Padding/impedance matching of a phono input
The standard RIAA preamp compensation is (closely) modeled as a 50Hertz low pass filter, with a 2-octave wide plateau from 500 to 2,000 Hertz..

So you want the opposite of that: a 50Hrtz High pass filter, with plateau.

And of course a 60dB attenuation before the High pass.