A ported subwoofer can lose control of the driver below the tuning frequency of the port
A high pass filter can be used to protect the driver from excessive excursion.There are many active filter designs available, and one popular one is the Linkwitz-Riley filter, which has a rolloff of 24dB / Octave.
A detailed discussion of this filter is available at the Linkwitz Labs website, which suggests the following circuit.
Discussions with the WinISD people confirm that the formula for calculating the component values is as follows
Frequency = 1/(2 * pi * R * C * sqr(2))
For example R=20K C=0.33uF should yield 17Hz
Continuing the example, here is the graph for the “Sidewinder”, which has a design frequency (-6dB point) of 17Hz
Be warned, the filter designer in WinISD has some faults - their circuit shows both resistors the same, rather than one being twice the value of the other as shown above. This has been acknowledged by the authors, so I guess it would be fixed in their next production release. Also it doesn't calculate the correct values, so use the formula above or double check with another package
I made mine on some "Veroboard" - the extra pots were part of the experimental setup and were removed
once the final values were settled on.
If you prefer not to make your own layout, you can buy circuit boards that are very similar to the above circuit.
Rod Elliot has a project that basically uses the same circuit in a 2 or 3 way crossover. See his page to get a detailed description of how it all works. He also has circuit boards available!
Bob Ellis at DiyAudio has designed full crossovers that include notch filter and Linkwitz transorm capability. Check the discussion thread to see if there are any boards left over from the most recent group buy.
The filter was powered with a simple +/- 15V supply kit from Jaycar. If you prefer, you can buy good regulated power supplies, such as this "open frame" unit sourced by an American builder.
Whilst we're dealing with filters, this would be a good place to give a plug to the AJ Crossover calculator page, where you can model all sorts of crossovers.
If you're good at maths, you can find a comprehensive article on filters at maxim-ic.com
My “wish list” for the ideal subwoofer processor includes:
- A low pass filter to augment that which the plate amp already has - this would allow a faster cutoff to protect from pipe resonances that a long port can be prone to. Measurements of the signal coming out of my surround amp suggest content above 120hz, even though this is the theoretical upper limit for Dolby 5.1
- An equalisation circuit could be included for those who wish to build a sealed unit. (See the Bob Ellis link above)
- Continous phase adjustment. All the Jaycar plate amps have a phase switch for 0 or 180 degrees shift. There is some debate over the merit of a greater level of control being required
- Adjustable preamp gain. Handy for those who are starting out and don't yet have a surround amp. Most DVD players appear to output their LFE channel at a level that is lower than you get from a surround amp, meaning that a boost can be required before feeding the subwoofer
- A “soft clipping” circuit. This would protect your creation from the over-aggressive mixing on some DVD's. This would be something like the Fast Audio Peak Limiter design at Rod Elliot's site
