Yes, if you retain the paper-cone B-209 you can certainly move the crossover points lower and obtain the benefits of moving the larger woofer out of the midrange band where it less well reproduces signal. The flower pot modification is
always a good idea because cross-modulation is distortion.
Tobin's commentary on the aluminum-cone B-209 is not about an error in new technology at all; it is about the
polarity inversion implemented by Bozak which made the midrange more pronounced. Whomever told you that Tobin felt the aluminum-cone B-209 ruined the Bozak sound is incorrect. See below for details.
Here's what I've previously written on the subject to explain the basic issues, slightly edited for clarity and space.
Absolute Polarity
The absolute polarity, commonly called "
acoustic polarity", and incorrectly as "
absolute phase", concerns the direction of compression or rarefaction wavefronts. In short, does a positive signal move the speaker forward or backward. The reason this matters is that our ears were optimized by evolution to prefer compression before rarefaction as this is how sound happens in nature.
This difference occurs because of the
Wood Effect. See
The Wood Effect: Unaccounted Contributor to Error and Confusion in Acoustics and Audio by Clark Johnsen (1988). You can do a search on the Internet for further reading.
Somewhat simplified, the issue is a difference in how the ear and brain process the different waveform types:
Compression Wave (0 —> Vmax —> 0)
versus
Rarefaction Wave (0 —> Vmin —> 0)
In this case
Vmax is the peak for the positive half of the waveform and
Vmin is the peak for the negative half of the waveform. They are used to convey the different halves of the waveform in an AC cycle.
Compression before rarefaction occurs in instruments and real-world sound; the ear/brain prefers this relationship. An additional complication is that the two waveform halves are rarely symmetric for real-world sound, and the speaker may exacerbate this asymmetry, distorting the sound.
You'll find that the following situations can apply:
(a) playback equipment reverses polarity (preamplifier or DAC) compared to recording
or (b) the recording's polarity was reversed in recording, playback equipment maintains it
or (c) both equipment and recording polarity are reversed
or (d) both equipment and recording polarity match
Some equipment contains a polarity reversal switch to restores proper phase. This is different than a switch to swap the polarity of only one speaker to make both speakers have the same polarity (i.e. the +/- at the amplifier matches that at the speaker), but not absolute polarity (i.e. both speakers move in the same direction, but do not prefer compression before rarefaction). So the polarity may be mostly correct or mostly incorrect depending upon your equipment, but even when you have the right phase for the equipment the individual recording may or may not be in absolute polarity with respect to playback equipment.
In summary, configure your speaker polarity for the majority of recordings and switch it for the minority, and rejoice in discovering new depths to sound.
Polarity Reversal in Bozak
Here's what I elsewhere wrote on this subject, which has some relevance here. Again slightly merged and edited for clarity and space.
Midranges may be wired with reverse polarity with the woofer and tweeter for better transient response and phase coherence. The ostensible goal is to minimize the non-linear phase shifts across the driver's spectrum. Some, as Betts below describes, prefer to take the hit in the center of the band so that is out of phase with the woofer and tweeter. Others have the high/low portions of the band be out of phase so that the main portion, where we hear the best, be in-phase. Except this is incorrect, because altering polarity does not alter phase, as the two are separate phenomenon. (I will post this in a subsequent message.)
The issue is accurately described as a polarity reversal not a phase reversal, which is incorrect, because (as above set forth) the rarefaction and compression waves are reversed when the driver polarity is reversed. But it also has phase-like characteristics because the driver direction is now reversed and thus 180 degrees out of phase. This makes the midrange more pronounced because the inversion of polarity alters the sound.
Consider why this works. The midrange has a driver on either side, whereas the woofer only has a driver on one side and we do not well hear phase shifts in low frequencies. The issue thus simplifies to enduring phase shifts in the upper end of the woofer region which would be more audible than the lower-end ones vs. phase shifts at the endpoints of the midrange compared to the adjoining drivers. So it appears to be an attempt to better merge the woofer-midrange and midrange-tweeter.
Bozak, like many manufacturers, chose the path of reversing the midrange to smooth the transition to woofer and tweeter and make the transition less pronounced, which had the side-effect of making the midrange more pronounced because of the reversal of polarity. Given the writings on the subject I do not think the latter was the intended effect, although it is often described as such.
Others prefer to maintain the midrange with identical polarity and use lower-order crossovers which add less phase shift of their own from component reactance.
The important fact is that Bozak made such a change and the Tobin modification undoes it. Bob Betts, Rudy Bozak's right-hand man and the co-designer of the famed Bozak 909/919 preamp and 929/939 amps among others, discusses the transient response and phase coherence:
www.bobsamerica.com/bozak-xoveranalysis.html
After much analysis, testing, and evaluation we made the decision to accept the phase shifts at the center of the midrange band, rather than at both ends of the midrange band. We then sat back and waited for the fireworks to begin. Oddly enough, there were amazingly few! This is surprising, since customers are usually very quick to express a complaint, but remain conspicuously quiet when content. The feedback from our dealers, reps and end-users was almost unanimously complimentary. One Sunday morning I received a phone call from Benny Goodman asking when I was going to come to his studio and "...make the new repairs that I heard at the factory?"This is not a "yes" or "no" or good or bad decision. It is a gray area of engineering that is associated with the subjectivity of how we listen, what we listen for, our personal tastes, our ear-to-brain calibration from live vs. reproduced experiences (or lack thereof), and the intuitive thought process. The condition (and dilemma) of where to place the phase incoherency is a trade off - neither is technically correct when applied to a passive crossover, but quite often, the intuitive thoughts of a semi-technical evaluator will weigh in favor of the in-phase condition, only because "it seems to make sense." Oddly enough, Bozak Inc. was one of the last manufacturers to effect the midrange phase reversal. At that time, according to our research of currently available 3-way speaker products, about 85% to 90% of the speaker industry had already made the "correction" changeover.
www.bobsamerica.com/bozak-xoveranalysis.html
If a speaker sounds like the sound is coming from inside the box, then it probably has poor transient capability, and if it sounds dull and lifeless, then the damping qualities are probably not very good. Here’s a standard test that engineers have used over the years: Play a known (to you), small jazz combo of acoustical (not electronic) instruments and listen to the bass drum and string bass. Are they separate and distinct? Or is the woofer just flopping around in rhythm to the beat? Bass is bass, but what is it that makes up the bass—can you tell by listening? So this is basically why two loudspeakers with similar “published” specifications can sound so vastly different. It would seem that no one ever offers specifications on IM, harmonic, and phase distortions. In my 40-plus years as an audio products design engineer, it has never ceased to amaze me how audio manufacturers perpetuate the virtues of “flatness”—linearity—without even so much as a mention of other physics characteristics. Over the years, amplifier producers have been forced, by public demand, to publish such numbers as intermodulation distortion, harmonic distortion, damping factor, etc. But remember that these figures are gleaned, gathered and garnered on a test bench using sine waves as audio signals and resistors, not loudspeakers, as loads. Seldom will you hear any discussions concerning “dynamic stability” or “transient reproduction.” Likewise, the loudspeaker industry suffers similar, if not worse, negative virtues.
