The Korf Blog

The inside story: our research, development and opinions

20 December 2018
Flexure Bearings Revisited, Part II
In the previous post, we have described a prototype tonearm based on a ready-made flexure pivot. Today, we'll go through the usual measurements and sonic evaluation.

And here's the vibration data. We've left the line for Prototype 5 on the chart as it uses the same arm tube. Also, the line for our flexure-based Prototype 2 is here too as it also uses flexures and the same steel tube.
What we see here is the typical vibration picture of a not very rigidly built tonearm. The main arm tube resonance is a lot less pronounced compared to the prototypes which clamped the arm tube tightly.

For the counterweight, the difference between stopper screw and a more rigid mounting is less visible.

The zone where bearing resonances usually are, at about 5-6Khz, is full of hash suggesting that either our flexure pivot rings at these frequencies, or the headshell and the accelerometer itself generate more sub-harmonics than usual. Given dismal rigidity of the prototype, I'm inclined towards the latter. To exclude the influence of the flexure pivot, we'll measure its natural frequencies in one of the coming posts.

Subjective evaluation
Here's our prototype on a TechDAS Air Force III in all its messy glory. How does our new arm compare to earlier flexure-based prototypes, and to the ball bearings ones?

It clearly isn't bad at all. I would say it's superior to a Prototype 5 with conventional bearings. The addictive smoothness that appears once you get rid of rolling bearings is there. However, it loses in dynamics to Flexure Prototype 2. Is the Riverhawk pivot to blame?

I don't think so. Remember, we've also changed the way armtube is held in the central part — from clamping to fixing it with a stopper screw. As the measurements above showed, it's a much less rigid connection. While it might be acceptable with softer armtubes, a thick-walled stainless steel one has exposed its limitations. I would try not to use such connections in the future.

Result: ★★★★☆

By the way, if you install more than one cartridge/tonearm a month, you need Dr Feickert's protractor. It allows for easy setup of pivot-to-spindle distance when mounting a tonearm, and setting up cartridge overhang with it is a breeze. I've used dozens of various protractors, and among those available today, Dr Feickert's is clearly the best.

(I'm not paid by him, I don't sell anything he makes, and I'm plugging it here only because of sheer admiration)


To sum it up, our second generation flexure prototype raised a question over the natural frequencies of a readymade pivot. We'll try to answer it with a separate measurement of the flexure itself at a later date. The mechanical configuration we chose for the prototype turned out to be not very good, and we'll try avoiding it in the future. But sonically, the Riverhawk pivot was at least as good as a single-sheet flexure we've used before — and it also proved as robust as ball bearings in day-to-day life.

In the next blog posts, we'll come back to measuring turntable's main bearing. The precision microphones have arrived, and the preliminary results look very promising!

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