Linkwitz-Reiley Low Pass Filter for Subwoofer Projects

By: Nathaniel "JojoD818" Dijamco / October 2003

Introduction:

A very effective approach in actively powering a subwoofer is to provide it's own power amplifier and an active low pass filter or some form of signal conditioning prior to amplification. This project is an extremely effective filter for any subwoofer project. It provides low inputs from preamps, tone controls, equalizers, etc. and a high input for directly connecting it to the output of your source amplifier or even a satellite/subwoofer (3.1) configuration by just adding a simple passive high pass filter for your satellites.

The circuit is simple and only consists of 3 stages, the input summing mixer and attenuator, phase correction, and the Linkwitz-Reiley low pass filter.

The input mixer takes it's input from either the high or low inputs and mixes the left and right channels into a single channel and is fed to an inverting opamp that provides phase correction. Attenuation is controlled by the 50K potentiometer that is actually the feedback resistor of the summing opamp.

The phase switch is provided to make your sub in or out of phase with your whole system.

Finally, the signal is fed to an 8th-order (48dB) Linkwitz-Reiley low pass filter and it's output is connected to your amplifier.

The circuit requires 1 dual opamp package and 1 quad opamp package, your favorite opamp should work fine with this circuit. Suggested supply rail voltage is a regulated +/-15V and as always, must be decoupled accordingly.

Values for resistor R are indicated in the schematic diagram, personally, I use 10 Kohms for resistor R for a cut-off frequency of about 112Hz. 

Schematic Diagram:

PCB Parts Placement Guide:

PCB Foil Pattern:

Actual circuit picture:

Another one in action: