The Korf Blog

The inside story: our research, development and opinions

23 July 2018
3rd and Final Flexure Prototype
With this blog entry, we wrap up the experiment with tonearm prototypes that use flexures instead of horizontal bearings. Here are the quick links to other articles in this series:

First Prototype
Second Prototype

Stainless steel spring in a second prototype started sagging almost immediately. Well, the bronze spring sagged immediately. Unfortunately, it isn't capable of supporting the arm's weight. The configuration we chose can only really work with a spring steel flexure.

Bronze spring a few seconds after installation. Alas, that's unusable.
I managed to keep the bronze flexure flexing long enough to do some measurements and a quick listen. Other flexure-based prototypes are on the chart for comparison: Prototype 1 is spring steel, 2 is stainless steel, 3 is bronze.
Bronze flexure measures almost the same as stainless steel, maybe slightly worse. No surprises here.

Subjective evaluation
The bronze flexure is disappointing not only structurally, but also sonically. This is the worst-sounding tonearm of the three. Dynamics are either absent or wrong, and there's a whole feeling of disjointed-ness and lack of musical authority.

It looks like a certain minimum of rigidity is required in a flexure bearing. If it isn't there, things go downhill fast.

Result: ★★☆☆☆

Summary of Findings
Structurally, single-leaf flexures cannot replace bearings in a consumer tonearm
In a way, it was a necessary experiment for me. I was ruminating on the idea of flexure bearings in tonearms for a long time, and the only real way to get this chip off my shoulder was to build a few and see how they perform.

Sonically, I was pleasantly surprised. Adding another spring into a system of imperfect springs didn't sound like a good idea. However, our low-frequency tests showed no difference between gimbal and flexure bearings (sorry we would not be publishing those — some things should remain under wraps for the time being).

Probably, in a tonearm bearing, initial sensitivity and lack of "sticking" is a lot more important than whatever returning spring force might develop with further deflection. And the lack of bearing chatter is a revelation. Once you experience sound without it, going back is difficult.
Structurally, it was a disaster. Single-leaf flexures are just not durable enough to replace bearings in a consumer tonearm. Of the 3 springs we used, 2 failed catastrophically during normal handling. One didn't, but was bent badly out of shape and all but unusable.

We have entertained many other single-flexure configurations, but none made it out of simulation.

What was left of stainless steel and bronze springs

Cutout of the simulated tonearm central part with vertical single-leaf flexure. Notice the distance between vertical and horizontal rotation axis.
Orienting the flexure vertically might decrease the static and handling loads on it, but will introduce unwanted longitudinal degree of freedom to the system. With a pulling force on the stylus being 1/5th to 1/3rd of downforce, possibility of movement along the tonearm axis is highly undesirable.

Also, with a vertical single-leaf flexure it is quite difficult to have a single point of rotation in vertical and horizontal planes. We wouldn't go into reasons why having them apart a la Dynavector DV 507 or Gray 216 is not a good idea — but trust us, it isn't.
I have not given up on flexures completely. There are other configurations that might be a lot more durable than a simple leaf spring. But this is a subject for a different experiment. Perhaps, maybe, one day...
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