My current speakers are B&W Nautilus 805s and have a -3dB point at around 43Hz. With 5.5″ drivers they lack the ability to move large amounts of air needed to reproduce the bottom octave of human hearing. I decided to build a subwoofer to cover the 23-43Hz missing from the sound system, and hence the name “Elusive Octave”
The main driver is a Pearless 830845 XXLS. This 12″ Nomex Cone driver has a large extension and is capable of moving incredible amounts of air. In retrospect the speaker has a high Q point and as such requires a large box for a sealed enclosure so the 830500 would likely have been a better candidate, but live and learn.
The driver is capable of 350watts continuous RMS with a Qts and Qms of ~0.45 and an Fs of 35Hz. When targeting a final Q response of 0.707 this necessitates a 120L sealed enclosure. Sealed systems are simpler to design and often result in tighter sounding bass. The 120L enclosure could also be designed to fit perfectly under an existing table so there was no great loss.
The box is constructed out of 20mm MDF sheets. The main mount for the driver has a second 20mm MDF sheet laminated behind it for additional support around the weak point. The long length of the subwoofer necessitated further bracing either side of the driver. For maximum stiffness the bracing was built as one sheet with centres routed out.
All parts of the subwoofer enclosure including the bracing were glued with wood glue and the screwed with long timber screws at 200mm intervals. This provided a very sturdy construction accessible only from the front of the enclosure. The inside of the enclosure was stuffed with PETE (Polyethylene terephthalate) or Dacron (trade name). This material dampens the inside of the enclosure making it appear larger than 120L to the driver.
The amplifier driving the subwoofer is a 2x 250watt unit configured as a bridged amplifier, and thus capable of driving 1x 500watt into the perceived 4Ω load. The main amplifier is followed by a slightly complicated preamp.
The preamp schematic shows several distinct sections.
- U1 sums the L and R channel, provides approximately 15dB of gain and has a fixed low pass filter with a cutoff at 120Hz.
- U2 provides a selectable invert. By grounding the +ve input via a switch it’s possible to change the gain from +1 to -1.
- U3 Provides a 2 pole low pass filter. By varying R12/R15 from 0Ω to 100kΩ we can alter the cut-off frequency from 350Hz to 12Hz.
- U4 is identical to U3 providing a total of a 5 pole low frequency response.
- U5 is the more interesting part of the preamp. This Linkwitz transform circuit cancels the existing frequency response of the subwoofer and provides new response in this case cancelling a 35Hz Q7 and replacing it with a 20Hz Q5 response. This works as subwoofers are still capable of extending their response below their critical frequency providing there is sufficient power and speaker extension available.
The end result… thundering bass. Oh and there’s a volume control on the preamp output too.
The final finish is a grey 2pac coat bluffed to a glossy finish. A square speaker cover was added.