During Construction--Standing up at Last
With little siblings:
Large Advent, KLH-6,
AR2a, Cambridge Six
That's Kloss Enough!
With EPI tweeter.
This site describes the combination of four Smaller Advent loudspeakers into a single large cabinet. The project was imagined as the answer to the question, "What would have happened if Henry Kloss built an expensive, large loudspeaker? Kloss, sketched below, was one of the first to realize that acoustic suspension loudspeakers (sealed box, no port) were to be a commercial success, because they produce low bass from a compact box. He helped establish Acoustic Research with Edgar Vilchur and later KLH (he is the K in that company's name), Advent and Cambridge. All these companies still exist, though Kloss died a few years ago. Kloss apparently believed that less is more, and his designs tended to be simple and affordable.
The best Kloss two-way systems (in my opinion, the Advents) could produce powerful 30-Hz tones. Contrary to what it says on the cheap "subwoofer" that came with your computer, 30Hz is not a subwoofer, but it is definitely woofing. At 30 Hz, you may get goosebumps and the wife may complain. The only musical instruments making sounds lower than this are a Bosendorfer piano and some organs. Difference frequencies--e.g., from massed string instruments not in perfect tune--also give musical information at very low frequencies, so there is some justification to plumb even deeper depths for bass response. Advents could also tweet (squawk) on out to perhaps 10K (maybe a little less--see the modern Stereophile review of the classic Smaller Advent and measurements below). The high end extension is said to have increased with successive versions, such as the New Advent Loudspeaker. Advent midrange was uncolored by standards of the day and still sounds pretty good (not according to my golden-earned friends, who think it "constipated", "slow" and/or boxy).
Through the 1960's and 1970's, other builders in New England copied the basic, simple formula for acoustic suspension design (EPI, Boston Acoustics, Snell, NHT). The Classic Speaker Pages website is devoted to these acoustic suspension speakers. One popular brand was EPI, where the resident guru was Winslow Burhoe (who later got away from acoustic suspension designs, as have most designers today). Kloss designed the Advent and the Smaller Advent to generate revenues for a very precocious entry into home theater. He was therefore interested in (what else) keeping things simple to maximize profit. The man's entire career seems devoted to getting 90% of the sound for 10% of the cost, but he chose wisely given this philosophy. Unlike Kloss, Burhoe was willing to entertain the thought of a larger acoustic suspension speaker system, the EPI-1000 which is the tall monster in the figure below.
Following Advent, all early EPI's were two-way designs. They used a highly-regarded tweeter. Two woofers, a 6" and an 8", were available. How does one account for the great variety of box sizes and configurations with just two woofer designs? It so happens that acoustic suspension speakers are tolerant of different box sizes, with larger volumes conferring less bass but smoother and slower roll-off as the frequency declines. Burhoe's original "module" (the 8-inch woofer and the 1" tweeter) went into the EPI-100 (bottom right in the figure, taken from the Human Speakers website, this being a company devoted to EPI and its descendants). Like most acoustic suspension speakers (and certainly the Smaller Advents that are the main subject of this project) the EPI-100 probably had a little hump in output in the 100-Hz regime that allowed it to deliver satisfying bass at lower frequencies (the EPI-100 reaches down to about 45 Hz, according to product literature). Acoustic suspension speakers roll off more slowly in the bass than ported speakers, so even though they may start rolling sooner they can end up producing more bass at very low frequencies.
Burhoe (and surely Kloss) realized that boosting the box size would start the roll earlier but make it even more gradual. This is desirable from a standpoint of bass tonality (all notes get the right emphasis) but there is also a change in damping (a certain tautness). As the box is sealed, the pressure changes of woofing are decreased while the cone motion is increased if one goes to a bigger box. If the excursions are too large, the woofer might be ruined. Fortunately, acoustic suspension woofers are designed for long travel (high Xmax). The New England designers built their own woofers, and they were rugged devices. The woofers all have the very low free air resonance (fs), high driver Q (Qts) and long excursion voicecoils (high Xmax) desired for sealed box applications. No silly cast frames (except the ritzy AR speakers) but you would have to work pretty hard to blow a Kloss or Burhoe woofer. Thus was born the EPI-150, which puts the Model 100 woofer in a bigger box. EPI claimed 35 Hz for the Model 150. Power handling would be a little worse in this system, despite its larger size. To improve power handling, one adds more woofers. To extend the bass to lower depths, one adds more air per woofer. The limit of this strategy is realized in the four-woofer EPI Model 1000, which has an interior volume of about 10 cubic feet for its 4 woofers, compared to about 0.8 cubic feet for the model 100 with its single woofer. There is more than double the air volume per woofer, but four times the number of woofers so you won't have to wear a catcher's mitt while listening to Pictures at an Exhibition. A frequency extension of 22 Hz was claimed for the EPI-1000. It is a big speaker, as the photograph below shows. That's a young Burhoe standing in-between one pair. The speakers are 75" tall.
The EPI-1000 had one eight-inch woofer on each side of the cabinet, at about ear's height. The rest is just to contain the air! I heard them now and again, at Schaak electronics in Minneapolis. It was a lousy setup (designed to draw college kids into the store and get 'em to buy a pair of Model 100's) but the sound was...well...eerie. I owned a pair of Small Advents instead, purchased in Mankato, Minnesota, right after high school graduation. I lugged one of them all the way across campus to an EPI clinic, to see how it would stack up. I think the deal was EPI would give you a set of speakers if your speakers actually beat theirs, so sure were they about the frequency response capabilities. The EPI's won...but only by making measurements way off axis, where its tiny tweeter had an advantage over Advent's larger driver, which is effectively a mid-tweeter. I remember that the EPI people were impressed with the SA's, but two years later I sold them anyway to get into hybrid electrostats (Janszen Z410's). That move was motivated by the distinct lack of high-frequency energy and "life" in the SAs. The decision was reached after hearing a live cover band playing in a bar; this event made it clear that the SAs did not have realistic treble presentation. This was probably a deliberate decision on the part of Kloss, who realized the amount of noise (tape hiss, pops and ticks on LP discs) that would accompany any signal from the sources available at the time. When the New Advent speaker was introduced, he wrote at some length about the decision to add more energy about 10KHz to take advantage of Dolby tape hiss reduction. It's ironic that (as memory serves) LP noise got worse about this time.
Fast forward to the year 2002, when an ad for a set of EPI-100's in Natchez, Mississippi, appeared on e-Bay. For $100 I finally bought a set, and they are fun, lively little speakers. They are currently used as rear speakers in a surround setup. By this time, I had acquired Kloss' last (I think) acoustic suspension two-way design, the Cambridge Model Six, as part of the same mid-fi home theater system. I had also experimented with modern ported systems (Mirage) and a passive radiator VMPS subwoofer. I like the bass out of the old acoustic suspension designers better than the ported sound. Some high-end speakers have returned to sealed designs, too. If only the bass could be deeper...
By accident, I found the Human Speakers website and was reminded of Burhoe's giant EPI-1000. I also found DirectAcoustics which is apparently Burhoe's company (he did Burhoe Acoustics in-between). The theory behind the huge EPI-1000 appears on these sites. I figured I could pick up more EPI's and build a set for fun. Now, a cabinet that large is a bit of a production. Lots of internal bracing would be required. Also, I wasn't sure I really wanted the omnidirectional sound pattern. I began to think about just putting EPI-100 speakers in their existing, well-braced boxes together into a larger speaker cabinet. This would be a kind of "hot rod" design--e.g., some of the old elements (drivers, parts of cabinets) recycled into something new and better. Holes would have to be cut into the sides so the woofers could access all the air. My love of things wooden confounded this plan. How could I trash the beautiful, real-walnut cabinets? OK, some newer EPI's had crappy vinyl cabinets, but who wanted to wait for those to appear on Ebay? The idea of going all the way back to my mid-fi roots and using the vinyl-clad Smaller Advents was born. These often appear for sale on e-Bay, and should produce even deeper bass.
The SA may have been Kloss' favorite. It uses a slightly more expensive woofer than the large Advent (according to Advent literature). Compared to the original large Advent, production savings came in the form of its vinyl-covered cabinet and absence of a low-pass section to the crossover. Also, the cabinet is made from thinner wood on some sides. The SA trades even more efficiency away for deep bass, and was engineered as a 4-ohm system to pull more out of transistor amplifiers and cheap receivers with which it was often paired. The SA produces absolutely amazing deep bass from a cabinet that used to be considered small (not compared to junky computer speakers). The SA is actually larger than the Cambridge Model Six, which was sold as a modern version of the KLH Model Six. This is just a case of marketing hoping that no one is paying attention. The SA is about the same size as the EPI-100, but uses a beefier woofer...and the famous Advent "fried egg" tweeter.
|An array of Kloss designs||
KLH-Large Advent Stack
|Cambridge Model Six||KLH Model Six||Small Advent|