Active subwoofer filter circuit

This active subwoofer filter system provides the frequencies required to run the subwoofer within its limits. The purpose of this unit is to prevent the subwoofer speaker or circuit from overloading at unsupported frequencies. This approach can ensure the safety of the subwoofer speaker or driver circuit.

This system consists of TL074 based active lowpass filter, highpass filter, and preamplifier stage. The lowpass filter is in 880Hz cutoff frequency. Highpass filter cutoff frequency can select from the rotary switch. In this given design, the highpass cutting frequencies are at 80Hz, 115Hz, 150Hz, and 180Hz. Use the calculator application shown here to use this system for frequencies other than those mentioned above.

Finished prototype of an active subwoofer filter circuit.

In this circuit, both lowpass filter and high pass filter uses a 3-pole Butterworth filter configuration. Here all the filter component values are calculated using the calculator application mentioned above.

This circuit is useful to match the incompatible amplifier systems and subwoofer systems. Our primary test setup consists of few Yamaha active subwoofers and a Denon AVR system. Without this control circuit, we can see that the audio system produces loud noises at very low frequencies and puts a lot of pressure on the subwoofer speakers. After connecting this circuit, the problem disappeared, and the system generated a clean audio output.

Rear view of the active subwoofer filter circuit prototype.

To minimize the PCB size, we design this PCB using SMD components. Most of the resistors and capacitors used in this PCB are select from the 0603 packages. For the op-amp, we use a dual in-line package so that the user can easily change it without any soldering. The most common quad versions of the op-amps can use with this circuit, including TL074, TL084, TL064, LF347, etc.

This circuit is designed to power using a dual-rail 12V power supply. In our prototype, we used a 230V to 15V+15V, 15VA step-down transformer with this circuit.

All the soldering, mounting, and alignment procedure shows in the video attached to this article.

As mentioned above, at the testing stages, we create two versions of this circuit. The initial version consists of two 4558 op-amps and a highpass filter with a fixed cutoff frequency. The calculator application also builds around this design.

Version 2 of this filter circuit uses the same design, but we include a highpass cutoff frequency selector to this circuit and used a more convenient quad version of op-amp.

To adjust the GAIN control, apply 100Hz, 2Vpp, waveform into the audio input. Next, connect the oscilloscope to the output terminal and adjust the control. When making this adjustment, be sure to set the GAIN control to the location where the noise-free output will receive. The complete alignment process shows in the video attached to this article. According to our observations, keep this control in low or mid-gain positions for best results.

Internal view of the active subwoofer filter circuit prototype.

The RV2 provides to adjust the current going through the power indicator LED. For most of the LEDs, set this to a mid-position is sufficient.

This project is an open design. All the circuits and KiCAD design files are available to download here. The source code of the calculator application is also available at github.com.

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