Subwoofer filter

High Quality subwoofer filter and equalizer

Aways wanted to have that cinema sound in the very low regime in your home-cinema?
Here's your answer!
The document below contains the schematic of a high-quality subwoofer filter and equalizer module. It has the following features:

- 6 channel input mixer with pass-through outputs
- Adjustable gain 0x to 5x
- Adjustable cutoff frequency in 6 steps from 31Hz to 100Hz
- Very steep cutoff by using 4th order (24db / octave) Linkwitz-Riley lowpass filter
- 8-band constant-Q equalizer from 20Hz to 100Hz
- Output phase adjust 0/180 degrees
- Output relay ensuring quite powerup, even when amplifier is already powered and connected

Why do we need a 4th order filter?

Many subwoofer systems don't have a very steep cutoff filter. This makes that the subwoofer is still not completely quite at frequencies above 120Hz. This WILL give annoying artifactes, which are especially noticable in dialogs. On the other hand, some other subwoofer systems have a quite low cutoff frequency to overcome this problem. This is however also not desirable for obvious reasons.

Why do we need an 8-band equalizer?

A subwoofer is one of the most difficult speakers in your home-cinema sound system to adjust. Since the wavelength of the soundwaves produced by the subwoofer are quite long, it is very sensitive for the acoustics in your room. Some frequencies are greatly attennated in the room, while other frequencies are amplified. To overcome this problem, a fine-enough equalizer is needed to cutoff resonant frequencies and boost aborded frequencies.

Why not using a fixed compensation network?

The performance of a subwoofer is not only a function of the speaker itself. It is very dependant on the surroundings as well. It is known that even moving furniture will influence the performance of the subwoofer. Therefore, it is very interesting to make the frequency response of the filter adjustable.

Schematics and PCB

Subwoofer filter baseboard (schematics + PCB)
Subwoofer filter equaliser (schematics + PCB)