EgglestonWorks Andra loudspeaker Page 3
But arriving at a bass response that matched the free-air characteristic of the other drivers was no easy task. Finally, Eggleston settled on using two 12" Dynaudio drivers in a configuration he dubbed "pressure-driven." There are two parallel chambers, with one 12" driver mounted in front of the other. The inner driver, which is in a heavily ported box, acts as a servo behind the outer driver—when the outer driver moves back, it doesn't have to compress the air in the cabinet because the inner driver moves in concert with it. The driver, even though its enclosure is sealed, gets the benefits of free-air-like operation (footnote 1).
The woofer's crossover consists of a heavy-gauge inductor in series with the two drivers, and there's also an RLC network in parallel with the drivers. Like any loudspeaker, the Andra has a resonance peak that manifests itself electrically as impedance magnitude. The RLC network serves as a nullifying circuit, cancelling the cabinet tuning so that the 6dB per octave crossover can do its job.
"It's funny," Eggleston commented, "but while 90% of the Andra's midrange tuning was done by ear, 90% of the woofer tuning was done with nearfield measurement. Bass is so room-dependent that the only way you can standardize performance is through measurement."
Eggleston's crossover is certainly unusual—the midrange drivers are run full-range, while the networks on the woofer and tweeter are vestigial. This makes it somewhat difficult to specify the precise crossover points. "Actually," Eggleston explained, "there's quite a bit of overlap. The rolloff on the high end of the midrange is very, very gradual—it begins around 3500Hz. The crossover point to the tweeter is about 3000Hz. We could make it tighter, but every time we've tried that we've compromised airiness and openness. The woofers run totally flat from about 20Hz up to 120Hz. The midrange is active from about 55-60Hz and up. Again, there's overlap, but we chose to do it this way for musical reasons—and after a lot of listening. This configuration is almost like an active system. Certainly on the midrange, you're connected straight into the amplifier."
Yet he remains modest about his unique design philosophy: "We haven't made any technological breakthroughs. All we've done is put together a speaker the best way I can think of. I want to strike an emotional chord with this speaker—that's it, basically."
A son faithful and true
Bill Eggleston has certainly struck an emotional chord with this listener. Let me just come out and say it: I love the Andra. Of all the speakers I've had in my current listening room, none has sounded better over a wide range of musical material.
To begin with, it played loud. I don't know if this is important to you. Hell, I didn't even know it was important to me until I began listening to Peter McGrath's wonderful four-channel digital recordings using EgglestonWorks' smaller Rosas as rear-channel speakers. You see, Peter likes to play stuff loud. Really LOUD. He claims he's merely listening at a realistic playback level, but I think he's trying to achieve that sense of there being no dynamic limit that is implicit in live music-making and almost completely lacking in reproduced music.
Playing the Andras at high volume did give me some of that exciting U-R-There sensation, but unlike a lot of loudspeakers, I didn't have to crank 'em to get the music out of 'em. In fact, the Andras reminded me of the Quad ESL-63 in that I was required to attempt to match the output of the speaker to what an instrument or band would produce live, or ruin the sense of reality. Is this a glitch or a feature?
Neither, really. I think it's a sign of how uncolored the speaker was. An acoustic guitar recording such as Enrique Coria's Latin Touch (Acoustic Disc ACD-23), played at a realistic volume, sounded amazingly like an acoustic guitar playing in a real space. But when I played it significantly louder than an acoustic guitar, it started to sound out of kilter—overtones were no longer in proportion to fundamentals, and the instrument took on an aggressive character.
Footnote 1: Actually, a compound pair of drive-units like this, which I first saw in Linn's 1976 Isobarik design, behaves very much like a single drive-unit with just over twice the cone mass, twice the driving force, and (if connected in parallel) half the impedance.—JA