091Audio has released HPDoctor, a free high-pass filter focused on precision in the low-frequency range. The plugin operates from 1 Hz to 150 Hz with fractional slopes from 1 dB/oct up to 24 dB/oct and offers seven filter types covering different sonic and phase characteristics. The idea and UI design were provided by Ronny Pries.
Where most high-pass filters stop at 20 Hz and provide a limited set of slopes and types, HPDoctor is intended for situations where a regular HPF is not precise enough.
Applications Include
- Removing DC offset without affecting the audible low end.
- Cleaning up subsonic energy before compressors.
- Shaping kicks and bass with resonance around the cutoff.
- Console-style high-pass emulation.
- Mastering cleanup where precision matters.
- Phase manipulation and alignment via the allpass mode.
Features
- Frequency range: 1–150 Hz.
- Slopes from 1 dB/oct up to 24 dB/oct (type-dependent).
- Seven filter types: Butterworth, Elliptic, Chebyshev I, Chebyshev II, Bessel, Ladder, Allpass.
- Optional “Const -3 dB” mode for consistent cutoff behavior across filter types.
- Full 64-bit double-precision internal processing.
- Click-free transitions for all parameter and filter changes.
Availability
HPDoctor is free and is available for Windows, macOS (signed) and Linux in VST3 and AU plugin formats.
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