Koetsu cartridges with Fidelity Research 64S tonearm
 

I've read with great interest the discussions about the Koetsu cartridges and the best arms to use with them. In a number of threads, the Fidelity Research 64S was named as an ideal arm for the Koetsus. However when I calculated the resonant frequency (RF) for this particular combination, the RF comes in well below the 8-12 Hz recommended range. For my calculations, I've been using the formula, RF = 159 / square root of (M x C) where M (mass) is the sum of the effective mass (EM) of the tonearm in grams, cartridge weight and hardware weight and C is the compliance measured at 10 Hz. I did not add headshell weight.

A typical stone body Koetsu is about 13 grams and compliance is 5 x 10-6cm/dyne @ 100Hz. Compliance of Koetsus are measured at 100 Hz. It is recommended that the compliance be multiplied by 1.5-2 to get an equivalent compliance reading at the 10 Hz standard used in the above formula. In my calculation, M is 13 + 35 (EM) + 1 (screws) or 49. C (compliance) is 5 at 100Hz or 10 (est.) at 10 Hz.

Therefore RF = 159 / square root of (49 x 10)
= 159 / 22.14
= 7.18 Hz
(adding the headshell weight would have lowered the RF even more)

So, why do the Koetsus sound so good with the FR 64S arms? I realize there are other factors to consider besides RF in evaluating the synergy between cartridge and tonearm. Any thoughts? Am I missing something?

FYI, I am using a Koetsu Vermillion with a VPI 12" 3D arm and get a RF of 11.10. Adding additional weight at the headshell would bring the RF down to a more ideal 10 Hz range.