STP distance. Is it critical ?

This might help:

http://www.audioasylum.com/audio/vinyl/messages/77287.html

You can see in numbers what difference there is.

I think the effective length of the arm will change if it is mounted somewhere other than its designed location. I might be wrong, but it makes sense that the arm would swing through a slightly different arc. The manufacturer's template would not line up quite right.

This might help:
The link did not come through correctly.

Here's another attempt:

http://www.audioasylum.com/audio/vinyl/messages/77287.html

I think this is how it works....

Consider the arc of the stylus as it swings across the record surface. The proper set-up establishes two null points where the cantilever is on exact tangent to the groove, the inner and outer grooves have a small out-of-tangent angle.

Moving the pivot point of the same arm away from the platter center changes the position of the arc across the record surface. The two null points are moved closer together and the tracking angle at the inner and outer grooves is greater. Sound quality suffers at the outer grooves, but the tracking between the two null points is a little better.

Moving the pivot point of the same arm close to the platter spindle moves the null points farther apart resulting in a greater tracking angle error across the middle of the record, but coming closer to tangent on the inner and outer grooves.

In both cases of "improper" set-up, the worst-case tracking error is greater than if the arm/cart is set-up as designed. The differences in cartridge alignment points is based on the location of null points, and the location and degree of tracking errors.

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

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

Nandric,

That's a good point regarding the SME V.

It would be nice to get some insight on the geometry model from the SME designer(s).

Bill

Dear Nandric,

I agree with Jonathan (who am I to disagree with such an expert!?).

The way I read the SAEC engineer statement emphasizes what is being discussed in the thread. Changing one of the three "dimensions" changes the other two for a given geometry. If the STP increases, so does the effective length, the cartridge offset angle shallows. Each geometry has a different ratio, but the same principles apply.

If one of the dimensions change without changing the other two, then the geometry is different.

Take a glance at this table to see how the dimensions are interrelated. This table is for a single geometry / location of null points. Other geometries will have different ratios.

http://www.audioasylum.com/audio/vinyl/messages/77287.html

BTW, the dimensions illustrated in this table vary a little from the ones provided my Micro Seiki for the MA-505 standard length arm (237mm, 222mm, 21 degrees 50 minutes) which uses a different geometry.

HTH,

Bill

Raul,

The issue with the SME V is that the holes for cartridge mounting do not allow for any other cartridge position or adjustment of the cartridge in the headshell. This would be OK if all cartridges used the same "mounting hole to stylus distance." Bit there is no standard value for the position of the stylus relative to the mounting holes in the cartridge. Therefore, as the position of the stylus relative to the mounting holes changes, then so does the effective length of the arm. Since the effective length changes, so does the STP distance. Increase the effective length and the STP, and the cartridge offset angle must decrease.

So the geometry of the SME arm changes most every time there is a change of cartridge. In some cases, it seems that the cartridge offset angle will not be a good match the effective length / STP.

Even though we're talking about VERY MINUTE differences, I wonder if that might be one of the reasons that the SME V arm doesn't seem to perform as well for some users.

Bill