This uses standard Sallen & Key low-pass filters, with two identical Butterworth circuits in series. Each section looks like this:

This has the transfer function

where .

This corresponds to the required Butterworth low-pass response

if and .

If we impose C_{1} = C_{2}
= C, and R_{2} =
a_{2}
R_{1}, then

and .

I used K=1.5, so R_{1}=2R_{2}.
At a crossover frequency w_{0}
=2 pi x 400Hz, and using C_{1}=C_{2}
=5.6nF+10nF+22nF=37.6nF, this gives R_{1} = 14965ohms
and R_{2} = 7482.7ohms I used 15kohms
and 7.5kohms, which give w_{0}
=2 pi x 399Hz.