091Audio has released 091HPDoctor, a free precision high-pass filter plugin for macOS, Windows, and Linux.
091Audio is a small plugin project by Andy Leuckert, a software developer from Chemnitz, Germany, whose day job involves post-processing of measurement data.
HPDoctor is his first release, and it’s a high-pass filter unlike any other plugin I’ve seen. Most high-pass filters stop at 20 Hz, and HPDoctor goes all the way down to 1 Hz. The idea and design came from producer Ronny Pries.
The 1-150 Hz range is the headline feature, and although it may not sound like a big deal, it makes a difference in specific situations. I think it will be especially handy when I’m working on cinematic booms and impacts, where the low end has to be crispy clean, and you want to surgically deal with subsonic content without touching the audible bottom.
A plugin that goes down to 1 Hz, like this one, is going to be a useful addition to my sound design workflow in that regard.
Apart from the extended range, HPDoctor offers seven filter types — Butterworth, Elliptic, Chebyshev I, Chebyshev II, Bessel, Ladder, and Allpass.
Each fukter mode has its own sonic and phase characteristics. Slopes go from 1 dB/oct up to 24 dB/oct (type-dependent), and there’s an optional “Const -3 dB” mode that keeps the cutoff behavior consistent when you switch between filter types.
Everything runs internally at 64-bit double-precision, and parameter and filter-type changes are click-free.
The applications for a high-precision HPF like this one are many. It’s suitable for removing DC offset without touching the audible low end, cleaning up subsonic energy before compressors, console-style high-pass emulation, mastering cleanup, and shaping kicks and bass with a bit of resonance around the cutoff.
I really like the look of this one. The interface is super clean, and the design is refreshingly simple. There’s no clutter and no trendy skeuomorphic textures. It’s just a straightforward utility that does its job.
Overall, it’s a nice little tool for cleaning up the low end. The one thing I’d love to see in a future update is a stereo-to-mono option for low frequencies, so you can quickly ensure everything below a chosen cutoff stays mono. That’d round it out nicely.
091HPDoctor is available as a free download (no signup required) in VST3 and AU formats for macOS (signed), Windows, and Linux.
Download: 091HPDoctor (FREE)
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Last Updated on May 15, 2026 by Tomislav Zlatic.
