APOGEE CENTAURUS

SLANT 6 LOUDSPEAKER

Robert Harley

The idea of mating a dynamic woofer to a ribbon midrange/tweeter is appealing on paper. Such a "hybrid" loudspeaker would have the many advantages of a dipole ribbon transducer, yet be more practical and af­fordable than full-range ribbon designs. Among the ribbon's great strengths is its narrow vertical dispersion (reducing the ceiling and floor reflections), contribut­ing to the ribbon driver's well-deserved reputation for transparency, terrific sound­staging, transient zip, and excellent reso­lution of detail. By adding a dynamic woofer to a ribbon midrange/tweeter, the system cost can be contained compared to a full-range design.

Despite the lure of affordable dynamic/ribbon hybrids, many such products have failed-musically, technically, and commercially. The biggest problem is mating the woofer's spherical, or omni directional, radiation pattern to the ribbon's dipole, or figure-eight, radia­tion. The differences invite audible dis­continuities at the crossover point. The 'seamless' crossover has been a long­ elusive goal for designers of dynamic/ribbon hybrid loudspeakers.

Few products have managed to pull off this technical sleight of hand. The $22,000 Genesis 11.5 loudspeaker, which has become my reference, is a rare exam­ple of a hybrid that works. Unfortu­nately, its cost puts it out of the range of most music-lovers. What the world needs is a loudspeaker that brings the transparency, soundstaging, and detail of a ribbon driver to a product with good bass performance-at an affordable price.

Apogee Acoustics made an attempt at this lofty ideal with their $1195/pair Centaur Minor loudspeaker. In my review of that product in January 1992 (Vol. 15 No. 1), I found that the Centaur Minor offered exceptional midrange transparency, great soundstaging, and a smooth tonal balance. There was no question about the ribbon's performance-Apogee has been at the forefront of creating excellent full-range ribbon loud­speakers for more than 15 years. But the dynamic woofer and enclosure half of the design was less well executed. Specifically, I criticized the Minor's lack of bass extension, its inability to play loudly, limited dynamics, and somewhat coloured bass, which was probably caused by cabinet resonance's. Nonetheless, the Minor was superbly musical, and particularly recommendable to those who value soundstaging, detail, and liquidity over the ability to play rock at high levels.

Four years later, Apogee has taken another crack at the affordable hybrid loudspeaker with their $1995/pair Centaurus Slant 6.

DESCRIPTION

The Slant 6's tall, narrow shape, small footprint, and slanted rear panel make the handsome, elegant loudspeaker less obtrusive in a room than most conventional box loudspeakers.

Essentially, the Slant 6 is a flat panel on which the ribbon midrange/tweeter is mounted with an offset woofer enclosure attached to the panel rear. The entire structure rests on a flat base. Two pairs of high-quality binding posts allow for bi-wiring, and four spikes are supplied for adjusting the rake angle (how far the Slant 6 tilts back) and for coupling the loudspeaker to the floor. A three-position toggle switch on the input-terminal cup selects between "high," "low," and "normal" woofer levels. This switch engages different power resistors in the crossover. Rather than adjust the overall woofer level up and down, the switch is more of a Q control that affects only the bottom two octaves.

The (24 litre) woofer enclosure is nearly as tall as the front panel, but not as wide, so it doesn't interfere with the ribbon's dipolar radiation pattern. The "Slant" name comes from the wedge shaped woofer cabinet that narrows to a point at the top of the front panel. This design adds volume to the woofer enclosure while making the loudspeaker more visually appealing. The slant design has other technical advantages, including discouraging resonance’s inside the enclosure by reducing the parallel surface area, and avoiding the abrupt transition from no cabinet to cabinet, which could affect the dispersion of the ribbons rear-radiated energy. The new woofer cabinet is also well-braced, with a figure-eight brace, two staggered cross braces, and a shelf brace-all made from Baltic birch, which is claimed to be more rigid than MDF Finally, the slant design provides better support for the front baffle than does a small box attached to the bottom of the panel.

 

A 6.5” cone woofer is reflex-loaded in the enclosure, with the port firing to the rear. Although the Slant 6's woofer is the same size as that used in the Centaur Minor, the new driver has a larger magnet a 1.5” voice-coil (instead of 1”). Double the power handling, and much greater excursion. Apogee's larger loudspeakers (including the Slant 8 and Mini-Grand) use the same family of driver, which are made by Danish manufacturers Vifa to Apogees specifications.

The heart of the Slant 6 is the 26" long ribbon transducer used to reproduce the midrange and treble. The driver is a refinement of that used in the Minor and Stage loudspeakers. It now has a magnet structure that extends the full length of the ribbon to increase power handling and dynamic range, and has a suspension to stabilize the ribbon during high excursions. This is a tiny device attached to die rear of the ribbon that keeps it from twisting during loud passages At low sound levels, the suspension has no effect The Slant 6's ribbon is identical to that used in the Mini-Grand

The system crosses over at 1kHz, with 6dB/octave slopes through the crossover region, steepening to 12dB/octave out of band. This allows the drivers to be wired in-phase for better time behaviour, while maintaining the power handling advantages conferred by steeper slopes. The crossover uses Apogee's custom-wound air-core inductors and metalised polypropylene film capacitors the same components used in the Studio Grand and Mini-Grand. No printed circuit board is used in the crossover; all wiring is hand-soldered point-to-point.

Overall, the Centaurus Slant 6 appears to build on the strengths of the Centaur Minor while addressing die latter's shortcomings. Moreover, unlike the Centaur Minor, the Slant 6 needs no base (which added $295 to the Minor's cost). For the $400 difference in price (figuring the Minor with bases), the Slant 6 looks to be a much better loudspeaker.

SETUP

Apogee's Jason Bloom setup the Slant in my new listening room last October. Since then, they've shared space with the Genesis II.5, another dynamic/ribbon hybrid with a dipolar radiation pattern.

Watching Jason set up and demonstrate loudspeakers was great fun - as anyone who has gone to a CES in the past 15 will attest ¹ Even after all this time, gets excited about finding the one tweak-sometimes a loudspeaker movement of 1"- that maps the sound into focus and best integrates the speaker's bass with the room

1 At a late - 1980s CES, Jason was playing both LPs and CD's for the first time at a Show (his previous demos had been LP only). A group of salesmen of from a mid-fi chain - many of them looked like kids to Jason, had wandered off course and found their way into Jason's room. On seeing him putting in LP on the turntable one of them said. "Look! That guy must really be an audiophile he's using a record player!"

I experimented with placement after Jason left, but ended up with the Slant 6es where he had left them. 60" apart (inside edge to inside edge), with the rear of the woofer enclosures 43" from the rear wall, and a listening distance of 11' (listening position to a point between the loudspeakers' front baffles). Very little toe-in was needed to bring the loudspeaker into focus-too much toe-in, or none at all reduced image specificity and affected the tonal balance.

Adjusting the rake angle is easy with the long front spikes and the short rear spikes-I tilted the 6es back considerably from a vertical position. (The greater the tilt-back, the less treble energy at the listening position.) Although Apogee recommends a reflective wall behind the 6es, I found that adding absorption smoothed the treble. Finally, I did nearly all my auditioning with the woofer switch in the "low" position, which produced the best quality of bass, at the expense of some weight and extension.

The Slant 6es were more fussy about setup than are most loudspeakers. There was only a small range of placement in which they excelled. In addition, the 6's treble balance varied greatly with listening height and the amount of tilt-back. You should be prepared to spend considerably more time with this loudspeaker than with most if you're to achieve the kind of performance I'm about to describe Finally, make certain the 6es we set up optimally at your dealer before you audition them?

SYSTEM

I used two amps to evaluate the Slant 6: the $12,000/pair reference-quality Audio Research VT150 tubed monoblocks and the $2355, more real-world McCormack DNA-1 Deluxe. Loudspeaker cables included 3' bi-wired runs of AudioQuest Dragon/Clear and an 8' bi-wired pair of Symo - the Apogee-recommended cable for all Apogee loudspeakers. When Jason was visiting we heard significant differences between cables - the Slant 6 appeared to be very cable-sensitive. I ended up doing most of my auditioning with the bi-wired Symo set, both because it seemed the best match, and because I could use it with the DNA-1 Deluxe as well as with the Audio Research monoblocks.

The preamp was the Audio Research LS5 MkII, fed from a Sonic Frontiers SFD-2 MkII processor (with HDCD decoding: again, see my follow up elsewhere in this issue), a Mark Levinson No.30.5 or a Theta DS Pro Generation V (all in balanced mode). The analogue front-end was a Well Tempered Turntable and Larry Pederson-modified WTA, fitted with lots of Marigo mods (Turntable Isolation System, Well-Damped Arm Clamp, Mango Mat, etc.) The AudioQuest AQ 7000nsx's output was amplified by a Vendetta Research SCP2B phono stage.

SOUND

My initial favourable impressions of the Slant 6 were confirmed by extended listening after Jason left. I found that, rather than sounding like an improved variation on the Centaur Minor, the Slant 6 was a very different loudspeaker. First, the Slant 6 was brighter and more forward in the upper midrange and lower treble corn~ to the Centaur Minor, In fact, I would characterise the Slant 6 as a bit on the bright side of reality-unusual for an Apogee product. Even with a low listening axis and a large tilt-back, die Slant 6 had a band of brightness in the lower treble that tended to emphasize detail. I wouldn't go so far as to call the Slant 6 etched or analytical but it certainly was lively and immediate - in contrast to the Minor's more laidback sound. Tape hiss sounded whiter and was more apparent through the Slant 6 than with the Genesis 11.5. Similarly, vocal sibilance was a little excessive and cymbals had a somewhat forward character. Further tilt-back decreased this character, but at the expense of top octave air and extension.

Although the DNA-1 Deluxe had an excellent treble presentation for a solid state amplifier of its price, these traits were greatly ameliorated by the super smooth, liquid Audio Research VT150s. Careful system-matching is therefore critical with the Slant 6 - avoid inexpensive solid-state power amplifiers and poor-quality digital sources that are inherently bright and dry in the treble.

The brightish character aside, die Slant 6's midrange was fabulous: open, transparent and uncoloured - the antithesis of boxy, dosed-in, or congested. Instruments and vocals hung in a large, transparent space that was totally detached from the loudspeakers. Female vocals in particular were well-served by the Slant 6s having a pure and open quality devoid of the resonance’s often associated with cone loudspeakers and their enclosures. With my eyes closed, the Slant 6 produced an uncanny sense of the vocalist appearing in my listening room. The speaker had no chesty coloration; no odd resonance’s to give a congested or "hooty" quality, and no sense of boxiness. Many $5000/pair cone loudspeakers don't approach this level of performance.

The tonal balance did, however, emphasize the upper midrange and slightly de-emphasize the lower midrange. This made midrange textures a little thin and threadbare, highlighting the harmonics over die fundamentals. For example, saxophone took on a slightly thin and reedy quality, rather than sounding warm, round, and rich. Piano had a trace of clanginess in the upper register, and woodwinds were missing some warmth and body. This character was mild, however, and wasn't a distraction to enjoying the music.

The Slant 6's soundstage was stunning, combining a feeling of expansiveness with pinpoint image specificity. Not only did the Slant 6 throw an amazing sense of depth, but it also revealed fine degrees of layering within that depth. Consequently, the depth wasn't just a washed-out morass, but a finely delineated and articulated resolution of a recording's spatial information. I could hear precisely the front-to-back location of instruments, all the way back into the soundstage's innermost depths.

"Festival Day in Seville" from the HDCD Sampler (Reference RR-S3CD), is a particularly good track for hearing soundstage layering. The Slant 6 did a superb job of decoding all the spatial nuances Keith Johnson managed to capture in this recording (particularly when HDCD-decoded). In addition, image outlines were razor-sharp and tight, surrounded by a transparent expanse of air and space. Images had a beautiful sense of bloom around them, making the music seem to hang in transparent, three-dimensional space in the listening room.

Significantly, the Slant 6 changed its spatial perspective with the recording, becoming small and intimate when the recording called for it-as on the terrific new Doug McLeod disc, Come to Find (AudioQuest AQ 1027). Overall, the Slant 6's resolution of spatial information was superb by any measure, but particularly impressive for a $2000 pair of loudspeakers.

The Slant 6's bass performance was vastly better than that of the Centaur Minor. The low-end extension was deeper, dynamics were more powerful, and the 6's woofer was able to play at higher levels without getting into trouble. When listening for pleasure, the Slant 6 had an excellent bass balance, with a good feeling of weight and power. The sense of weight came, however, from the bass and midbass, not the low bass.

Listening critically to the bass, I was aware of the lack of deep extension and somewhat warm and "plummy" midbass-you wouldn't mistake the Slant 6 for a scaled, over damped design. But when I got out of the analytical auditioning mode, the Slant 6's compromises were so well-concealed that I was able to enjoy the music and forget about the loudspeakers. In fact, a visiting friend found the Slant 6's bass performance one of the loudspeaker's best qualities. Many loudspeakers with an overly ripe midbass tend to irritate over time and intrude on the music. Not so the Slant 6 - its overall bass balance and performance was musically satisfying, even over a wide range of demanding music.

I also found the bass dynamics excellent. The low-tuned toms on John McLaughlin's Que Alegria (Verve 837 280-2) had good punch and impact. Moreover, I was able to play the Slant 6 very loudly and not hear woofer problems. Even the bass-drum whacks on Trittico (Reference RR-52CD) didn't bottom-out the woofers, although the bottom-end extension was a little lacking-as you'd expect from a $2000 pair of loudspeakers. Note, however, that I've been spoiled by the $22,000 Genesis 11-5, with their four 12" servo-driven woofers powered by a dedicated 800W power amplifier. Nonetheless, the Slant 6's bass articulation, extension, and dynamics were much better than those of the Centaur Minor.

I heard two qualities from the Slant 6, both of which are very hard for cone loudspeakers to achieve: transient speed and lack of overhang. So many loudspeakers smear the energy of a transient attack (such as the pluck of acoustic guitar or the sharp edge of percussion instruments) over time, muting the attack and prolonging the decay. The result is a lack of fife and immediacy, loss of detail, and another due that you're listening to reproduced music. The Slant 6 excelled at presenting the dynamic nuances of music.

The Slant 6 did an excellent job on Jorge Strunz's and Ardeshir Farah's Frontera (Milestone MC13-9123-2), conveying the attack of the two acoustic guitars without sounding over-etched. The sound of the guitars was devoid of muddiness or overhang. As a result, I was able to hear each guitar as a separate instrument, and every note was clearly articulated - even during the high-speed dual leads. Another benefit of this transient zip was the superb resolution of low-level detail. The Slant 6 was highly resolving of recorded detail, although the detail was on the forward side of reality owing to the somewhat brightish presentation.

Finally, the integration of the dynamic woofer and ribbon midrange was excellent. Listening to acoustic bass, piano, cello, and other instruments whose range traverses the crossover point revealed a smooth, relatively seamless transition.

Apart from the critical auditioning, I spent many an enjoyable evening of music with the Slant 6 (particularly when driven by the VT150s). In this fundamental ability to involve the listener in the musical experience, I have nothing but praise for this affordable loudspeaker.

MEASUREMENTS

JA measured the Slant 6 in Stereophile's test lab and presented the graphs to me after I had completed the auditioning and written the rest of the review.

The 6's calculated B-weighted sensitivity was 83.2dB for a drive level of 2.83V at 1m - a rather low value. The impedance plot (fig. 1) shows a minimum impedance of 3.78 ohms at 120Hz with a 4ohm dip at 2kHz - and an impedance of less than 5ohms above 1kHz. The port tuning can be seen as the impedance dip at 37Hz, implying reasonably good LF extension.

Fig. 1 Apogee Slant 6, electrical impedance (solid) and phase (dashed) (2 ohms/vertical div.)

The impedance curve suggests that the Slant 6 is a moderately demanding load on a power amplifier. When the impedance curve is considered along with the lowish measured sensitivity, the Slant 6 is clearly not a candidate for use with 30Wpc British integrated amplifiers. Instead, a fairly muscular amplifier with the ability to increase its power as the load impedance drops is recommend.

A small tubed amplifier may be a good match tonally, but won't deliver the dynamics and bass extension of which the Slant 6 is capable - Incidentally, the 150Wpc (rated power into 8 ohms) McCormack DNA-1 Deluxe showed no signs of strain even during loud passages and high playback levels. The lower-powered, $129 DNA-05, reviewed by TJN in February, may be an ideal match for the Slant 6 in price, tonal balance and power output.

Fig.2 shows the responses of the port, woofer, and ribbon individually measured on the ribbon axis. The port output at 37Hz is coincident with the woofer's minimum-motion point (the dip in the woofer trace). Although specified at 1kHz the crossover transition actually appears at 1154Hz (the point where the woofer and ribbon traces are equal in level. Though the woofer roll off shows no significant break-up modes, even well out of its passband, there are a suspicious peak and dip in its output in the upper midrange. Finally, the ribbons response is smooth and flat, but with a slight depression in the top octave.

The Slant 6's output averaged over a 30-degree horizontal window combined with the complex sum of the nearfield woofer and port responses (fig.3), shows a smooth overall response. The low-frequency -6dB point occurs at 30Hz - a respectable figure for a single 6.5" woofer, although this suggests the woofer is being driven fairly hard at the lowest frequencies at high SPLs. A slight rise in the bass spanning three octaves (40Hz-320Hz) may be partly responsible for the plummy quality noted during the auditioning We can also see also see an excess of energy in the band between 1kHz and 3kHz (exactly the band in which the car is most sensitive, both to quiet sounds and to small amplitude variations). Which may account for my perception that the 6 has slightly exaggerated detail, a tendency toward brightness and an emphasis on instrumental harmonics rather than fundamental. Note that I didn't find the Slant 6 bright per se, but that I heard an emphasis in a certain band that increased the perceived amount of musical detail and emphasised instrumental harmonics over fundamentals.

The suckout between 600Hz and 900Hz is a crossover effect caused by the driver outputs not adding in-phase directly in front of the ribbon. (The microphone was positioned 37" from the floor, almost exactly half up the ribbon. for all these measurements.)

Fig.4 Apogee Slant 6, horizontal response family at 53", normalised to response on ribbon axis, from back to front: differences in response 90° - 5° off-axis on woofer side of baffle: reference response: differences in response 5° - 90° off-axis on ribbon side of baffle.

Fig.4 shows how the Slant 6's response changes over the horizontal axis with the on-axis response subtracted from all the traces so that only the differences in response to the loudspeakers sides are shown. Moving off-axis on the ribbon side of the baffle, the midrange suckout fills in; on the woofer side, the suckout worsens. By placing the ribbons on the inside edges of the speakers (the way the 6es are meant to be set up), this suckout will not be a factor in this speakers' direct sound. However, the coloured off-aids response will be reflected to the listener by the room's side walls, which possibly contributes to my feelings of a slightly lean balance overall. As usual, some side-wall absorption will smooth the Slow 6's tonal balance. Note that the 6 has only moderate dispersion above 5kHz, which will add to the depressed top-octave oft-aids balance.

Fig.5 Apogee Slant 6, vertical response family at 53", normalised to response on ribbon axis 37" from the floor, from back to front: differences in response 45° - 5° above ribbon axis: reference response; differences in response 5° - 45° below ribbon axis.

Measuring the response as a function of vertical aids produced the graph of fig.5. Again the only the differences in response are shown (the straight line reference it at a height of 37").

We can me a dramatic rolloff in the ribbon's response as we get outside a very narrow (± 10 degree) vertical window. You don't need MLSSA to know this about the Slant 6 - just stand up when listening and you can hear the instant drop in upper midrange and treble energy This ribbon's very narrow vertical dispersion -due to its vertical dimension being larger than the wavelengths of the sounds it produces - results in less reflected energy from the floor and ceiling, which is a good thing. Fig.5 confirm the necessity of getting just the right relationship between tilt angle and listener height to Fig.6 Apogee Slant 6, step response on ribbon axis at 53" (5ms time window, 30kHz bandwidth) achieve a smooth tonal balance.

The Slant 6's impulse response (not shown) was clean, with none of the ultrasonic ringing often seen in loudspeakers with metal-dome tweeters. The step response (fig.6) is more revealing of the 6's time behaviour: the ribbon and woofer are wired with the same acoustic polarity (the ribbon's output is tile first spike, the woofer's output the later hump), with the ribbon leading the woofer in time. Although wired with the same polarity, the two drivers share the same flat baffle, which means that the two drivers' acoustic centres aren't perfectly aligned, resulting in the ribbon energy leading the woofer energy. As the suggested setup has the listener off-axis on the ribbon sides of the baffle, this will result ill an even larger time lead for the ribbon axis. Finally, the cumulative spectral-decay, or waterfall, plot of fig.7 shows the Slant 6's ribbon to be very well-behaved. The Slant 6 stores very little energy, instead decaying quickly and cleanly, as seen by the white space and lack of garbage in the lower traces. This is to be expected from a low-mass ribbon driver, although not all ribbons decay this cleanly. Apart from some energy-storage problems at the top of the woofer's passband, the waterfall plot correlates with my impression that the 6 had superb transient performance.

Fig.6 Apogee Slant 6, step response on ribbon axis at 53" (5ms time window, 30kHz bandwidth)

Fig.7 Apogee Slant 6, cumulative spectral-decay plot at 53" (0.15ms risetime)

Nothing in the Slant 6's measured performance surprised me. The drastic change in the amount of ribbon energy at the listening position as a function of the vertical axis confirms that the 6's tonal balance is very dependent on the amount of tilt-back and listening height. Be prepared to adjust the front spikes, listen, and adjust again in an iterative process to get the optimal tonal balance.

CONCLUSION

The Apogee Centaurus Slant 6 gets an enthusiastic thumbs up. This loudspeaker does things you just don't hear from $2000/pair cone loudspeakers. Its midrange transparency, openness, resolution of detail, and soundstaging were first rate. I was also greatly impressed by the Slant 6's transient ability-the sound was quick and detailed, with no overhang or slowness. Many much-more-expensive speakers aspire to the Slant 6's level of transparency. Although not the Slant 6's strong suit, the bass was musically satisfying and well-balanced, though lacking the clarity and extension of some loud speakers in the $2000 price range (ill Snell Type D, for example).

Careful system-matching and setup are critical to getting a musical sound The Slant 6's tendency toward brightness suggests that forward, dry, solid state amplifiers should be avoided, sweet-sounding tubed amplifier of sufficient power, or a solid-state amplifiers with a clean treble (such as the McCormack DNA-1 or DNA-0.5), is ideal. Also, be prepared to spend some time experimenting with placement and angle to get the best performance fro the Slant 6.

Along with the identically priced CS1.5 and the Martin-Logan Aerius, Apogee Slant 6 joins a select group of loudspeakers in this competitive priced range worthy of a strong recommendation.

 

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