091Audio has released 091HPDoctor, a utility plugin focused on precise low-frequency processing. At first glance, it may look like a niche tool, but it targets the kind of detailed cleanup and shaping work that standard high-pass filters often do not handle well.
A High-Pass Filter That Reaches Down to 1 Hz
Most high-pass filters stop at 20 Hz, but HPDoctor goes down to 1 Hz. That alone makes it more useful than usual for jobs like DC offset removal, subsonic cleanup, and very controlled low-end shaping.
The plugin covers frequencies from 1 to 150 Hz and offers slopes from 1 to 24 dB per octave, depending on the selected filter type. In practice, that gives users much more room to make careful moves instead of forcing broad or overly steep cuts.
Seven Filter Types With Different Behavior
091HPDoctor includes seven filter types: Butterworth, Elliptic, Chebyshev I, Chebyshev II, Bessel, Ladder, and Allpass. That is a strong range for a utility plugin like this, especially because the differences are not just technical on paper. They can matter when phase behavior and low-end feel are important.
An optional -3 dB Const mode is also included, which helps keep cutoff behavior more consistent across the available filter types.
Built for Practical Mixing and Mastering Jobs
091Audio presents HPDoctor as a focused utility, and that seems fair. It is meant for specific tasks rather than broad tone shaping. Those tasks include removing DC offset without touching too much audible bass, cleaning subsonic energy before compression, shaping kicks and bass with resonance around the cutoff, and handling mastering cleanup where more precision is needed.
The allpass mode also opens the door to phase alignment work, giving the plugin a bit more depth than a basic corrective filter.
Features
- Low-frequency processing ranges from 1 Hz to 150 Hz
- Slopes from 1 dB/oct to 24 dB/oct
- Seven filter types
- Butterworth, Elliptic, Chebyshev I, Chebyshev II, Bessel, Ladder, and Allpass modes
- Optional Const -3 dB mode
- Full 64-bit double-precision internal processing
- Click-free transitions for parameter and filter changes
- Suitable for DC offset removal, subsonic cleanup, and phase work
Plugin Formats
The plugin is available in VST3 and AU formats for Windows, macOS, and Linux. The macOS version is signed.
Free Download
091HPDoctor is available now as a free download from the 091Audio website.
