Eminent Technology LFT8B’s reversing woofer polarity

To turn the LFT-8b in an absolute giant-killer (actually, even in stock form it is), use a pair of GR Research/Rythmik OB/Dipole Subs in place of the LFT’s 8" woofers. Unlike "normal" subs, the OB/Dipole will play up to the 8b’s x/o frequency of 180Hz.

Until you do that, there is a remedy for the resonance in the 8" woofers output (in the 1k-1.5kHz range, way above it's x/o frequency), heard as a low-level "whistle". "Davey" has posted on The Planar Speaker Asylum a simple, cheap low-pass filter that suppresses that resonance to the point of inaudibility. It is just a single cap and resister, placed across the woofer's terminals (on the back of the woofer, not the speaker terminals).

Scot, here’s the deal with GR Research, Rythmik, and the OB/Dipole Sub. GR Research is a 1-man company (Danny Richie) specializing in two things: 1- open baffle speakers; 2- kits for DIY speaker builders. Danny is also well known for his crossover expertise. Rythmik is also a 1-man company (Brian Ding), known for it’s patented Direct Servo Feedback subwoofer design and the sealed and ported subs incorporating the servo design.

Danny was already offering a sealed sub when he learned of Brian’s then new servo system, and thought combining an open baffle woofer with the servo system would make a killer sub. Danny contacted Brian (they are both located in Texas), the two put their heads together, and the result is the OB/Dipole Sub.

Both men and companies sell the sub, in kit form only. Rythmik lists the sub on their website in the "Custom Installation" section. The listing shows the A370 plate amp and 12" woofer that comprise the kit, not installed in the requisite OB frame into which the kit is installed (the consumer is responsible for the latter---see below).

GR Research lists the sub on it’s website in the "DIY Subwoofer Kits" section. In that section, all the various versions of the OB/Dipole Sub (1- economy plate amp and one woofer; 2- that plate amp and two woofers; 3- the more advanced A370 plate amp and one woofer; 4- the A370 plate amp and two woofers; and 5- the A370 and three woofers) are pictured, again as on the Rythmik site just the plate amp and woofers, no OB frame.

In each of the OB sub listings on the GRR site, links to diagrams for both W- and H-style OB frames are provided (just click on the highlighted link), as well as a link to a woodworker in Canada who is making fantastic H-frames (for one, two, and three woofers) in flat-pack form. The frames are CNC cut out of 1" (baffle) and 1.5" (top/bottom/sides) thick MDF, with dadoes and alignment dowel pins to make assembly very easy (only wood glue and clamps required).

For lots of info, pictures, and discussion of the OB/Dipole Sub, head over to the GR Research section on the Audiocircle Forum.