Most mixers have that fixed 75/100Hz high pass filter for cleaning audio from extra low rumble. What I need is an sweepable HPF covering at least 10Hz - 1kHz region. The slope should be quite deep so that I can make the sound lighter without totally killing it. CEM3320 (http://electricdruid.net/cem3320-filter-designs/) would be ideal but I don't like to use this now obsolete part. So here is my LM13700 OTA-based 24db/Oct HPF. It's made from four serial 6db/Oct OTA-filters. With a switch you can choose between 12db and 24db slope. I have simulated it with LTSpice and it seems to work. Here is my preliminary front-panel and PCB for Eurorack. Here you can find some interesting stuff about OTA's in two parts (Understanding and Using OTA Op-Amps.pdf, Understanding And Using OTA OP-Amps PartII.pdf).