Raul, This is a kind of student-teacher situation; me
being the student asking the teacher if he is not'forgeting something'. We assume that all designers start with pivot-spindle (exact)distance. For the eff. lenght however they must assume éxact position of the stylus. As J. Carr stated there is no standard position or norm for the stylus. That is why those 'slots' in the headshell are needed. To compensate the (lack) of the standard.Ie to get the stylus where the designer ímagined its position.

Regards,
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.