general tonearm design question


Many popular tonearms are designed with with the fulcrum for the vertical axis at ~70 degrees (e.g. Rega, SME), rather than perpendicular (e.g Origin Live).

Doesn't the former design cause the needle to track to the outer groove as the counter weight swings downward?

...or does the cartidge/tube somehow counter this?

Would a counter weight mounted at 90 degrees to the fulcrum, yet the arm tube set at 70 degrees be the solution to this, or am I wrong?

(The reason I started wondering about this was due to the varous Rega counter-weight mods. I would think that a lower center of gravity would exacerbate this problem. Furthermore, wouldn't a lower center of gravity only be effective on a uni-pivot design?)

your thoughts?
128x128popluhv
Nsgarch and Dougdeacon both give great answers here, and I agree that having the vertical bearing axis perpendicular to the armtube rather than the cantilever is a flawed design.

Here's the part I don't understand. Go to Origin Live's Web site here:

http://www.tonearm.co.uk/index.htm

and scroll down a bit. There's a top view view of all the OL tonearms. Except for the least expensive model (an unmodified Rega RB250) all of their arms have the vertical bearing axis perpendicular to the armtube, not the cantilever. Their most expensive arm is something like $5-6K, so building at right angles shouldn't be needed for hitting the price point. I'm guessing they think the design affords some advantage, but I can't see what that might be. I'm puzzled.

David
David, I can't explain OL's design, it doesn't make sense to me. Maybe Doug has some thoughts. . . . . .

I mean one can always make excuses like: if a record is sooo warped that if the height variation (from flat) would significantly throw the azimuth off, you couldn't play the record anyway, etc., etc. But I put my trust in the wisdom of the designers like SME and others. I'm not really sure what happens to unipivot arms when the stylus hits a bump. Anyone?
.
I saw the same thing on the Origin Live website. As a matter of fact, I have an OL Silver tonearm on my Basis 1400. I could see no change in cantilever/sylus azimuth position when I used a mirror on a few different positions on the platter.
Origin Live must know something about Tonearm design.
The OL design doesn't make sense to me either. I owned a Silver. It was an excellent arm for the money, really excellent, but it didn't match top level arms for clarity and resolution.

Personally I wouldn't spend $5K (or even $2K) for an arm with this flaw, since the sonic effects are of a type we're sensitive to. It's possible *some* rock listeners wouldn't care, since there are many rock LP's which cannot support too much clarity and resolution. With classical, OTOH, the more resolution the better. I just remounted my UNIverse last night after checking out a friend's cartridge in our system. The tiniest azimuth change I can manage on the TriPlanar's really excellent adjustor is instantly audible. Knowing what's possible, we wouldn't be happy settling for less.

I'm not really sure what happens to unipivot arms when the stylus hits a bump. Anyone?
Most unipivots, it seems to me, rely heavily on stylus/groovewall pressure to help stabilize the arm in the azimuth plane. After all, they're quite "floppy" until the stylus hits the groove.

This has always seemed like a serious design flaw to me. Our poor stylus, cantilever and suspension have plenty to do following groove modulations without having to help stabilize a structure that outmasses them by 100x or more. (Paul and I both think this is a major reason we dislike a certain arm that I won't offend SS by naming. Let's not go there again!)

Someone once tried to convince me that the azimuth rocking of an unstabilized unipivot keeps the stylus in perfect azimuth relative to the changing groove angle on warped LP's. That's nonsense of course. It would be true only if the stylus were capable of rocking the arm to match the groove angle with ZERO time lag and ZERO overshoot, which is patently impossible. The mass of the arm and the compliance of the suspension will always create a delay between the input of uneven groovewall pressures and the readjustment of the arm angle. By the time the arm reacts the warp would have changed the groove angle, you're always behind, and then the arm would overshoot. Adding outrigger weights would not necessarily help, and it could easily make matters worse depending on warp angle, warp frequency, cartridge compliance and the rotational inertia of the arm. That's too many uncontrollable variables. Stability may be a compromise, but it's the best sounding compromise to our ears.

The Basis Vector and Graham Phantom are exceptions among unipivots of course. Each stabilizes itself, in its own way, leaving the stylus/cantilever/suspension to do their primary job. I think this is a major contributor to their increased clarity vs. their unstabilized cousins. The Phantom's non-contact method of stabilization seems especially ingenious, since it preserves the design advantages of a unipivot while eliminating the disadvantages. Kudo's to Mr. Graham on that one.

Signed,
Anyone...