300 Hz and 500 Hz High Pass Filter

The RDL ST-HP3 is a compact, fixed high-pass filter designed for horn loudspeaker protection in balanced line-level audio systems. It attenuates low-frequency content below 300 Hz and 500 Hz, providing two filtered outputs without requiring field adjustment. The module accepts a 20 kΩ balanced line-level input at +4 dBu and supplies two 150 Ω balanced line-level outputs at +4 dBu. Its defined filtering, compact dimensions, and flexible 24 Vdc power configuration make it a focused solution for integrating low-frequency attenuation into an audio signal path.

Ideal Use Cases

The ST-HP3 is intended for systems where low-frequency content must be reduced before a signal is used with horn loudspeakers. Its separate 300 Hz and 500 Hz outputs accommodate two filtering requirements from a single line-level input. This configuration is useful when an installation calls for a fixed response rather than adjustable filter controls. Because filtering requires no field adjustment, the module is also suited to installations where consistent settings and a straightforward commissioning process are priorities.

Why Choose This Model

This model combines two specified high-pass responses with balanced +4 dBu input and output connections. Its attenuation curve is 18 dB per octave, while rated headroom is greater than 18 dB. THD+N is specified at less than 0.01%, and insertion loss is less than 2 dB. These published figures provide clear performance parameters for determining how the module will fit into a balanced line-level system. The dual-output design also avoids the need to select or calibrate an adjustable cutoff frequency in the field.

Installation and Integration

Integration centers on connecting the balanced source to the 20 kΩ input and using the appropriate 150 Ω output for the required filter response. Both the input and outputs are specified for +4 dBu line level. The ST-HP3 operates from 24 Vdc at 30 mA and supports either a ground-referenced or floating power arrangement through the RDL SupplyFlex power input configuration. A power supply is not included. The listed recommended supplies are the PS-24AS for the USA and the PS-24AX for international use.

The enclosure measures 0.7 inches high by 3 inches wide by 1.6 inches deep, equivalent to 1.7 by 7.6 by 3.9 centimeters. Its specified ambient operating range is 0° C to 55° C. Since the filtering is fixed, installation does not require cutoff-frequency adjustment after the signal and power connections are completed.

Performance Highlights

The 300 Hz output has a specified frequency response of 300 Hz to 30 kHz within ±0.5 dB, with a -3 dB frequency of 225 Hz. The 500 Hz output has a specified frequency response of 500 Hz to 30 kHz within ±1.0 dB, with a -3 dB frequency of 450 Hz. Both responses use the stated 18 dB-per-octave attenuation curve to reduce content below their respective operating ranges.

Additional performance specifications include greater than 18 dB of headroom, THD+N below 0.01%, and insertion loss below 2 dB. Residual noise is listed as less than 90 dB when referred to +4 dBu. Together with the balanced input and two balanced outputs, these specifications give system designers concrete signal-level, impedance, and filter-response information for integration planning.

What Makes It Different

The defining feature of the ST-HP3 is its combination of fixed 300 Hz and 500 Hz high-pass filtering in one compact line-level module. Instead of presenting adjustable filter controls, it provides predetermined responses that require no field adjustment. The two output options make the filtering choice explicit, while the SupplyFlex configuration accommodates ground-referenced or floating 24 Vdc power. This focused design directly addresses low-frequency attenuation for horn loudspeaker protection without adding unspecified processing features.

Final Assessment

The RDL ST-HP3 is a purpose-specific choice for balanced +4 dBu systems that need fixed low-frequency attenuation below 300 Hz or 500 Hz. Its dual outputs, 18 dB-per-octave curve, compact enclosure, and no-adjustment operation provide a clearly defined approach to horn loudspeaker protection. It is best matched to installations that can supply 24 Vdc at 30 mA and require one or both of its documented filter responses.