I had this project saved for later publication, but the first delivery seems to have succeeded quite well. Analog guru Duke Aguiar, who sells the LPT interface cable to the Audio Precision APIB bus (with a USB solution in the works!), has received and favourably tested my recreation of the AES17 compatible 20 kHz brick-wall measurement filter for Audio Precison System One/Two.
Before I knew what hit me there was already a review post up at DIYaudio!
The performance is stellar, like the original AP and much less costly
Brick-wall 20 kHz lowpass filter compatible with Audio Precision System One/Two
Now available for order – webshop here!