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Philosophy: Bearing vs. Unipivot

I was looking at the VIV Rigid Float arm on AG today, and a couple of things struck me. Obviously, the primary one is that there is no arm offset on this arm, no overhang, no need for anti-skate. But, secondarily, I can really follow Ivor Tiefenbrun's thoughts on the Linn turntable: any relative motion between the stylus and the groove harms the sound. Stable bearings, whether gimballed or not, solve this situation. But there are so many unipivot arms out that are well-thought-out and well-regarded, including the aforementioned VIV.that there must be something more going on with them than there used to be, because there just HAS to be more relative movement with a unipivot arm than one with bearings. Someone please educate/enlighten me...
benjysch
dougdeacon
3,338 posts
 
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.
atmasphere's avatar
atmasphere
7,068 posts
 
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.
pryso
1,501 posts
 
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.
atmasphere's avatar
atmasphere
7,068 posts
 
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.
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