Tonearm Geometry and Setup

Nandric, you said:

why are we not free to choose the zero points on the record radius independant of the tonearm used?

You are, of course, as Dertonarm says, free to choose whatever null points you wish. The origin of the null points for the geometries used are based on the inner and outer radii of a record which give a constant, the linear offset, which in turn gives an offset angle and effective length. To be strictly correct, any calculation is only accurate for one record, and is, to a greater or lesser extent, a compromise for all others.

So, a designer of tonearms has to decide first and foremost what are these inner and outer radii. Are they IEC or DIN, or the median of his own collection, or of a surmised potential buyer? In the past, 7" singles had to be considered so the range of possible radii was extended if they had to be accommodated as well as LPs.

In my case, the Odyssey arm used nulls similar to Lofgren A/Baerwald DIN. That was a design choice, as was the nominal effective length of around 230mm. If I was designing an arm today, I would make it have an adjustable offset angle in addition to an adjustable base, thereby allowing the user to choose the alignment. Also, the effective length used to be governed by the space available on turntables. This is less so now, so longer arms are more common, and they have advantages in reducing tracking error and skating forces.

(Though, as an aside, an 8" arm plays a 7" single with less distortion than a 12" arm playing an LP)

All these features in an arm are merely for facilitating setup, they do not define the arm except in the single case of the hypothetical standard LP, whatever it may have been.

Some corollaries of this are:
1. Some LPs may be more useful than others at showing the effects of particular alignments.
2. When evaluating an alignment choose a track at a distortion maximum not a null.
3. When comparing alignments you shouldn't use the same LP, as it will probably suit one alignment better than another unless you wish to use that LP as your standard.
4. You will have LPs with loud passages at the beginning of a side which may sound worse with certain alignments which favour end of side.
5. The cartridge offset angle need not be the same as the tonearm headshell offset, nor the mounting distance be the same as the specified tonearm alignment should another be used.

In all this it is illustrative to play with the fantastic calculators on Vinyl Engine which allow a reverse engineering of the geometry to try non-Lofgren alignments and look at how small are the variations in distortion with small changes in nulls. And how quickly the distortion rises on the inner grooves with LofgrenB. All a lot easier than calculating in the old way...

John
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Nikola

A system like the Sony would be ideal especially if it was programable, such that the force could be adjusted to vary at different points - I don't know if this is possible.

What is perhaps more interesting is that a mechanism of that kind might be used "in reverse" to measure the varying inward force trying to rotate the arm, and the settings stored for the next time the record is played.

As you have steered the thread towards antiskate, can I recap and condense what I said in another thread?

The skating force basically follows the tracking error curve of the arm (not the tracking distortion curve) which varies across the record. staying more or less constant within a few percent of around 25% VTF, reducing slightly during the first half of the side.

Depending on the overhang used and the arm effective length, at the inner null the force starts to increase rapidly. So it would appear that the ideal would be to follow this curve, reducing slightly then increasing.

However, it has been shown that there are other factors which increase the friction force as the radius decreases, which would skew this result and make it more likely that the force should at least be constant then increase, or even gradually increase all the way across the record. When I designed the RP1 that's what I did, as I thought that, like tracking distortion it seemed to be worse towards the inner grooves, and less of an issue further out, and should be weighted that way. My tonearms had an antiskate mechanism, using lever and thread, which allowed for varying the force, and the ratio to some extent across the record, or it could be removed completely. But it was not any more sophisticated than that - it was all "suck it and see". So, really, a tonearm should perhaps give the user the option of trying different things.

The actual amount of antiskate needed is very much dependent on stylus profile and downforce, as the skating force is proportional to VTF. Obviously, with enough downforce the stylus will never meet a modulation large enough for it to lose contact with the outside wall, or even throw it up out of the groove. However there will still be unequal forces on each side of the groove unless there is some kind of compensation. Does it exist as a force. It exists, yes, it definitely does. Whether its effects are noticeable to the listener is one thing, but why some people prefer no antiskate, or removal of the mechanism, that is for them to say. But the force is still there, and it has to be dealt with somehow. As Dertonarm says, a longer arm has less skating force than a shorter one. In that, we agree.

So as I see it, the bottom line is that there is a turning force (a torque) which tries to rotate the arm inwards. This torque has the effect of reducing the VTF on the Right channel and increasing it on the Left. Without compensation, one channel has more VTF than the other.

J
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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
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Dear Nikola,
My comment a while back was rather tongue in cheek, not myself having a surfeit of decks.

I make do with one set up and don't worry too much about it, though I do like my anti skate set up well...

"Best" and "bad" are a different matter. Especially regarding distortion, as is well documented. So 1% (or 0.5% or 0.1% or less) can be regarded as acceptable or not. I suspect that vertical mistracking of one form or another rather than distortion per se due to the horizontal alignment is the main culprit regarding inner groove distortion, that and poorly set up anti skate.

However if I had four decks all identical, I may well set them up with different alignments and mark the albums appropriately. Or even completely different decks to suit different types of music. Why not?

But I doubt it. I would probably sell three of the decks and buy a guitar...
Regards,
John
..

Nikola, I normally use a test record with tracks of increasing level, and listen with headphones, as that way I can hear better when there is mistracking. You have to hear the mistracking to know when it is equal on both channels. then re-adjust the VTF.

Having said all that, if, with whatever set-up, the sound feels solid and the image doesn't wander or get a bit watery, then fine. But I still feel more comfortable knowing that the forces on the stylus are equalized as best they can be, as the skating force is always there as long as you have a VTF
John