Infinity IRS Epsilon loudspeaker
Cary Christie, one of Infinity's co-founders and the only one still working with the company (though now as a consultant heading his own, independent Christie Designs), has been quietly building the foundation for a whole new generation of Infinity high-end designs (see my interview with Christie elsewhere in this issue). Some of his worknew electromagnetic induction drive-units used in a monopolar arrangement with the back wave absorbedwas incorporated into the Infinity Renaissance. Now, with the new IRS Epsilon, we can take a better look at what Christie and Infinity have been working on for the past several years.
The Epsilon is a massive yet sleek one-box loudspeaker which makes use of updated drive-units that echo longstanding Infinity design concepts: planar drivers and servo-controlled woofers. Its 150 lbs per side seemed like more as we unboxed and wrestled it into my listening room. The Epsilon doesn't take up much floor space, and looked reasonably svelte once in its ready-to-crank position, but it definitely doesn't take easily to being moved once it's set upand this has as much to do with sound as with size.
The Epsilon's bass chores are handled by a heavy-duty 12" driver with a 3-lb ceramic magnet, over-large (8") spider, 1.5"long voice-coil, and greater-than1" peak-to-peak excursion capability. The cone is composed of Injection Molded Graphitea combination of oriented graphite fibers and polypropylene which, according to Infinity, provides an optimum blend of strength, rigidity, mass, and damping.
This driver is mounted in a relatively small sealed enclosure. To get the desired extension and low-bass distortion, a servo feedback network is used. But while an amplifier is an integral part of most servo subwoofer designs (as in Velodyne, Genesis, Mirage, and earlier Infinity designs), the Epsilon requires separate woofer amplification. This is a mixed blessing, as we shall see.
The Epsilon system includes both a Servo Control Unit (SCU) and cables to link the SCU to a special input on the back of the speaker cabinet. This in turn is connected to an internal servo network which is linked to an accelerometer mounted at the apex of the woofer's cone. The accelerometer senses the motion of the cone, which is then compared with the amplifier output. If the two disagree, a correction signal is generated by the servo, which not only lowers bass distortion, but boosts the bass output to correct for the typical low-bass rolloff found in any real-world loudspeaker/cabinet configuration.
The system preamplifier's outputs are connected to the SCU inputs; outputs from the latter feed the amplifiers for the woofer and the top-end sections of each loudspeaker (see block diagram). The Epsilons must be bi-amped, therefore. The signal destined for the woofer amplifier is low-passfiltered and modified within the SCU, as called for by the Servo Control system. The full-range and unmodified top-end amplifier also passes through the SCU, but only as a hookup convenience. The crossovers for the upper-range drivers are passive, and located within the Epsilon's enclosure.
Infinity Epsilon, system block diagram showing servo connection (one channel shown).
The upper-range drivers that take over from this Servo-Controlled woofer at 150Hz are similar in design to the EMITs (ElectroMagnetic Induction Tweeter) and EMIMs (ElectroMagnetic Induction Midrange) of past Infinity high-end designs. But both the drivers and their applications have undergone considerable refinement and modification. Their diaphragms are made of laminates of polyimide film, pressure-sensitive adhesive (with damping properties), and an etched, aluminum voice-coil (footnote 1). The EMIM and L-EMIM drivers are about 1½ times the thickness of a human hair; those of the EMIT are less than half that. The EMIT also incorporates a specially developed fabric acoustic filter for smooth horizontal-dispersion characteristics.
Though many audiophiles are fans of dipole-radiating loudspeakers, this type of design has its problems: placement sensitivity, dipolar cancellation of low frequencies, and frequency-response anomalies resulting from the rear radiation as it bounces off the front wall, then combines in a time-delayed fashion with the output from the front. Infinity avoids or minimizes these problems by absorbing the rear radiation of the naturally dipolar EMITs and EMIMsa technique they first used in the Renaissance loudspeakers. The entire top half of the cabinet, above the sealed-subwoofer enclosure, is an open baffle filled with absorbing material designed both to damp the rear radiation and prevent it from reflecting through the thin, low-mass laminate diaphragms. The sole exception to the monopolar, front-radiating design is in the mid- and upper treble; a second EMIT is mounted at the top and rear of the enclosure to enhance the speaker's power response in the upper two octaves.
The Epsilon's cabinet is substantial. The woofer enclosure is made of high-density fiberboard: 1" thick, except for its 3"-thick (!) front baffle, a thinner, vertical extension of which is used to mount the remaining mid- and upper-range drivers. This sub-baffle is moderately sculpted and covered with acoustic felt to minimize diffraction. The front baffle is framed in an attractive Santos Palasander wood finish resembling rosewood. The grillecloth isn't readily removable.
According to Infinity, premium-quality parts and high-quality internal cabling are used in the crossover network. Each driver has its own isolated, glass-epoxy crossover circuit board. A unique aspect of the Epsilon's design is the use of two 9V batteries per loudspeaker to bias the joining point of a series/parallel set of capacitors in the tweeter network; this is said to noticeably improve the top end. The battery bias is applied through a high (5 megohm) resistor, limiting the demand on the batteries. As a result, the batteries' active life expectancy is equivalent to their shelf lifeabout two years.
The Epsilon's rear panel has two pairs of top-quality WBT input jacks, and control switches to adjust the relative balance of the EMITs (three positions) and EMIMs (two positions each). The audible effect of these controlsspecified as no more than 1dBis subtle but significant. Less subtle but no less important are the three controls provided on the SCU for overall woofer level, bass contour, and midbass contour.
The Epsilons' large, adjustable feet cover equally heavy-duty spikes, which should be used except where there's risk of damage to tile or wood floors. Large locking rings, which secure the feet or spikes after the loudspeaker is leveled, are the best-thought-out such devices I've ever seen.
Connecting all the pieces of the Epsilon systemSCU, amplifiers, and loudspeakerswhile a bit complex, is fairly straightforward. Nevertheless, you must know with absolute certainty the polarity of the amplifiers driving the subwooferswhether or not they invert phase or, in the case of the balanced hookup, which pin is referenced as positive. A switch on the back of the SCU is set by the user or installer to correspond to the non-inverting or inverting nature of the woofer amplifiers. If set incorrectly, the feedback loop won't operate properly, possibly resulting in damage to the loudspeaker, the amplifier, or both.
Footnote 1: The current running through this flat, etched voice-coil interacts with magnets placed on both sides of the film to cause the diaphragm to vibrate.