A sliding SME base

 Here are photos, kindly sent by Chris, of his RP1 on his Thorens.
It is using an SME adapter, ostensibly for Ortofon arms, which comes from Hong Kong.
The ebay link is here. I have also seen them advertised in clear anodising (silver finish). Another black option is here

It looks good and isn't too expensive. As a finishing touch, an option is to paint the silver knob black, or get a gold plated one.

The SME Protractor

As I don't own one, I don't have a vested interest in promoting SME or otherwise, but I have been sent a pic of their protractor for the SME V, so I can comment on it.

So let me say that SME owners can be assured that the concept is not at all crazy, nor inaccurate.

Although, like all protractors, it depends on a number of things.
 
The first is that the SME arm is correctly designed and the headshell is as specified at the appropriate angle for a Baerwald IEC alignment at a particular nominal effective length (as given in the SME spec for their various arms)

Hopefully this is so. Then, as the quality of SME manufacture will be top class,  we assume the arm is made to the specification.

The second thing is that the cartridge and therefore the cantilever is set exactly on the headshell axis. For this a depth gauge should be used to align the the cartridge body, if it is cuboid, although the cantilever is a problem, as always (but that should really be an issue for cartridge manufacturers). So it is possible to align the cartridge to within 0.05mm along its length, ie 0.15 degrees or better. This is at least quantifiable, not guesswork.

The third thing is that one has to align the arm tube edge with the outline of the arm tube on the card. This isn't so clear - they should have omitted the outline of the arm entirely, and simply drawn some closely spaced lines at the same angle as the arm outside edges, as the principle is to align the arm outside edge parallel with a line, any line, at that angle when the stylus is on the null point. If a cartridge has a longer (or shorter) cantilever, the angle of the arm, and hence the edges (even though they taper) remains the same but moves sideways.

Using the arm tube as a guide, it is possible to obtain an accuracy far greater than with a cantilever view - perhaps as much as a factor of 10 (assuming generator on the same axis as cartridge and cantilever on same axis as generator.... we have to assume something ...!)

The fourth thing is that the protractor is made  accurately.

So, therefore, the potential for correctly setting up an SME using their protractor is very high. The areas where it is lacking are the same as other protractors, namely the accuracy of manufacture of arm and protractor, but it scores in angle alignment accuracy, as in this area it is potentially more accurate. Also any errors in the protractor will be minimised by the fact that the arm is optimising alignment at the base not at the headshell.

Of course it never hurts to double check using an accurate two point protractor with suitable nulls. But this applies to all protractors.

SME Geometry

When an arm is being designed many designers have a particular effective length in mind, be it around 9" (230mm approx.) or 12" (300mm approx.). Together with figures for minimum and maximum recorded radius, the cartridge offset and tonearm mounting distance can be calculated from Lofgren´s equations.

This approach might be called the fixed effective length + fixed offset angle approach. In this case the arm must have a facility (such as headshell slots) to adjust for variations in the distance between the cartridge mounting holes and the stylus, which varies with manufacturer. This is essential, as the effective length and cartridge offset angle are fixed for the mounting distance of the arm, which on most arms is generally not adjustable and its accuracy is dependent on the installer. This illustrates the disadvantage of this design approach, because if the mounting distance is wrong and not adjustable, the effective length should be different from the specified length , and the offset angle should also be different. This can be dealt with by moving and twisting the cartridge in its slots  and setting up using a universal two point protractor with appropriate nulls, or a protractor based on the Dennesen principle.

A second approach is the nominal effective length + nominal offset angle approach. This approach is based on the fact that there is an interesting consequence once the minimum and maximum radii are chosen, typically IEC or DIN. In calculating each alignment,  (Lofgren A/Baerwald, LofgrenB, Stevenson) these radii are constant, and the formulae are such that the mean of the nulls is therefore constant also. This is worth a closer look.

Arm geometry is such that the ratio of this mean to the effective length is the sine of the offset angle. In other words, from the mean of the nulls divided by any given effective length you can get the offset angle for that length.

This can be imagined as a right angled triangle, with the effective length being the hypotenuse. The short side is  the mean of the nulls, and the angle opposite, the offset angle.

This means that the short side of this triangle remains constant, so that as the effective length increases, the offset angle decreases. The corollary of this is that  if the third side of the triangle increases the effective length increases and the offset angle decreases.

All arms have the above triangle embedded somewhere in their geometry.

The short side is called the Linear Offset and is commonly disregarded (or often misinterpreted) as applying only to the angle that the axis of the bearing for vertical movement  makes with the arm tube, and important only inasmuch as it allows the vertical movement axis to be perpendicular to the cartridge cantilever.

However, unless one appreciates the importance of Linear Offset,  it is impossible to have a complete grasp of tonearm geometry.

Traditionally, SME arms are fixed headshell arms, with a fixed cartridge mount and fixed headshell angle (as opposed to fixed cartridge offset angle). The later 3009 and 3012, V, V-12, and M series are designed for a LofgrenA/Baerwald IEC alignment (the early Series I arms and the newer 300 series are different - the early arms used Stevenson, and the 300 series uses an alignment close to Baerwald DIN). This means that there is a specific linear offset which pertains to them, (and all other arms, of whatever effective length, with this same alignment). Therefore for a nominal effective length of 233.15 (SME V) or 308.81 (V-12) the linear offset is  the same - 93.45mm.(91.54mm for the 300 series). The headshell offset angles are different because the effective lengths are different. With the cartridge square in the headshell,  the correct cartridge offset is obtained.

In terms of the embedded triangle,  all that has happened in the longer arm is that the cartridge and hence the stylus has moved along the third side, thus extending the triangle. The cartridge has not twisted. It does not need to,  because its offset decreases as the effective length increases, as it must do to maintain the correct angle for the alignment.

This can cause confusion among designers and commentators (some of them well known ones who should know better) when faced with cartridges of varying mounting hole to stylus distances. If one does not realise that the arm´s effective length is nominal, it is mistakenly assumed that, because there are no slots in the SME, the cartridge must twist, because the heashell angle is fixed. However, in a cartridge with, say,  a longer cantilever, all that is happening is that the stylus has moved along the third side of the embedded triangle and the real cartridge offset angle has changed appropriately as the effective length increases.

To illustrate the point, if the cantilever was made longer and longer, with the cartridge square in the headshell of the SME V,  the stylus would eventually end up at a point where the effective length and the cartridge offset was the same as the V -12 (or any other arm of that length using the same alignment). The same would apply to the early 3009 and 3012, as they have the same linear offset.

Of course, with a given alignment, an effective length and an offset angle, there is an appropriate mounting distance/overhang given by the equations. This distance is automatically arrived at using a two-point protractor, or one specific to the arm, the mounting distance on the SME being adjusted using the sliding base to square off the nulls which are LofgrenA/Baerwald IEC (for the V). When this is the case, the mounting distance must be correct for the effective length, whatever it may be.

The advantage of the sliding base in the SME means that mounting distance/overhang can be precisely adjusted using the nulls on a protractor. There is no need to actually know the effective length or mounting distance, as long as the arm base has been located such that it is within a few millimetres of  215mm (i.e. the approximate mounting point for the nominal 9" arm). The sliding base then sets the precise distance to square the nulls. The issue of cartridge variations is unimportant. One other point often not appreciated, is that the Lofgren B  IEC alignment is easily achieved by simply sliding the base towards the spindle by approx 0.5mm thus altering overhang without having  to re-adjust the cartridge mounting to also reset the offset as would be the case with a slotted headshell and fixed base.

The SME´s disadvantage is that other alignments (which have different linear offsets) depend on being able to vary the cartridge offset from the set value, and it has no facility for this other than the clearance between cartridge and headshell mounting holes. Depending on the sizes of screws and holes involved this can be a degree or more, which is often sufficient.

Of course, this argument applies to slotted headshells also, as the cartridge still has to be twisted for other alignments, and so depends on sufficient clearance in the slots. Had one hole of the SME been extended into an arc this would make the arm more versatile.

Confusion with the SME also arises because SME themselves do not make clear that the effective length and offset angle are nominal values, and that a two point protractor, or one specific to the arm, is required to set up the arm in the absence of a means to measure the actual physical distance from the arm pivot centre to the stylus when the arm is set down on a record with correct VTF. This distance, of course, is necessary if any arc protractor is to be used with the SME.

It would be  pointless SME supplying an overhang gauge or specifying a fixed pivot to spindle distance. The whole point of the SME approach involved not having a fixed overhang, because a specific effective length would have to depend on a cartridge having exactly the mounting hole to stylus distance to achieve that length in the absence of headshell slots.

All this is partly because of the focus, in forums and by some manufacturers, on precise mounting distance being paramount, leading to the proliferation of arc protractors,  (and the assumption they are more accurate, as opposed to easier to use) than two point protractors, and partly, perhaps, because of a lack of appreciation on the part of current SME staff of the subtleties and advantages of the original design, thus giving the impression by specifying the nominal effective length to two decimals, that the arm is of the fixed effective length type.