Tone, Tone, Tone !

@mijostyn, you sound like the guys on the Home Theater Subwoofer sites. They too consider the SPL output capability of a sub as the only criteria with which to assess it’s quality. Linkwitz and Richie/Ding designed their OB/Dipole subs for the reproduction of music, not car crashes and bomb explosions ;-). OB/Dipole subs are not for everyone, and obviously not you. Each to his own!

I still have a pair of the original HSU subs utilizing cylindrical enclosures. Also a pair of KEF B139 woofers each in it’s own quarter-wave transmission-line enclosure, and a pair of sealed subs with 15" woofers. I bought the latter as a kit, and designed my own 4 cu.ft. enclosure: 18" w x 24" h x 24" d, inner cabinet (cross-braced every 5") separated from the outer cabinet by 1/2", that 1/2" filled with sand. Got that idea from Danny Richie, who posted plans for his 12" sub on his GR Research website.

A 3dB boost at 20Hz is NOT a shelving circuit. A shelving circuit is a progressive low-pass compensation filter, not a static figure (6dB/octave, the boost therefore increasing as frequency decreases), which exactly compensates for the 6dB/octave front-to-back cancellation inherent in dipole speakers and woofers. Without it, an OB/Dipole sub WILL exhibit declining output with frequency, and of course not perform optimally. That's just a poor or incomplete design, not an insolvable weakness in the design. ALL designs present their own challenges to be solved by a speaker designer.

Siegfried and Brian Ding did just that, and their OB/Dipole subs do NOT exhibit the failing of a missing bottom octave. Linkwitz didn't chose to go with an OB/Dipole sub out of blind allegiance to some "pet theory". Give the a little more credit than that! He (along with Russ Riley) invented the Linkwitz-Riley filters, fer cryin' out loud!

The dipole-cancellation shelving circuit is well known to professional speaker designers, and is included in the DSPeaker Anti-Mode 2.0 Dual-Core. I wonder if the fact that both Gradient and DSPeaker are located in Finland has anything to do with that?

@mijostyn, what were the dimensions of your OB/Dipole sub frames? Were they H-frames, or W-frames? Linkwitz chose to go with the W, Danny Richie with the H (though he provides plans for both on his GR Research website).

Did you incorporate the mandatory shelving circuit (6dB/octave boost below 100Hz) to compensate for the endemic dipole cancellation? The OB/Dipole version of the Rythmik plate amp does, and the sub absolutely reproduces the bottom octave. Honest! ;-)

Siegfried Linkwitz was nothing if not a methodical scientist, measuring everything. He published all the test results of his measurements on the LX521 loudspeaker, including those of its’ OB/Dipole woofer. That woofer too reproduces the bottom octave, and the LX521's electronics include the OB/Dipole-mandated shelving circuit. That shelving circuit is NOT optional in an OB/Dipole sub, it is mandatory.

Speaking of transparency.....it was when I read Harry Pearson say in a review of a loudspeaker that it was "transparent in the same way the original is" that I realized he didn’t actually understand what the term transparent means. If the original were "transparent", you wouldn’t hear it! That's like saying a photo copy is as transparent as the original. If the original were transparent, there would be nothing to see or copy! 

Transparent means what J. Gordon Holt said it did: the ability of a component to be invisible, like an open window. Lack of transparency (JGH sometimes likened that to a layer of what he called "scrim" placed between source and listener) is any veiling, adding of texture, change in vocal and/or instrumental timbre and color, reducing of dynamic contrasts, hardening, or any other artifact added to the original; Doug Sax performed bypass tests to evaluate the transparency of any piece of gear he was considering for use in his studio. If the insertion of the component was absolutely undetectable, it was perfectly transparent. If it added any of the above, it wasn’t. Interestingly, in spite of their unequalled transparency, Sax didn’t choose ESL loudspeakers for monitoring his recording and mastering work.

@mijostyn, Siegfried Linkwitz disagreed (R.I.P.) with you ;-). Have you actually ever heard an OB/Dipole sub, or are you speaking in purely theoretical terms?

Rythmik's Brian Ding, even though collaborating with Danny Richie on the OB/Dipole Sub (it is more Danny's baby than Brian's), finds it to sound too "lean" for him. One complaint about subs mated with planars is their tendency to sound too 'plump", a little fat and tubby. That can certainly not be said about the OB/Dipole, and one reason why it works so well with planars. 

There are those who still feel the best bass QUALITY they ever heard was that produced by the two bass panels of the Magneplanar Tympani loudspeakers; the original T-I up through the final Tympani, the T-IVa. The T-IVa (upon which the new MG30.7 builds) also contains the great Magneplanar ribbon tweeter, and a "pretty" good (;-) magnetic-planar midrange driver. Not quite as transparent as ESL’s, but what is?

One aspect of ESL’s (in fact, all planars) that cannot be ignored is their line-source sound propagation characteristics. Their wave-launch is completely different than that of a point source, and it would appear a person prefers one or the other. When I replaced my Tympani T-I’s with Fulton Model J’s in 1974, I learned I was a line-source man. As with everything, ya gotta learn and chose your priorities, ’cause ya can’t have it all.

@prof, I too have heard that "ghostly sound" from planars (most recently a pair of Maggie 1.7’s), but that can be and often is a result of comb-filtering caused by the back wave of the speaker bouncing off the wall behind it, meeting up with the front wave, and causing frequency-related cancellation. Planars are less effected by sidewall reflections than are point source loudspeakers, but much more effected by those from the front wall.

That an OB/Dipole sub cannot produce anything below 40Hz is complete and utter nonsense, assuming it is constructed properly---in an H-frame (GR Research/Rythmik) or W-frame (Linkwitz. See below), to prevent front-to-back dipole cancellation. Anyone who has heard the Gradient made for the 63, such as yourself, can attest to that fact. Robert E. Greene reviewed the Gradient/QUAD 63 combination in TAS, and reported no lack of bass below 40Hz. Same with those (such as myself) who have actually heard the GR Research/Rythmik OB/Dipole Sub.

Remember too that Siegfried Linkwitz employed an OB/Dipole woofer section in his outstanding LX521 loudspeaker, and it also had no problem reproducing the bottom octave. Does anyone really believe an engineer with as much knowledge of and talent at designing loudspeakers as had Siegfried would let one out of his lab if it had no bottom octave output?!

What IS true is that the output in general of an OB/Dipole sub is quite a bit lower than that of a sealed sub using the same driver (for instance, it takes four of the GR Research/Rythmik OB’s to equal the output of a single Rythmik F12G). But that is unrelated to it’s bottom octave---it is frequency-unrelated. It is for that reason some OB/Dipole sub owners use them in multiple sets, stacked atop one another. A Google Images search will lead you to pics of two, three, even four OB/Dipole Sub stacks. Not cheap, but SOTA never is. ;-)

On the other hand, the Gradient SW-57, made for the original QUAD, WAS deficient in the bottom octave. But then, it employed a pair of 8" woofers, and they can play only so low, whether in an OB/Dipole design, sealed, ported, or infinite baffle. The LX521 uses a pair of 10" Seas aluminum-cone woofers, the GR Research/Rythmik a pair of 12" paper-cone woofers designed by Richie and Ding and custom-manufactured for them. The same woofer, with an aluminum cone, is the one Rythmik installs in their F12 sealed sub (the F12G has the paper-cone woofer. Confused yet? ;-).

@prof, knowing how well the OB/dipole Gradient sub integrated with the QUAD 63, you may be interested to know that there is now a contemporary OB/dipole that does everything the Gradient did, and more. It was developed as a team effort between Danny Richie of GR Research and Brian Ding of Rythmik Audio, and it is really special. It is comprised of a pair of 12" servo-feedback controlled (Rythmik’s reason d’etre) woofers installed in an OB H-frame. It is THE sub for dipole loudspeakers.

Danny Richie showed at RMAF for a few years, using a pair of the OB/Dipole subs at the front of the room and a pair of sealed subs (Rythmik F12G’s) at the rear. His room was voted "Best Bass At The Show" three years running.

Feeling that in Popular music (non-classical), the melody and vocalist are my number one priority (I am hesitant to say that, as it may seem to minimize my love of harmony and counterpoint, as well as the chord structure of songs), the lifelike reproduction of singers and their vocals has to be my personal number one in the reproduction of recorded music.

J. Gordon Holt was a stickler for the reproduction of vocals without what he called "vowel colorations" (he single-handedly created much of the audiophile vocabulary, though Harry Pearson loved to take credit for that accomplishment) . When I read a review of a loudspeaker by JGH in the very first copy of Stereophile I received in 1972, I immediately knew what he was talking about when he used that phrase. We hear voices everyday, and if a loudspeaker (or recording) produces an un-natural tonal-timbral sound, we instantly recognize it as such. How anyone can listen to music through a "colored" loudspeaker is a mystery to me.