Philosophy: Bearing vs. Unipivot

Groove spacing is in fact what I mean when speaking of horizontal speed. Many records are in fact cut with a constant groove spacing but most are not. At any rate, the arm can be made to track the variable groove spacing with no worries.

Ralph, I respect your broad knowledge so could you explain "horizontal cutting speed"?

Obviously the rotational speed of the platter in cutting (and playback) should be constant. But groove spacing is not (a challenge for tangental arms). As I understand the process one task in mastering is to monitor loudness and frequency and adjust spacing since the groove dimension (width) is not constant. That would seem to complicate things.

If I have mis-information, kindly explain.

I owned a Rabco for years. I set it up with an opamp to sense the feeler contacts to reduce noise (I got tired of the feelers not making good contact, which would result in the arm lifting off at random points on the LP...) At the same time I also integrated the output response so it would ramp up slowly and ramp down slowly.

The result is that the motor was always turning. This reduced the "error" (BTW Rabcos have **way** less error than radial tracking arms) by about 1.5 orders of magnitude- the arm would find the horizontal cutting speed of the LP and track with it. It was really easy to do.

Benjysch, thanks for clarifying. Parts of my post addressed azimuth instability, so hopefully not a total waste. Some unipivots address this successfully, others not so much.

I'm no van der Rohe (thanks for the comparison) but I am a stickler for details in technical discussions. I draft and negotiate legal contracts for a technically oriented multi-national, where nerdliness is next to godliness!

P.S. You gotta love that Rabco. A fun design, even if it never achieved groove tangency except by random chance. I still have my ST-7 and it works perfec... umm, as well as it ever did.

I want to thank you all, and to Doug have just a couple of comments for clarification. I understand that one part moving on the point of another constitutes a bearing. Perhaps I should have specifically named ball bearings, but then folks may have thought that I didn't know that knife edges were also bearings. Having a stylus captured between both vertical and horizontal ball bearings means to me that, apart from micro movement from bearing slop, the stylus will maintain azimuth, and will not tip to the right or left in the groove. That right or left tip is what I meant by relative movement. The same thing can be said for the old Rabco servo driven arm; it was always in front or behind the angle the groove was cut at. With this relative motion is a loss of information from the groove.
I know that there are many unipivot tonearms that are astonishing, so obviously these problems have been scotched, if not killed. Like you, and architect Mies said, God is in the details.

?? I'd be interesting in seeing how that applies here.

I don't like unipivots because the bearing is so easy to damage. I've never seen an older one come through the shop that did not have a damaged bearing. It is the Achilles heel of unipivots.

Its not like gimballed arms are perfect. Damage to their bearings is a problem too. Some manufacturers solve it by leaving some slop in the bearings, which can cause coloration. Others like SME, use larger bearings that are harder to damage (and have more sticktion). Triplanar uses a small bearing that is ultra-hard, in fact the hardest made, which is why they have consistent performance year in and year out.

I also prefer gimballed arms because they hold the cartridge in better locus with respect to the groove itself.

Ockham's razor is just as sharp as ever.

In response to Ivor, Naim made the ARO. One of the simplest forms of Unipivots. The ARO till date fascinates more Linn-ies than all the Linn tonearms put together :-).

Dougdeacon has articulated a far better answer than I could, and I very much concur with his final phrase about the devil being in the detail. I think that arms such as the two Caliburn ones (Cobra and Copperhead) are both 9 inch uni-pivots which illustrate that execution is the most important thing. I personally was a bit ham-fisted when I first ever used a unipivot moerch and was put off as a result. Some say gimballs are more evenhanded overall, it may be due to not being able to pivot in the azimuth.

"Perhaps that was what you were thinking of" might be what I read, when there were just a few unipivot arms(Maywire, etc.). That could have been the basis for Ivor's argument, back in the day.

...there just HAS to be more relative movement with a unipivot arm than one with bearings.

A broadly construed assumption that lacks clarity of formulation and overreaches in its conclusion.

First, did you ask yourself what constitutes a "bearing"? Perhaps not, because this statement implies that unipivots have no bearings. That is untrue. A unipivot consists of a "cup" and a "needle", one sitting on top of the other. These perform every function of a bearing:
1. they allow movement between two components that are in contact with each other (the armwand and arm mount);
2. they limit the direction and range of that movement;
3. they minimize friction (dependent on materials and design); and
4. they minimize "chatter" (an undesirable type of movement that causes the bearing surfaces to lose and regain contact. In short, a unipivot has bearings.

Second, you posit that unipivots allow "more relative movement" than tonearms with (other types of) bearings. Umm... what kind of movement? Relative to what?

With regard to chatter, unipivot advocates argue that a unipivot allows LESS of this relative movement than captured bearing designs. The argument is based on the fact that in a unipivot, the entire weight of the armwand is supported on a single point. In captured bearing designs, the weight of the armwand is supported by (at least) two points. Increased mass-loading of the bearing interface increases the amount of energy required to induce chatter. All else being equal (which it never is) the unipivot is a clear winner in this respect.

OTOH, a unipivot (in its pure form) is free to rotate about the tonearm's longitudinal axis, which results in changes in azimuth at the stylus/groove interface. In this respect, captured bearings may allow less relative movement. Perhaps this is what you were thinking of?

Of course, unipivot designers have adopted various measures to control/limit/prevent this undesirable movement. The success of these measures varies from absolute (eg, the secondary bearing of the Basis Vector, which arguably makes it no longer a true unipivot) to virtually non-existent, with many iterations in between employing means that range from mechanical to inertial to magnetic.

As an aside, unipivot advocates would argue that the design provides less total friction between the bearing surfaces. This of course varies with the design and materials.

FWIW, I've heard world class performance from both captured bearing and unipivot designs (far exceeding anything made by Linn, BTW). As with most high end audio gear, the devil is in the details of materials, design and construction.

Hope this was helpful.