You can find examples for the math on the web.
If you to try it paralleling two 2SK389 FETs should give you both excellent current and voltage noise figures (paralleling two devices gives you only 30% of voltage noise reduction). Depending on the source impedance it may not be necessary at all.
Did I answer your question? It is eleven at night and I am just rambling on.
is the 2SK389 obselete?
what about a bunch of matched 2SK170s? I noticed that they are/were used in Freds 992.
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