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The original design is from L.E. Larsen and has been published in L'Audiophile.
It's a Sallen Key type.
The basic formula for such a filter:
for a second order filter (12dB)
for a fourth order filter (24dB)
This is true for OP amps because they have a gain at 1, an infinite input impedance and an extremely low output impedance, but a tube cathode follower is not that close to an OP amp:
As the formulas and explanations given in the article were particularly obscure, I don't give them here ...
This schematic gives a 7KHz crossover frequency and slopes with a 12dB attenuation.
This schematic gives a 5KHz crossover frequency and slopes with a 24dB attenuation.
This power supply is identical for both schematics.