Tonearm Geometry and Setup

Dear John, If the designer is free to choose between Lofgren and non-Lofgren geometry than he obviously will do this according to his own preference or assumptions regarding the (potential) customer.But this way he already preselected the 'optimal adjustment' for the customer?
Kessler&Pisha stated in the context of 'Overhang' that many
Japanese designers 'overlooked' the dependance from or correlation with the eff. lenght. So, for exapmle, Dyna 505 has 15mm overhang while 17.164 should be optimal. Ie
the eff. lenght corresponding with 15mm overhang is 274mm
(10.787 in.). This lenght will not fit the turntables disigned for 9" tonearm ( Tonearm geometry and setup; Audio, January 1980). My 'intuition' is : you can't have
it both ways; the choice for the designer as well as the choice for the user. Ie the choice of the designer limits the choice of the user. As far as I know our choice for
Bearwald means the least distortion for the whole record
radius (aka 'avarage') while Stevenson means the least distortion near the lead out 'part'.
This means to me 'one way or the other' and this precludes
our 'free choice' or at least limits our choice. Otherwise
I am not able to make any sense of 'different tonearm geometry'.

Regards,


.

For those who think to be good in math but also think to
have good nerve there is an article from Stereophile by
Keith Howard on the net: ARC ANGELS; Optimizing Tonearm Geometry. Look for : tonearm geometry.
I myself should keep my promisse to myself made after
my high school : 'never ever mess with math '.

Regards,

What a cool question.

Ebm, To which question are you refering?

Dear Nandric,

If the arm has a range of adjustments then the alignment can be whatever the user chooses - in that sense the arm does not drive the geometry.

The Dynavector arm in your example could be mounted a couple of millimetres closer to the spindle and so have the "correct" overhang, as overhang is simply effective length minus mounting distance. The question then is: can the cartridge be aligned to the "correct" angle?

Had the headshell a facility for doing this, then there is no problem. The more scope for adjustment, the more the onus for choosing an alignment falls upon the user, as it should, given that it is his or her own records that are going to be played.

For me, as a design factor, the chosen alignment is the least of it, there are other far more relevant aspects to consider which are more important to the function of the arm. Of course there are always useful features which might conflict with other desirable features, perhaps, for example, ease of adjustment versus rigidity.

Regarding choice of alignment, LofgrenA/ Baerwald gives the lowest distortion for the 3 maxima, and Lofgren B, lowest average distortion across the side. Stevenson IEC is a special case of LofgrenA, in effect with an inner radius of around 55mm, making the inner null at the IEC minimum. Choosing different null points means that there will be an equivalent LofgrenA or B to match, except that the inner and outer radii to obtain the lowest maxima or lowest average distortion may vary from normal. For example, Dertonarm's Uni-DIN uses nulls of 109.52 and 63.49. This is equivalent to inner and outer radii of 58.4 and 129 for Lofgren A, which shows that beyond 129mm the distortion increases for an LP towards the outer radius.

If you have a wide ranging collection of LPs, then it is reasonable to use an alignment which takes all this into consideration. Personally, I would chose an alignment such as LofgrenA DIN, as it is a reasonable compromise, and try to align the cartridge as well as possible. Then forget about it (unless your favourite old record distorts on the inner groove. At which point you make a null there, and play with the calculator until you get something that is acceptable for the rest of your collection...)

As an aside, in this regard, with the potential of computer generated graphics, I am surprised that we are all still aligning to the nulls, given the surfeit of protractors and super protractors on the market.

I'd have imagined that by now we should have had a protractor with the adjustment grids at the minimum and maximum radii, offset to the appropriate angle to give the nulls in the correct location for the chosen alignment. The further apart the alignment points, the more accurate the alignment Perhaps one of the younger computer literate gurus could oblige.

John
.