MartinLogan Quest Z loudspeaker
Looking at the progression from Aerius to Quest, a definite trend is apparent. First, the woofer increases in size, as does the electrostatic panel's radiating area. Second, the crossover point is approximately halved each step of the way, from 500Hz for the Aerius, to 250Hz for the Sequel, and 125Hz for the Quest. Third, the price increases by about a thousand dollars for each step upward in size.
The Quest's transparent electrostatic panel is curved in the horizontal plane (a M-L trademark) to give increased HF dispersion. The constant-charge diaphragm is segmented, using horizontal foam strips, to break up the inevitable LF drumhead resonance. The perforated stators (another M-L trademark) are finished in a black insulating paint. The woofer is a 12" moving-coil unit—identical, in fact, to the woofer used in the Monolith—and is mounted in its own sealed enclosure. The woofer and panel crossover networks, each 12dB/octave, are individually accessible via dedicated binding posts so that bi-wiring, or even bi-amping, is easily accomplished.
The "Z" version of the Quest differs from its predecessor in having a more benign impedance magnitude, making it an easier load for most amplifiers. For example, the dip around the crossover frequency to below 2 ohms has been significantly reduced to about 3 ohms (footnote 1).
Two frequency-contouring switches are located on the interface plate on the back of the speaker. The bass-contour switch provides a –5dB cut (more extreme than the –3dB cut provided on the original version of the Quest) below 200Hz, to give a measure of room-placement flexibility (footnote 2). As the speaker-wall distance decreases, the bass becomes more prominent. Thus, if you're forced, because of domestic considerations, to place the speakers closer to the wall behind them than is optimal, the bass-contour switch offers some hope of restoring a reasonable bass balance.
The presence-contour switch is also a two-position switch that gives you some control over the upper midrange and is useful for taming the brightness of under-damped rooms or for taking the edge off a bright front-end. Said to effect a 2dB change centered around 2.5kHz, measurements of my samples showed a 1–2dB increase in response from 1.5–4kHz with the switch set to "+2dB." However, the response is actually flatter with the switch in this position, so I prefer to think of it as actually giving a 2dB-deep valley in the "flat" position.
Hybrids: the promised land
The obvious promise of a hybrid loudspeaker is that it combines the best of two worlds—the bass extension and punch of a dynamic speaker with the midrange transparency, speed, and detailing of an electrostatic panel. However, the hidden promise is a speaker that does much more. For example, a full-range ESL suffers from poor sensitivity due to the need to space the stators far enough apart to allow the diaphragm sufficient excursion to reproduce bass frequencies at high levels. The size of the panel also escalates to intimidating proportions to maintain decent bass extension in the face of dipole or front-to-back cancellation in the lower octaves. Relieving the panels of responsibility for reproducing bass frequencies allows the designer to optimize the design for greater sensitivity. It also makes it possible to use a curved diaphragm for increased high-frequency dispersion.
However, there are three basic problems standing in the way of a successful integration of two disparate technologies.
The first has to do with driver "speed." The common (mis)conception is that a dynamic driver isn't fast enough to keep up with an electrostatic panel, the overall presentation lacking cohesiveness. The disparity in speed constantly calls attention to itself, giving the impression of a sea of upper-range detail emerging from a sluggish low-frequency foundation.
Such a picture, while useful for a taste or flavor of the problem, is technically misleading. Dynamic woofers have received a bum rap in audiophile circles. There is no reason that a cone woofer's acceleration or risetime could not equal that of a lightweight diaphragm. It's a question of force over mass—as Newton discovered three centuries back—and ML proudly makes mention of its woofer's risetime in its literature. But this is all a red herring. Woofers don't need speed. Yes, Virginia, tweeters do, but, most emphatically, woofers don't. Spending their time in the basement, as it were, reproducing a bandwidth limited to just the first several hundred hertz or so, woofers don't require large accelerations.
What they do need is control. It's both boxy resonances and high-Q bass alignments that ring like a doorbell that have given box speakers and, by extension, dynamic woofers, bad reputations. It is the lack of pitch definition, precision, and detail in the bass range that is often confused with the issue of woofer speed. Start with a rigid, well-damped enclosure, then add a suitable woofer to give an alignment with good transient response and an in-band response that's free of resonances, and you have a bass system worthy of partnering an ultra-light and speedy stat panel. The sealed woofer enclosure of the Quest does not operate as a boomy bass reflex; with a well-damped, closed-box alignment, its transient response is stat-ready.
The second problem involves the crossover region, where the dynamic woofer and stat panel overlap in output. When the units speak in tandem, their different sonic signatures tend to detract from the illusion of a single voice. The cohesiveness of the soundstage can be seriously compromised by mixing different types of transducers or even different cone materials. MartinLogan has minimized such problems in the Quest by keeping the crossover frequency below the critical frequency of 200Hz and by using second-order slopes to narrow the region of driver overlap.
The third obstacle to the optimal integration of woofers and electrostatics involves the inherently different radiation patterns of these transducers. While the woofer acts as a point source, the panel—at least within a listening distance of 15' or so—acts more like a line source. A practical consequence of this is that the tonal balance of the hybrid changes with distance. This is because the bass intensity falls off as the square of the distance from the woofer (ignoring room modes for the moment), while midrange intensity only falls linearly with distance from the panel. As you back away from the hybrid (at least in a large room), the woofer loses loudness much more quickly than does the panel. What this means is that you must be willing and able to adjust the listening seat to hit that zone where the bass and mids are in proper balance.
Setting up and moving the Quests is definitely a two-person job. With help from my 17-year-old son, Daniel (footnote 3), they entered the reference room. ESLs need a lot of room to breathe properly, and the Quests proved to be no exception. About a third of the way into the room is where they ended up, toed-in toward the listening seat and with the Sound-Lab SALLIE absorber panels located about three feet to their rears. I picked the optimal listening position while listening to a reproduction of a double bass. The key was to move in and out until the midbass popped into balance with the lower midrange.
Because the Quest is so easy to bi-wire, there's no excuse not to do so. The sonic payoff is worthwhile. I used TARA Lab's RSC speaker cable. Interconnects were primarily the Expressive Technologies IC-1. I tried a host of power amps, including the Jadis JA-200 monoblocks, the Air Tight ATM-3, and the Classé 700 monoblocks.
Footnote 1: Apparently, the Quest was doing so nicely in the marketplace that dealers were alarmed when word got out that MartinLogan was about to modify the speaker. I'm told that there is absolutely no need for concern. The "Z" crossover modifications in no way impacted the frequency response or the sound of the speaker.—Dick Olsher
Footnote 2: The crossover may be modified in the field to vary the degree of bass cut. This requires changing out a couple of resistors in the crossover network. If you wish to experiment in this regard, contact your dealer for details.—Dick Olsher
Footnote 3: I want to acknowledge Daniel's help, because we often take for granted the good things that teenagers do.—Dick Olsher