Arm geometry and null points

Halcro, I normally look at the tonearm alignment situation from the other direction. Lofgren A, Lofgren B, Stevenson and other alignments can be said to be a set of null points at specific radii that are selected (usually by the tonearm designer, in some cases by you) for certain priorities, such as end-of-side distortion, lowest peak distortion or whatever. If the priorities are different, the radii of the null points will likewise be different.

Once you (or the tonearm designer) know what priorities you want to optimize for, and the null radii have been picked (Lofgren A, Lofgren B, Stevenson etc.), you can calculate the overhang and angular offset required to hit those radii within the constraints of the tonearm's effective length (the sum of the overhang and spindle-to-pivot distance).

Usually, as the tonearm length increases, hitting the same null radii will require the overhang to be shorter and the headshell offset angle to be shallower.

Practically, unless you are using an SME/Graham type tonearm base which can be moved fore and aft, or a baseplate with an offset hole of the kind found on many vintage Japanese turntables, you will have little freedom to alter the spindle-to-pivot distance (for example, if the dealer drilled the tonearm hole in a slightly different location from what the tonearm manufacturer intended). You may therefore need to recalculate and adjust the overhang and offset angle to compensate for the discrepancy in spindle-to-pivot distance and accurately hit your target null radii.

IME, many vintage Japanese tonearms (Audiocraft, Yamaha, Micro-Seiki, FR, Ikeda etc.) do not use Lofgren A or Lofgren B, and so if you want to use these alignments, you may need to recalculate the tonearm geometry.

regards and hth, jonathan carr

The respective null points will be at the very same locations - no matter whether 9", 10" or 12" (or anything in between). The derivation from that zero error arc however will be the less the longer the effective length of the given tonearm. Each of the respective different length tonearms however have to be adjusted for overhang and azimuth independently - according to the geometry desired.
As Jcarr pointed out in his post, several japanese tonearm designers went for very "individual" calculations in their tonearms geometry. The FR-60 series tonearms in particular should be recalculated before put to use. This will better their geometry by a magnitude. The same is indeed true - as mentioned by Jcarr - for many tonearms from the 1970ies and 1980ies (especially so, but not only of japanese origin).

Cheers,
D.

Null points (Baerwald and otherwise) are defined by measuring a specified distance along a radius centered on the TT spindle. The tonearm has nothing to do with their location.

Null points do not change whether you adjust spindle-to-pivot or overhang correctly or incorrectly. They do not change regardless of which tonearm you use, or even if you use no tonearm at all! The null point is the null point.

This literal answer is so simple that I suspect you intended to ask some other question than the one you actually wrote. :-)

Here's another way of putting it: with a single pivot tonearm, the stylus scribes an arc. This arc will have at most two points where it intersects the straight line which corresponds to zero tracking error

The various schemes set these two points at different disc radii according to formulations which attempt to minimise either simple errors or the consequences of these errors.

Different tonearm lengths change the curvature of the arc but the position can always be adjusted so the arc has the same two intersection points..

This last statement assumes the radius of the arc is not made impracticably small.

Mark Kelly

Thanks guys, that's what I thought.
The reason I had to ask Doug, is that I have just bought from Yip the Mint Tractor made for the Graham Phantom II standard arm which is, as you have said, the best template on the market IMHO.

For my other arms, the Copperhead and the DaVinci 12" Ref, I have the WallyTractor and the Feickert respectively.

I wanted to use the Mint Null Points inscribed lines to align the styli in my other arms after I had set up the correct Spindle to Pivot distances and Overhang adjustments using the other templates and I asked Yip if the null points were in the same positions and he did not know!!??...a bit worrying?

Regards
Henry

Henry,

Still not sure why you had to ask moi in particular, but thanks for clarifying.

Yes, you can use the Mint's null grids after setting S2P and overhang by other means. As Mark noted and you obviously understand, the Mint's arc won't be useful for any other arms, but the null grids will. Neither the null points nor the intercept angle of the grid (cantilever) at those points ever varies.

I wanted to use the Mint Null Points inscribed lines to align the styli in my other arms after I had set up the correct Spindle to Pivot distances and Overhang adjustments using the other templates and I asked Yip if the null points were in the same positions and he did not know!!??...a bit worrying?

Is it possible that he said that simply because he didn't know which geometry these other tools uses? Löfgren A and B for example have null points in different places.

Halcro, I share the same opinion as Jfd01. Probably Yip simply doesn't know what alignment was used for the design of each of your tonearms.

A 9 inch arm and 12 inch that are both designed for Lofgren A alignment will have the same null radii.

OTOH, a 9 inch arm designed for Lofgren A alignment will have different null radii from another 9 inch arm that is designed for Lofgren B or Stevenson alignment.

Practically speaking, the null radii have nothing to do with the arm length. The null radii have everything to do with what alignment was used during the design of the tonearm, and that is determined by the priorities (or whims) of the tonearm designer (to keep things simple, I'm leaving tonearm installation errors out of the discussion).

hth, jonathan carr

Hi Jonathan and Doug,

I made it clear to Yip that all arms were set using Baerwald geometry.

I think maybe he wasn't keen on me NOT buying 2 other Mint Tractors for the other arms?

Regards
Henry

Yes, Yip may have responded from his own economic agenda. Or he may reasonably believe that an accurate arc for setting overhang is essential to optimal alignment, and that skimping on a $110 tool when optimizing a $10,000 tonearm is irrational. Or both, these two explanations being in no sense exclusive.

You of course have an economic interest in that you'd prefer not to buy more Mints (rather as I surmised in my initial response). That interest of itself is neither more nor less valid than Yip's, though he or others might question its rationality as noted above. It's almost interesting that you asked us (me) a naive technical question while concealing the economic agenda, precisely as you're supposing Yip did with his answer. Perhaps you should check with your mirror.

You may knock me over with a feather, Henry, but hardly with this. Still, since you asked me personally may I mail an invoice for consulting services? Like you and (perhaps) Yip, I forgot to mention my $110 fee. ;-)

Sorry Doug, but John Ellison beat you to it on Vinyl Asylum with his one word answer......Yes!

For that he charges nothing but your verbosity and 3rd degree deserve some recompense? :-)

No.