This uses cascaded Sallen & Key Butterworth high-pass filters, as in the low-pass filter. Each section looked like this:

This has the transfer function

where .

This reproduces the required Butterworth HP response

if and .

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

.

I used K=1, so R_{1}=2R_{2}.

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