GR Research NX-Extreme

m-a, I didn’t mention it, but the Rythmik A370 plate amp (the amp that is included in the GR Research/Rythmik OB/Dipole Sub kit includes a continuously-variable Phase/Delay control, providing phase rotation from 0 degrees to 180 degrees (0-16 ms). That phase rotation creates a time delay exactly the same as physically moving the sub further from the wall!

In fact, that phase control is found on all the Rythmik plate amps, including those installed in sealed and ported models. I would not buy or own a sub without it. While I myself like the GRR/Rythmik OB/Dipole Sub, a few found it to sound too unlike what they think a sub should sound like ("thicker", "plumper", with more "weight") . One who feels that way is the owner/designer of Rythmik, Brian Ding! For those people, Rythmik offers many sealed and ported subs, including the F12 (a single 12" driver in a 1.5cu.ft. enclosure with a 600w amp) and F15 (a 15" in a 2cu.ft. box, same amp).

Lots of info---including unusually technical---available on the Rythmik website. For discussions about the OB/Dipole sub, take a look in the GR Research section of the AudioCircle Forums. 

Sure @martin-andersen, as with all dipoles, any of the GRR open baffle loudspeakers and subs can be placed as close as 3’ from the wall behind them (you call it the back wall, but if you’re facing it, it’s the front wall ;-) . 4’ feet is even better, 5’ better yet.

Sound travels at around 1’ per millisecond, and what we want to achieve is a 10 millisecond difference at the listening position between the direct sound from the front of the speakers/subs and that coming from the rear after being reflected off the "front" wall. A minimum of 10 ms is what is required for the sound of two acoustic events to appear to be separate from one another. 5’ spacing automatically creates that 10 ms delay (5’/ms from the rear of the speaker/sub to the wall, 5’/ms back to the speaker/sub), That 5 ms difference makes the rear wave sound like an acoustic event separate from that coming from the front of the loudspeaker, rather than part of the direct sound, a "smearing" of the latter. By the way, Danny Richie recommends the use of diffusion rather than absorption on the wall behind dipole loudspeakers. I concur.

As subs handle frequencies with very long wavelengths, the 10ms difference is not nearly as important for subs as it is for the shorter wavelengths/higher frequencies handled by the loudspeakers. Far more important is the phase relationship between the front and rear waves produced by each sub. The front and rear waves are in opposite polarity (180 degrees apart) relative to one another, and as the long wavelength bass frequencies "wrap around" the sub OB baffle, they meet on the two end sides of that baffle and cancelled out (one of the reasons dipole subs don’t energize as many room modes as do non-OB subs); + 5 added to - 5 = 0.

The rear wave also of course travels to the wall behind the sub, is reflected back towards the sub, and when that rear wave meets up with the front wave can create an either in-phase addition or an out-of-phase subtraction of lower frequencies. OB/dipole sub users need to experiment with sub placement to optimize the phase interaction between the front and rear waves. That might sound like a daunting task, but when you hear a good OB/dipole sub you understand why it’s worth it!

Oh, I neglected to comment on the 4 ohm version of the GR Research woofer. GR Research offers a 4 ohm woofer, but not for use in open baffle applications, only sealed and ported. The non-ob version has not been optimized for ob use, and the two are not interchangeable. So the non-ob woofer is available in 4 ohms only, the ob woofer in both 8 and 16 ohms, but not 4.

Another point: the GR Research 4 ohm woofer is exactly the same as the 12" woofer found in the Rythmik F12, except the GRR has a paper cone, the Rythmik an aluminum. Rythmik sells the F12 in a version with the paper cone woofer, model F12G. GR Research, being a company aimed at DIYers, offers the F12G as a kit only. You get the woofer and the Rythmik plate amp, and build the enclosure yourself. Or buy the version Jim Salk offers, with the F12 kit installed in an incredible box of Jim’s design and build. His enclosure has the best bracing I have ever seen!

Is everyone now thoroughly confused? ;-)

@maplegrovemusic , here are a couple of pointers about building a GR Research kit:

Glue: I recommend Titebond III Ultimate. It gives you longer "open" time before starting to harden than do Titebond Original and II. Use more glue than you think necessary; the MDF that the flat pack enclosures are made of soaks up glue. Use a small paint brush to spread the glue evenly (you can get a bag of small aluminum-handle brushes at Harbor Freight). After you tighten the clamps, wipe off the glue "squeeze out" with a damp sponge.

Clamps: I really like the Jorgensen Heavy Duty (1,000 lb.) clamps. Lowe’s has the best price I found: $21 for the 36" online (Lowe’s doesn’t stock them, but their shipping is free and fast). You’ll need at least four, more is better. If you want to spend less, Harbor Freight has some okay clamps that are really cheap.

Before gluing, do a "dry fit": assemble the enclosure, using masking tape to keep all the panels together. Looking at it assembled will help you think in an organized fashion. Once you apply the glue, a feeling of panic may overtake you! Prepare, think ahead, and remain calm. If this knucklehead can do it, so can you ;-) .

Read through as many "build" threads on the AudioCircle GR Research Forum as you can. You will learn how other guys went about their builds, the mistakes they made, and what they advise. You can ask them questions; Danny’s customers are a really great group of fellas!

As for finish, check out the black spray paint John Deere makes and markets (yeah, the tractor company!). It’s sheen is what’s called "Hot Rod Black" in the custom car world. Really cool, chicks dig it ;-) .

There are a couple of You Tube videos documenting how the GR Research loudspeaker flat packs (precut MDF panels you glue together and finish) are assembled. You don't need any woodshop tools except for some wood clamps (and glue, of course). The flat packs are very easy to put together, made with alignment dowels (just like IKEA furniture) and "biscuits". I love that you can finish them (or have it done for you, as in an automotive body shop for a Wilson Audio-quality paint job, or cabinet shop if you want a genuine wood veneer finish) any way you want.

Build threads are also posted on the Audiocircle GR Research Forum website. Join the fun! Once you get used to the idea, you realize it can take your hi-fi involvement to a new level, like super-charging your car's engine with a bolt-on blower kit. Ultra-high performance for the price of mass-produced mediocrity.

Danny Richie designed and has manufactured his own drivers (including the NEO3 planar-magnetic tweeter), and his cross-overs are the best in the business. Don't wait for a review of a GR Research product in Stereophile, it ain't gonna happen. But lots of speaker companies pay Danny to design the x/o's for their loudspeakers, He is a leader in the open baffle DIY market, highly respected by his peers, of which there are few. Um, does this make me a fanboy (I hate that term)?