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The Korf Blog

The inside story: our research,
development and opinions

3 September 2021
Fourth Time Lucky
We have shown you 2 prototypes of our flexure bearing tonearm. First one a year ago, and the second one this March. The third one, V3, should have been the series prototype, the last one before small batch production. But it wasn't.

Here is a story of what went wrong and how we fixed it.

The parts for the V3 prototype reached us on the 11th of June. By next Tuesday the arm was ready. I really liked the way it looked.

We measured it, and the results were even better than V2 (they are presented further down below). Now we only needed to do a brief listening test, and start accepting preorders...

A brief listening test stretched into week-long detective work. The V3 prototype sounded awful. Not merely inadequate, no. Full-blown horrible. $100 automatic turntable horrible. There was overblown sloppy bass, screeching highs and the mids so recessed it sounded like the lead singer is in another room. Behind closed doors. This was a major humiliation.

What did we change from V2? Sonically, the part that matters most is probably the counterweight stub. We have increased the stub diameter from 10 mm to 12 mm to make the counterweights themselves a bit more manageable. The stub's mass went up from 15 to about 60 gram. This proved fatal.

To check our guess, we have printed the same stub out of ABS. It weighed 8 gram. With a plastic stub, the arm sounded ok. Not brilliant, not the way we wanted it to, but a lot better than with the original stainless steel one.
Why does the high mass counterweight stub ruin the sonics?
Why does the high mass counterweight stub ruin the sonics? It cannot be explained neither by rigidity (it was great, see below), nor by effective mass (it remained absolutely same). Possibly, the answer lies in energy storage. The heavyweight fixed stub is an essentially perfect store, while the moveable counterweight is touching the stub in just a few points that may serve as a kind of a mechanical filter.

Measurements
Here's the vibrometry of V3 with steel and ABS counterweight stubs, plus V2 for comparison.

Isn't the steel-stubbed version just perfect?

We have a sort of a mantra here at Korf Audio that we repeat at least once a day.
Measurements are not performance
When they are done right, they might correlate with performance. Done wrong, they are useless. But they are not performance. We strive to maximize performance, not measurement results. If we need to sacrifice measured perfection to obtain better sound, we don't hesitate. Performance always comes first.

On to V4
Back to our CAD files we went. The diameter reverted to 10 mm, and we incorporated a few tricks to minimize the energy storage. In case we were wrong with our storage assumption and the problem was purely with the mass, we ordered a stainless steel and an aluminium stub. They arrived in early August, and with trembling hands I put a steel one on. Will it cure the problem, or does the answer lie elsewhere?

I needn't have worried. The steel stub sounded fantastic. We had all the magic of V2 back, plus a lot more!

I have tried an aluminium one later. It isn't bad either, but the steel one has a clear edge in both dynamics and resolution.
I love analogue audio. Its endless ability to surprise is a large part of its draw. Every time I become a little bit too sure, it finds ways of humbling me. But we stay on course, and we are almost there.
It is such a happy feeling to have something I designed finally sound exactly the way I want
As you can see we're currently using an old Telefunken counterweight, but the new ones are on the way. I am finally satisfied with all the tolerances in various connections, and the tonearm feels nice and tight. It is such a happy feeling to have something I designed finally sound exactly the way I want. Especially after the whole year of frustration with Covid, broken supply chains, flaky machinists and self-inflicted design setbacks.

We will soon be opening a preorder book for the first small series of 10 tonearms. If you want one, please make sure you have subscribed, and keep an eye on your inbox.

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