[sdiy] tube filter topologies...

[Nick Zuccaro]

hey there, i'm scheming up a tube LPF and am to the point where
i'm checking out some topologies to see if they're implementable
with tubes or not. one thing i'm leaning towards is doing a
sallen-key type of filter, but this relies on having an op-amp
type of gain stage. i've seen the schematics for the 2-tube
opamps <philbrick/EB> and was wondering if anyone has acutally
built one and done any characterization on this type of design.
seems to work ok in theory, there won't be a whole lot of gain
like the solid state opamps, but the bandwidth doesn't need to
be very great for a lpf and there is very little external gain
needed from the stage itself, so initally things seem to work
out. i also checked out the schematic for eric's ts-3 tube
filter, and that looks like it's a multiple feedback type of
circuit. is this because the tube op-amp type of implementation
is more of a science project than something that is actually
realizable? i was just kicking around a few things and thought
i'd see what other people have done previously...thanks, Nick

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