Tripple Passive Filter


Propust is a compact triple passive filter with fixed cutoff frequencies for highpass and lowpass stages. It is a simple RC filter with 2 stages for HP and one stage for LP. There is a drop in signal loudness. No power connection is required.

Clean up your final mix and make room for each sound

Simple fixed filters can help you clean up your final mix and make room for each sound, or they can be used for filtering noises and other signals when generating drum sounds, for instance. When the signal goes thru both HP and LP stages, it creates BP.


  • triple highpass 2-pole filters (cutoffs are approx. 7kHz, 3.5kHz, and 220Hz)
  • triple lowpass 1-pole filters (cutoffs are approx. (1.5kHz, 4.1kHz, and 880Hz)
  • passive design > signal drop + no power connection
  • handmade locally by Bastl Instruments in Brno, Czech Republic

Technical details

  • 2HP
  • 15mm deep
  • passive design – no power needed
  • pdf user manual download

DIY kit

You can build your own Propust. Go through the complete assembly guide below and make sure that you understand all the steps.

  • bill of materials (BOM) google docs
  • schematics and board diagram pdf