Simplistic NJFET RIAA

The ferrite can't be anywhere near saturation since there is no low-frequency current loop through it. Shielded cable would reject the vast majority of the distortion even if it did saturate.

However without an understanding of RF behavior and propagation, choosing a ferrite that won't either resonate or do nothing at all is about as straightforward as Feng-shui. Each situation often needs a different ferrite. So I'm guessing a lot of people get the wrong ferrite over and over again and then decide they are all bad.
 
Like using single chip double 2SK170 theoretically. I have seen differences in THD and its rising trend between the two types in a dif input high loop feedback line preamp though. They seem less capacitive but also less linear than the Toshiba. Of no consequence in open loop phono input stage. Maybe less final input capacitance if interested for MM.
 
46dBs
 

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Folded as used in FSP is indeed very strong front end, one of its characteristics so to drive the passive filter well, but a little later I will look into it to see if I can give you values for about 20 dB so to help you make a pre pre proper.

P.S. You miss a resistor (R13)