Pawel/Ensemble PA-1 & Reference loudspeakers Page 2
Pawel Acoustics was established in a small Swiss village in 1981 to produce high-end audio equipment for both the consumer and professional markets. A physical examination of the product makes it clear that the Ensemble loudspeakers are crafted in the tradition of Swiss perfectionism. Designer Harry Pawel and his brother Markus are described as "well-known Swiss electronics wizards." Harry is in his 30s, sings in a choir, and is also active in professional recording. I was a bit reluctant to accept the "electronics wizard" label until I found out that at age 17 Harry built his own ESLs with direct-coupled tube ampsboth of which are still working today. Not bad for a teenager.
The driver complement of each speaker is very similar and consists of a 5" woofer running fairly wide open, a ¾" soft-dome tweeter, and a KEF SP139 passive radiator. In fact, the passive radiator takes up almost the entire back panel of the enclosure. Both woofer and tweeter are custom-made for Pawel Acoustics the old-fashioned wayby hand. The tweeter is claimed to possess very linear characteristics, excellent dispersion, and a lightning-fast rise time of 12 microseconds. The woofer cone is made of a synthetic foam material sandwiched between a few-micron-thick aluminum foil and paper. The diaphragm is thus ultra-rigid yet extremely light. The exact composition of the synthetic foam is a trade secret, but the manufacturer insists that it has nothing to do with ordinary Styrofoam. As Bob Graham puts it, it's good to know that the woofers are not "reconstituted coffee cups."
The quest for diaphragm rigidity is not based on esoteric considerations, but on a fundamental understanding of the mechanical stress that a woofer undergoes. This stress has been compared to that of a supersonic aircraft coming out of a power dive. Let's calculate the maximum acceleration a woofer is subjected to while undergoing sinusoidal motion with a peak excursion of only 0.1mm at 1kHz. I get a peak acceleration of 1078ft/s2. Dividing by 32ft/2 (which is the acceleration at sea level due to gravity, or 1g), we arrive at 33.7g worth of acceleration. That's enough to black out even the most experienced jet pilot. The peak acceleration is linearly related to the peak displacement. Thus, a 1mm peak excursion for this woofer corresponds to an acceleration of 337g. And at 3kHz, a 1mm displacement yields a peak acceleration of 1011g. No wonder diaphragms break up!
I find it intriguing that the Ensemble woofer emulates the principles put forth by Barlow over 30 years ago (Wireless World, December 1958). Barlow essentially attributed the poor performance of conventional speakers to the sorry state of drive-unit development.
He goes on to say that "The unsatisfactory performance of the direct-radiator moving-coil speaker has recently stimulated interest in other types of transducer, such as the ribbon, electrostatic, Ionophone and corona-wind speakers. In these, the driving force acts over the whole of the diaphragm (whether of metal, plastic, gas molecules or ions), so that diaphragm breakup does not occur." He makes the following prophetic prediction: "...if the diaphragm of the moving coil speaker could be made more rigid, its performance might compare with those of other systems." Barlow proposed a sandwich construction of expanded polystyrene with aluminum skins, because, in bending, the maximum stress occurs at the surface. Thus, by concentrating a stiff and heavy material at the surfaces while using a lighter and weaker material for the core, the sandwich ends up being far stiffer than the same total weight of either material used separately. Such a sandwich can be hundreds of times stiffer than a similarly sized paper cone. Leave it to the Germans to be in the avant garde of driver technology. Barlow mentions a German driver, the "Zellaton," vintage 1957, that already used a metal-foil cone backed with foam plastic and varnish, while Barlow's diaphragm first made its appearance in the Leak "Sandwich" loudspeaker.
To take full advantage of the very rigid diaphragm, the voice-coil former should also be very rigid. An aluminum alloy former is used in the Ensembles to achieve mechanical stability and provide a good heatsink. The voice-coil is made up of high-purity edge-wound copper wire. A large magnet is used to increase acceleration factor and efficiency. The woofers are tested as they come in to the factory, apparently in very much the same way that "Stradivari chose his violins"by listening. It is on the basis of voicing that the quality of the woofer is determined by an experienced listener.
In the Reference, the woofer is further enhanced. The woofer "cone" construction is a pure geometric cone. There is no dustcap; the cone keeps receding to a point. It is well known that the strength of a vibrating surface depends on its contour, and a conical vibrating surface is stiffer than that of a plane. In fact, conventional paper and plastic diaphragms are not usable in a plane geometry; there's little stiffness to reduce lateral flexure. The figure I've seen is that a cone is about 140 times stiffer than a disc of the same diameter and thickness. The Reference extends the philosophy of the PA-1 to its logical conclusion. Its woofer is more rigid yet, with better linearity, and there is also less noise from the voice-coil coming through the cone.
The crossover is placed around 2.5kHz and uses impedance-compensated first-order high- and low-pass filters. The layout and positioning of the crossover have been optimized to avoid magnetic interference. Crossover parts are individually tested and matched for the tightest tolerance.
The cabinets are made of a sandwich of several layers of high-density wood in order to damp panel resonances. The speakers are finished in either a wood veneer (my samples were Swiss cherry) or a multi-layer lacquer. The input connectors are recessed banana jacks. This was the only aspect of the finish I don't much care forI would prefer high-quality binding posts, especially considering the cost. There is a bi-wiring option available with Neutrik Speakon connectors, which I have not tried. This option has to be ordered with the speakersit is not retrofittable.
According to Ensemble, Inc., the basic design idea was to make a loudspeaker that excelled in the areas of tonal neutrality, transparency, harmonic integrity, dynamics, and soundstage reproduction. Toward these ends, designer Harry Pawel opted for a small two-way system because it represents a point source. Considerations of time alignment and pulse response were taken into account, as were the practical considerations of good sensitivity and unproblematic impedance.
Because the Ensembles are designed for a full-bodied balance while mounted on stands, they will integrate well with a wide variety of rooms. However, as with all minimonitors, a basic tradeoff must be made between deep bass extension and imaging excellence. Placements close to the wall will provide deeper bass, but at the cost of deteriorating spatial resolution because of increased early wall reflections. For most folks, as for me, the interest in a minimonitor is based to a large extent on imaging, so the decision to optimize for soundstage reproduction was an easy one, in my room translating into a placement about a third of the way into the room from the front wall.
The classic isosceles-triangle stereo configuration worked well with both the PA-1 and the Reference. The speakers were positioned at two corners of the triangle, with liberal toe-in so that their listening axes crossed over in front of the listening seat.
The listening axis is defined as the center line of the front baffle, and the manufacturer recommends that it be at the same level as the listener's ears. Of course, the listening height is going to depend on the stands one uses, and with most stands it's quite easy to end up with the listening axis well below ear level. I experimented with several stands before JA loaned me a pair of 24" Celestion Si stands. These turned out to be the match for the Ensembles. With too low a stand, the balance becomes tilted toward the lower registers and imaging precision suffersprobably due to floor reflections interfering with the direct speaker radiation.
While the Celestion stands offered the optimal listening height and, being filled with a mixture of lead shot and sand, are inherently quite inert, there was the problem of firmly coupling the enclosure to the top of the stand. At about 15 lbs each, the speakers tend to rattle or dance on top of the stands when the enclosure resonances are excited. It would be far preferable to have this energy coupled to the stand and dissipated either in the stand or the floor. This could be done most effectively by rigidly anchoring the enclosure to the stand. I'm a firm believer in integrating the stand and enclosure. One way to do this would be to screw the stand to the enclosure itself. The importer, Bob Graham, is hard at work on a stand that would intimately mate to the Ensembles. To complete the perfect union, it would also offer an appropriate listening height.
As an interim fix, I placed a cinderblock atop each enclosure to weigh down the speakers and thereby secure a tighter box/stand coupling. It doesn't look pretty that way, and several visitors have either verbally or visually expressed puzzlement over the presence of ugly bricks atop the speakers. But one of the advantages of having a listening room that isn't in one's home is the freedom to adopt utilitarian solutions without much argument.