PFMOD HP-Sub (High Pass Subsonic)

The Harrison Laboratories PFMOD HP-Sub (High Pass Subsonic) is a specialized passive inline crossover designed to act as a rumble filter for subwoofers and low-frequency systems . It protects speakers from ultra-low frequencies that are below the human hearing range but can cause mechanical damage or excessive cone excursion .

Core Functionality & Purpose

The HP-Sub is a "subsonic" or "infrasonic" filter that removes extreme low-end signals (typically below 20–40 Hz) that often contain unwanted noise or "rumble" .

• Protection: By filtering out frequencies below a ported enclosure's tuning frequency, it prevents subwoofers from "unloading" and hitting their mechanical limits (over-excursion) .
• Efficiency: It reduces wasted amplifier power spent on reproducing inaudible low-end notes, thereby increasing available headroom for the audible bass frequencies .
• Audio Quality: Ideal for vinyl setups to eliminate low-frequency tonearm rumble or for car audio systems to tighten up the bass response .

Technical Design

Like other Harrison PFMODs, the HP-Sub is an active crossover simulator that is entirely passive, requiring no external power and introducing no electronic noise .

• Selectable Frequencies: Many HP-Sub modules are selectable via internal factory jumpers, typically offering points such as 20, 25, 30, or 35 Hz .
• Crossover Slope: Standard models typically feature a 12 dB per octave slope . However, specialized "Subsonic Cable" versions can offer a steeper 18 dB per octave slope (e.g., the 37 Hz 18 dB model) for even faster attenuation of unwanted low frequencies.
  
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