Carver Amazing Loudspeaker (Platinum Edition) Page 2
The Q of the Amazing's bass alignment is around 1, considerably higher than the 0.5–0.7 range most designers opt for. Don't expect tight, well-defined bass from such a design. This, according to Bob, is just the sort of bass quality he likes—"a warm, rolling bass."
The ribbon assembly flanks the woofer array on the inside edge of the baffle. If you look closely you can see that the planar "voice-coil" consists of four strips of pleated aluminum foil running the length of the assembly. The strips are joined top and bottom to form a continuous loop. The aluminum is glued to a Kapton backing stretched over a particle-board frame and clamped on all sides with a foam suspension. The foil is sandwiched between two layers of magnets located front and back. There's a total of 36' of ceramic magnets per side that provide the magnetic push-pull force for the ribbon. The Kapton is claimed to be very strong, dimensionally stable, and capable of tolerating extreme temperatures—a perfect recipe for a voice-coil former. The glue used is said to provide damping for the internal resonances of the aluminum.
Being a purist, I naturally feel obliged to point out that, strictly speaking, this sort of design is not a ribbon in the classic sense in at least one respect: The foil operates in the peripheral or leakage field of the magnets instead of being suspended directly between the north and south poles. For most folks, Apogee included, this sort of design qualifies as a ribbon, so I'll let it go for now.
The ribbon operates full-range above about 100Hz, with useful output to about 20kHz. There's a second-order crossover network at around 100Hz. However, I have hedged about the details here because the design changed before my eyes as the review progressed (more about that later). While there are no crossovers in the critical midband, there is at least one pot (hedging again) in the signal path. Sample 1 of the Amazing featured an upper-midrange control, while sample 2 had no less than two pots, the additional pot being a high-frequency control. Something unique to Sample 1 were two small foam rectangles mounted top and bottom on the back side of the ribbon. I've dubbed them "training bras" because, according to Bob, their function is to provide some damping for the ribbon until it breaks in—after which they may be discarded (though this is not spelled out in the manual).
The bass resonance for the ribbon is claimed to be 75Hz. In Santa Fe, after an extensive break-in period of over 100 hours, we measured a bass resonance centered at 150Hz. At Santa Fe's 7000' altitude, granted, I would expect the resonant frequency to climb because of the reduced air load; for the ribbon, I would even be willing to concede the possibility of a 25% increase. But a 100% increase cannot be explained by blaming Mother Nature, as Bob is inclined to do. I would rather point an accusing finger at Carver's quality-control procedures.
I'm also worried about the selection of a crossover frequency so darn close to the ribbon resonance. The extreme phase shifts that accompany a resonance make it a bitch to design a network with flat amplitude response in the crossover region. As a rule of thumb, one should choose a crossover point at least an octave away from a major driver resonance. A driver can be modeled as a bandpass filter. In its pass band, removed from the bass resonance and HF rolloff, a driver is known to be minimum-phase and thus much more amenable to conventional filter design. Did Carver succeed? Well, in my own measurements and his, there is a notch in the response between 100 and 150Hz, apparently due to the crossover. Of course, there's the possibility that this is caused by a room mode, but I rather doubt it.
Bob provided me with measurements he made in a hi-fi store at about 4 meters on-axis, comparing the response of the Amazing to that of the Quad ESL-63. Bob figured that since the Quad has no bass crossover, its response would be seamless and would highlight any problems with the Amazing's crossover. With both speakers and the mike in the same physical location, the response curves superimpose very nicely at 75Hz, but there's a 3dB notch in the response of the Amazing relative to that of the Quad, centered at 110Hz.
Virtues of directivity
Conventional box speakers radiate omnidirectionally in the bass. As signal frequency increases, however, the polar response narrows significantly. By the time the tweeter tweets above 5kHz, the response is pretty much confined to a cone with a half angle of 30°. All conventional tweeters beam sooner or later, and may be said to possess increasing directivity as a function of frequency. Looked at as a whole, the directivity of a garden-variety loudspeaker resembles the shape of a pyramid: low-directivity or broad radiation in the bass while taking on an increasingly narrow radiation profile at higher frequencies. In contrast, a dipole radiator has a much narrower radiation pattern, at least through the lower octaves. The pattern resembles a figure eight, with radiation lobes to the front and back and very little side-directed energy. One advantage of the latter pattern is that it significantly reduces early lateral room-boundary reflections and their resultant colorations. There is simply less energy splashed onto the side walls, floor, and ceiling.
I firmly believe that a dipole radiator represents a significant step toward solving the room/loudspeaker interface problem. Early reflections are anathema to accurate reproduction of the soundfield captured by the mikes at the concert hall. These reflections, even if spectrally similar to the direct sound, interfere with it to produce a comb-filter effect that colors perceived timbres. Reflections arriving at the ears during an initial 10ms window—after the direct sound has stopped—are the most troublesome because they are higher in amplitude compared to later arrivals and are not well discriminated against by the ear/brain system. Room treatment is therefore essential to suppress the room's sonic signature. After all, do you want to hear your room's reverberation or the original soundfield?
I've had good results with a dead-end/live-end sort of treatment with the speakers located at the dead end of the room. I'm therefore mystified by Carver's references to the beneficial effects of 6ms reflections when the Amazings are placed 3' from the front wall of the listening room. These reflections are claimed to increase the spaciousness of the soundstage. Well, I would have thought that rear-wall reflections or possibly a delayed rear channel would have done that. But in my opinion, Carver's suggestion is a recipe for destroying the accurate transduction of the original soundfield on the recording. I'm making a tacit assumption here that the soundfield captured during the recording session is worth preserving. This may be a poor assumption in the case of multi-miked pop recordings, in which case the additional adulteration of the room may actually be desirable. In the case of properly executed purist recording techniques, the recorded soundfield is rich with spatial clues and incorporates the right blend of direct/diffuse sound. That's the sort of information I'd like to preserve in my listening room. Unfortunately, this is much more difficult to do than I've so far indicated.
It's a fact of life that most of us listen in semi-reverberant environments. Room treatments are most effective at higher frequencies, say above 5kHz. At much lower frequencies it's very difficult to dissipate sound energy very quickly. As a result, unless you're glued to the loudspeakers, a significant portion of the sound energy at your listening position is due to room reflections. A critical distance may be defined at which the reverb energy just equals the direct sound energy. The basic problem is that this critical distance changes with frequency. It would be highly desirable to keep the ratio of reverb to direct energy constant at the listening seat. But because directivity or beaming typically increases with frequency, this goal is impossible to achieve. Recall that the average speaker is omnidirectional in the bass. It pumps lots of bass energy off-axis; together with the fact that room absorption is ineffective in dissipating bass energy, this assures that reflected energy is bass-rich and treble-shy. As directivity increases, more energy is concentrated on-axis and the direct component of the total sound increases. Finally, above about 5kHz, the absorption of highs by room surfaces, together with the on-axis treble beaming, combine to make the treble sound almost completely direct.
This is an important concept to understand. It explains, for example, why two speakers that measure exactly alike on-axis sound different—in the same room. The answer, of course, is different directivity patterns which change the character of the room reverb. Line-source tweeters like those in the Amazing are by their nature more beamy than a typical dome tweeter. Thus, if you've optimized your system for a dome-based loudspeaker, substitution of a line ribbon at the same position will result in a noticeably brighter balance. Why? Because the direct/reverberant sound ratio starts to increase at a lower frequency for line sources. This predominance of direct sound will emphasize overtone structures in the upper octaves, imbuing timbres with a bright, steely character. The solution is to either move farther back from the speaker or experiment with treble rolloff (footnote 1).
So what response is "correct" for a quasi-line-source design like the Amazing? It would be a definite mistake to equalize the Amazing to be flat on-axis in the near field. I learned this the hard way. I tried it, and had to duck the razor blades the Amazing hurled at the listening seat. Three meters away, the sound was excruciatingly bright. An even bigger blunder would be to equalize the Amazing to be flat at the listening seat. The superimposition of a reverb curve deficient in highs over a flat direct-sound curve naturally results in a listening-seat balance featuring high-frequency rolloff. That's the correct tonal balance. An attempt to flatten this composite curve by jacking up the direct-sound contribution will significantly brighten timbres. Trust me, you'll be reaching for the cotton balls.
Footnote 1: For a more technical exposition of these concepts, see Professor Han's "Frequency Responses in Acoustical Enclosures," AES Preprint 2452(F-3), 1987.—Dick Olsher