The HT75's most useful frequencies of 40 Hz and 16 kHz were chosen for general-purpose band limiting.
The filters are 18 dB/octave Butterworth alignment with each section having relay bypass.
Balanced inputs and outputs are on-board but can be removed and linked to provide unbalanced connections or to cascade filter boards.

Transfer Filter TF1 to replace Neumann HT75.

Transfer Filter TF1 to replace Neumann HT75.
For someone wanting additional frequencies multiple boards can be cascaded.
Two boards can also be controlled in parallel for Modulation plus Preview.
The relay supply and ground connections are isolated and floating from audio ground.
An on-board regulator can be used with 24V relays to operate the filter from the +/15V (30V) audio supplies.
An additional application for the TF1 is a playback warp filter.
Component value changes in the HP section convert it to an 18 dB/octave 18 Hz filter.
The initial run of boards have been sold.
Watch this space for availability of the next run.