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Paul, I do not think that “before or after RIAA losses” requirement has any merit as no matter what kind type RIAA filter is being used it always will lose around 20dB. I do not think also that there is a “safest path” or “reasonably affordable”. In my view any topology of RIAA filters are in the same cost, even the LCR RIAA filters are in the same price scale.  The 5K or 10K LCR would be expansive but 600R is tolerable. What I mean by tolerable is that the cost of the 600R LCR deluded in the cost of total corrector of compatible sound. To make a properly sounding feedback or RC phonostage is hard and when you do it then the cost of 600R LCR filter is well consumed into the cost of the whole project.

How difficult is it to find 437As good enough for a phonostage? For commercial applications is it a dead tube. WE might produce it again when they stated to redo the tubes but the existence of the Russian 6C45P pretty much killed the idea. For none-commercial applications it is a fine tube and if somebody has some stash of them then why do not use them?  And hard to get them new, they are expensive and mostly those 437A that let go are the noisy one, why would anyone sell good 437A?

It would be interesting to learn how much the 437A is better then 6C45P. The 6C45P are OK if they drive a lot of current. If do not use the stupid pulse version that everyone look like use and find them from 60s than the 6C45P might work out. I afraid that to find the 6C45P from 50s would be as hard as to find the 437A.  I do not know anyone who used 437A in phono. I think in Stefano case the 437A in input stage and output stage would behave differently.  Give him a few months and I am sure he will experiment with 437A, 417A, 6C45P, 3A/167M in different configuration and will decide what to use. Output tube in Thomas Mayer phono might be pretty much anything.

BTW, the stand alone LCR phonostages are not the optimum configuration in my view. It would be fun to have a full preamp with one two inputs with LCR RIAA, then you can optimize the amount of stages and do not use the preamp output stage as phonostage’s output stage, effectively dropping one active stage.

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