Subwoofer Crossover Filter

The RDL ST-CX2S is an active subwoofer crossover filter designed to derive a subwoofer signal from a stereo source. Its two balanced line-level inputs, balanced line-level output, adjustable low-frequency boost, and specified low-noise, low-distortion performance provide a focused solution for systems that require dedicated low-frequency signal processing. The output bandwidth extends from 22 Hz to 80 Hz, while higher-frequency content is attenuated by more than 45 dB at 400 Hz.

Ideal Use Cases

The ST-CX2S is suited to audio systems that need a subwoofer signal generated from a stereo line-level source. It can serve as an active crossover component where balanced +4 dBu signals are used and low-frequency output must be isolated from higher-frequency program material. The adjustable boost is useful when the desired bass response calls for additional output at the lower end of the operating range. With two 50 kΩ balanced inputs and one 150 Ω balanced output, the unit is configured for integration into balanced line-level signal paths.

Why Choose This Model

This model combines active crossover filtering with adjustable subwoofer equalization in a dedicated component. Gain is unity at 63 Hz and can be adjusted from flat response to +5.5 dB at 32 Hz. This provides a defined range of low-frequency adjustment without requiring a separate equalizer for the stated boost function. The ST-CX2S also specifies THD+N below 0.05%, input common-mode rejection greater than 60 dB at 30 Hz or above, and residual noise below 90 dB when referred to +4 dBu, with 95 dB listed as typical.

Installation and Integration

Connection is based on two balanced line-level inputs rated at 50 kΩ and a balanced line-level output rated at 150 Ω. Both input and output levels are specified at +4 dBu. The power requirement is 24 Vdc at 30 mA, and the input can be ground-referenced or floating through the RDL SupplyFlex power input configuration. A power supply is not included. The recommended options are the PS-24AS for the USA and the PS-24AX for international use.

The specified ambient operating range is 0°C to 55°C. Before integration, the installer should account for the required 24 Vdc supply and the balanced line-level connections. The adjustable boost can then be set according to the required low-frequency response. The provided source data identifies engineering functional block, front panel, and end panel CAD files in American DXF format, along with a PDF specification sheet.

Performance Highlights

The ST-CX2S delivers an output bandwidth of 22 Hz to 80 Hz with adjustable boost. High-frequency attenuation exceeds 45 dB at 400 Hz, helping concentrate the output on the subwoofer frequency range. Headroom is specified at a minimum of 18 dB at 30 Hz with maximum boost, with 22 dB listed as typical. These values define the operating margin available when the low-frequency enhancement is fully applied.

Signal performance includes THD+N below 0.05% and input CMRR above 60 dB at frequencies of 30 Hz and higher. Together with the stated residual-noise performance, these specifications support the product’s role as a low-noise and low-distortion active crossover component.

What Makes It Different

Rather than providing only fixed low-pass filtering, the ST-CX2S incorporates adjustable low-frequency equalization. Its gain remains at unity at 63 Hz while allowing adjustment up to +5.5 dB at 32 Hz. This combination gives the user a direct way to shape the subwoofer range within the unit’s stated operating parameters. The balanced stereo inputs, balanced output, active crossover function, and flexible ground-referenced or floating 24 Vdc power configuration are integrated into one dedicated model.

Final Assessment

The RDL ST-CX2S is a purpose-built choice for creating a balanced subwoofer signal from a stereo source while providing controlled low-frequency enhancement. Its 22 Hz to 80 Hz output bandwidth, greater than 45 dB attenuation at 400 Hz, adjustable boost up to +5.5 dB at 32 Hz, and documented noise, distortion, headroom, and CMRR specifications make its capabilities clear. For installations using balanced +4 dBu line-level audio and an appropriate 24 Vdc supply, it offers a defined active crossover and equalization function.