STX Audio STX851A Digital Bass Processor

The STX Audio STX851A is a car-audio digital bass processor designed to restore and shape low-frequency content before it reaches an amplifier or crossover. It is intended for systems that need more control over bass character than a standard head-unit tone control can provide. The processor works with RCA signal connections and can be used in factory-integrated or aftermarket audio systems.

Its digital bass-restoration circuit is designed to reinforce low-frequency detail that may be reduced in recorded or reproduced music. This can help create a fuller bass presentation while retaining a controlled signal path. The STX851A is suited to listeners building a dedicated subwoofer system or refining the low end of an existing amplified setup.

Two front-panel tuning controls let the installer tailor the bass effect to the vehicle and system. The SWEEP control selects a center-frequency range from 27 Hz to 63 Hz, while the WIDE control adjusts the range of frequencies affected. Together, these controls allow the restoration effect to be set for different subwoofer enclosures, speaker systems, and listening preferences.

The STX851A uses balanced RCA inputs rated for signals up to 15 V RMS, with specified noise rejection above 62 dB. Its stated frequency response is 10 Hz to 35 kHz, with a 133 dB signal-to-noise ratio and 0.0025% total harmonic distortion. These specifications support use in a signal chain where preserving clarity and minimizing unwanted electrical noise are important.

A dash-mount remote control is included for convenient bass-level adjustment from the driver's seat. The remote includes an LED indicator, and the processor also incorporates a PFM subsonic filter to reduce very low frequencies that may consume amplifier power unnecessarily. Operating from a 12V DC electrical system with a 150 mA current draw, it is designed for inline installation ahead of downstream crossovers or amplifiers.

Key Features

• Digital Bass Restoration: Reinforces low-frequency detail in the audio signal.
• Adjustable Sweep: Selects bass center frequency from 27 Hz to 63 Hz.
• Wide Control: Adjusts the breadth of frequencies affected.
• Balanced RCA Inputs: Helps reject noise in car-audio signal paths.
• Dash-Mount Remote: Provides in-seat bass-level adjustment with LED indication.
• PFM Subsonic Filter: Reduces inaudible low-frequency content before amplification.

Design & Installation

The STX851A is a compact inline signal processor built around a digital bass-restoration circuit and analog rotary adjustment controls. Its balanced RCA input design is specified for up to 15 V RMS input signals and more than 62 dB of noise rejection. The included remote is designed for dash mounting, placing bass adjustment within reach of the driver.

Install the processor in the low-level signal path before the crossover or amplifier, using compatible RCA interconnects and a 12V DC, ground, and remote-turn-on connection. Set the bass level conservatively first, then use the SWEEP and WIDE controls to match the system's subwoofer response and vehicle acoustics. The PFM subsonic filter can be useful in systems where extremely low frequencies are not needed or may place unnecessary demand on the amplifier and speakers.