Genesis II.5 loudspeaker system
Building such ambitious loudspeakers as the Genesis products is a risky proposition. Big loudspeakers often fail miserablyany flaw seems to be magnified by the loudspeaker's lofty aspirationsthe bigger they are, the harder they fall. Moreover, the criteria for what constitutes good performance rises proportionally with the loudspeaker's price. For several tens of thousands of dollars, the loudspeaker had better deliver the musical goods.
I thus took delivery of the $22,000, half-ton Genesis II.5 system with a combination of excitement and trepidation. The excitement was over the prospect of having a world-class loudspeaker in my home; the trepidation was the very real possibility that the II.5 would have some unacceptable shortcoming. Because a loudspeaker of this ambition requires several months to fully evaluate (and because they're so hard to move), I knew the Genesis II.5s would dominate my system for some time.
The question was whether the II.5 would become a monstrosity I wanted out of my room as soon as possible, or a product that would become an essential part of my musical life. There would likely be no middle ground.
For your 22 grand you get two 6'-tall, 28"-wide panels with integral twin dynamic bass drivers and a woofer power amplifier. The II.5 is a tall, thin panel in which three 1" round planar tweeters are mounted to the inside edge of a 4' ribbon midrange unit. This panel is flanked on either side by curved rosewood-finished "wings" that define the II.5's shape and size. A sealed box attached to the bottom rear of this assembly holds two 12" woofersone front-firing, one rear-firing. The entire system rests on a flat, triangular-shaped platform. A narrow black grille covers the drivers, with a separate grille covering the rear-firing woofer.
The system has two controls on the rear of the woofer enclosure: one adjusts the midrange crossover frequency to the tweeters, the other controls the level of the single rear-firing tweeter. Input is via a pair of high-quality binding posts.
The II.5's woofer enclosure also has a Neutrik input jack for connecting the woofers to the Genesis servo amplifier, which is included with the system. (Servo-driven woofers are explained below.) Note that the Genesis woofer amplifier doesn't include the crossover for the midrange and tweeters. Instead, you connect the woofer amplifier to the II.5 via the dedicated cable, and drive the midrange and tweeters directly with an amplifier of your choosing. Your preamp, however, must have two stereo outputs: one pair to drive the woofer amplifier, one pair to drive the midrange/tweeter amplifier.
The Genesis is a true dipolar design, radiating energy to the front and rear of the loudspeaker. There is a single rear-firing tweeter to complement the ribbon midrange's inherent dipolar nature. Dipoles radiate sound in a cardioid (heart-shaped) pattern both to the front and rear of the panel, producing a null at the sides.
Because the ribbon midrange is a line source, it has a very narrow vertical dispersion; ie, it radiates very little energy above and below it. The result is less reflected energy from the floor and ceiling. Dipoles are also different in that they require space behind them to work their bestthe II.5s need to be placed well out into the room for optimum performance. Moreover, compared with a point-source box loudspeaker, the rear wall's acoustic properties have a greater influence on how a dipole will sound in the room.
As with Arnie Nudell's first loudspeaker designthe Servo Statik I of 25 years agothe II.5 uses servo-driven woofers. A woofer servo operates by comparing the drive signal sent to the woofer with the woofer's actual motion. An accelerometer attached to the woofer's voice-coil sends a signal back to the servo amplifier via a custom cable supplied with the II.5. Differences between the drive signal and woofer motion are a form of distortion, and can be corrected electrically by modifying the drive signal. In essence, the servo system forces the woofer to behave in a predetermined way. In the II.5, only one woofer carries an accelerometer, with the amplifier assuming both woofers are behaving identically (footnote 2).
A servo system can also extend a woofer's low-frequency cutoff point, regardless of the enclosure size. Indeed, the II.5's woofer enclosure seems far too small for a pair of 12" drivers. In fact, the Genesis woofer in its enclosure has a resonance frequency of 70Hz. Without a servo system, the low-frequency extension would be severely compromised (a sealed enclosure produces a rolloff of 12dB/octave below resonance). As the woofer begins to roll off naturally, however, the servo system simply drives the woofer harder and harder to increase the system's extension. This is how the II.5 can achieve a 3dB point of 16Hz from such a small box.
This technique places extraordinary demands on the woofer and the power amplifier driving it, particularly at very low frequenciesthe lower the frequency, the more current the servo amplifier must force through the woofer's voice-coil to produce an equivalent sound-pressure level. The II.5's woofers, designed by Arnie Nudell specifically for these rigorous conditions, are made of two aluminum cones bonded together with a damping compound. This "tri-laminate" design is reportedly so stiff that the cone is virtually impervious to flexing (footnote 3). With a whopping 2"-diameter voice-coil and a 30-lb magnet, the driver can produce excursions of a full inch. To further increase bass output, the II.5 system uses a total of four of these servo-driven woofers.
The midrange is a 4'-tall ribbon driver made from a 0.001"-thick layer of Kapton. The term "ribbon" as applied to the Genesis midrange is actually a misnomer; in a true ribbon, current flows through the diaphragm itself. The more correct term for the Genesis midrange is "planar magnetic," a design in which a conductor is bonded to the diaphragm. Despite the semantic distinction, the Genesis ribbon has all the advantages conferred by such low moving massprimarily excellent transient ability.
Footnote 1: At the time of this writing, Genesis is just starting to ship the much smaller $12,950 Genesis V. See my interview in this issue with Genesis founders Arnie Nudell and Paul McGowan.
Footnote 2: The $70,000 Genesis I uses six 12" servo-driven woofers per channel, each with its own accelerometer. Yeow!
Footnote 3: A woofer cone tends to flex as a voice-coil pushes it forward, because it's being driven from a small point at the cone center where the voice-coil is attached. This tendency is exacerbated in a servo-driven woofer because of the high excursion required. Moreover, an accelerometer placed on the voice-coil doesn't know what the rest of the cone is doing. This is why the II.5's woofers were specially designed for this application.