INNERSOUND EROS HYBRID ELECTROSTATIC LOUDSPEAKER SYSTEM Harry Pearson, reviewer

The Absolute Sound, Issue 112, May 1998, Harry Pearson, reviewer

It's an odd thing about speaker systems.  You can respect them, without ever really warming up to them in that let-it-all-hang-out way that you could, say, do with an old beloved friend.  It isn't that you don't know they're good  some of them are  but somehow there's just not that emotional click.  And let's say that, like the early Wilson Watts or the early Aerial Acoustics designs, this is how I feel about some of today's best, most distinguished designs.  I hear all the things they're doing right, and I'm impressed (critically), but it's still just a speaker, Ingrid.
Then there is the really impressive, but still analytic, system that drives the critic in you bonkers  this is the speaker, like the Von Schweikert VR-8 or the new Genesis APM  that you can depend on to impress your cognoscenti pals (including the highest of the High End designers), that shows you plenty and manages to keep you in delighted surprise.  With this sort of speaker, you find yourself listening not to it per se, but back through it to the components that have preceded it in the system.  You like it, you respect it.

And, every once in a great long while, there comes along a speaker such as the EROS and it's a kind of love at first sight, warts, wrinkles and all.

I first heard the EROS in January when I was, still virginal to the intricacies of navigating a Consumer Electronics Show, just about to abandon hope of hearing anything at all interesting.  Then, voila, late one Saturday afternoon, someone dragged me into InnerSound's room, a modest enough establishment in one of the Alexis Park's suavely painted dungeons, and there it was.

Roger Sanders, the designer of the entire EROS system, has the electronics portion of the speaker system (the woofer-amplifier/crossover module) custom-built in London, to his specifications, while the actual speaker is made in Atlanta Georgia.  This electrostatic hybrid costs $4500, which begins to seem like one of those rare High End bargains when you consider that, in addition to the loudspeakers, there is an electronic crossover and 200 watt-per-channel amps included to drive the (highly modified) transmission line woofer.  And if you're a big spender, you can always substitute a bass amp of your own choice and run it through the crossover.

What impressed me, other than its great sense of aliveness and presence, was the integration between the panels and the woofer system, which as far as these ears could tell, was virtually seamless.  Doubly impressive, if you consider as an example some of the workaday Martin-Logans, which also accomplish a seamless blend in the crossover region; however, below that band of frequencies, and further down in the woofers' operating ranges, you not only can hear the Martin-Logans' (relative) distortion and coloration, but how slow they are in comparison with the upper panels.

Not so with the EROS, which somehow, manages to sound both quick, uncolored, and ultra-low in distortion as its transmission line woofer system descends into the depths, and descend into the depths it does.  Actually, that's not what really did take me by storm  it was that these stylish-looking devices actually were musically involving.  And I mean involving.

I had dragged along one of those hideous large  wallet-sized CD carrying cases, loaded with all of my then favorites, from Bela Bleck's Uncommon Ritual (on Sony) to The Absolute Sound and Hearts of Space's collaborative sample/test disk and I forgot about the blush wine (spare me) InnerSound's owner Raj Varma had inserted between my fingers, about the other folks in the room (they were there), about High End vermin and varmints, about Las Vegas, the works.  I found myself having something as close to a Ball in Hell as you'd like to imagine, and that was from getting off on the pleasure of the music.  Came back again even later that afternoon.  And the next day I put the iron lock on expertmentarian Robert Harley and dragged him to hear the speakers.  Alas, Varma and company had inserted a cold Electro-companiet preamp into the chain and the speaker sounded . . . well, like it has cold transistors in the chain.

Undaunted, I ordered up a pair.  (And so did the e'er contrary Dr. Robert E. Greene, who ought to be commenting on this review one of these days.)  Let's get the warts out of the way.

This is a one-person speaker system, unless you can line up listening chairs in front and behind your listening position so that others don't sit off-axis and miss the top octave on this system.  Even so, without being at the exact focal point of the speaker's angled field, you'll miss something of the witchcraft involved in its soundstage and image projection.  It doesn't sound bad, or anything approaching that, off-axis, it just doesn't sound as if Merlin had been spreading twinkledust over it.  That is to say it doesn't sound as magical as it can when you're in the exact spot  I'm sorry, by the way, to use that word magical (which has been worn to death in audio prose), but I can think of no other synonym that quite captures the EROS' true especialness.  That said, let's get off the warts for the while and take a look at the speaker's strong points.

It can play loud.  And, even with its 90 dB efficiency, it won't tax, in the smallish dimensions of Room 2, the ability of the 23 watt Viva Aurora (single-ended triode mono-blocks).  You want to get down tonight?  Well, you can with the Viva and this system pity, though, the Vivas are four times more expensive than the integrated speaker system, such a musical blending they make.  When I say loud, I mean soul-satisfyingly loud when the occasion calls for that.

Strangely, like so many electrostatics, including the Martin-Logans, the EROS is forgiving of compact disks.  I am beginning to wonder if the very speed of an electrostatic's response, its transient quickness, doesn't somehow synergistically mesh with the transients reproduced by digital in a totally serendipitous way. Even on home theater, electrostatics handle digital dialog with a clarity and articulation you simply won't find on any moving coil speaker system I know of.  And while I haven't yet tried it, I'd bet the EROS would be a humdinger in a home theater setup. (You'll notice the yet and get the hint of things to come.)  On many a pure moving-coil loudspeaker system, the compact disk simply doesn't sound quite right in the way it can on an electrostatically-based design.  (Don't shoot me, I'm just the reporter.)

More surprising, at least for me, were the most extended high frequencies, which don't roll-off so sharply and dramatically as some 'stats (try Martin-Logan's Request or the Quad, old or new).  It just keeps on going up, letting air and a kind of sweet breeziness into the top octave that translates down into bloom and that kind of cream-like sound on massed violins.  Even a small section of violins, if indeed that section has poised intonation.  It will surprise no one to learn that the transients, from the speaker's crossover point of 450 Hz upward, are as good as they come and without that peculiar, somehow inherent electrostatic credit-card coloration.  Instruments being reproduced in the soundspace do not have that disembodied quality, that lack of palpability that sometimes affects very fast ribbons and 'stats.  

Some part of this palpability may be the result of the woofer, which runs up well into the range of the soprano, and still has considerable output at 440 Hz., just about dead-center of the midrange, thus giving some heft and body to the electrostatic panels (which, come to think of it, look much like those of a Martin-Logan, except for the two bands running down the center of the element, and the fact that the panels are flat, rather than curved, as they are on most every other ESL).

The EROS did not require much break-in, at least not after the panels developed a charge.  Give or take, say, an hour or so, depending on the weather.  Then it's off to the races.

S. Markwell thought the InnerSound bass amplifier was somewhat soft and substituted a Plinius SA-50 solid state unit in its stead.  (This is easily accomplished and level matching is permitted, via a volume control on the front panel of the crossover box.  Well, not so easily accomplished if you do not read InnerSound's expertly instructive book on setting up the system and placing the speakers correctly.  InnerSound advises that you not listen for bass level in setting the volume of the bass amp, but rather for the purity and clarity of mid-frequency voices and musical material.  That's the way to get the bass level right, and on that point, the author of the point has it exactly so.) SLMI decided to reserve judgment on the matter of ultimate bass amp quality until we could do much more listening  I could and would and will say that the InnerSound amp is worthy of the package, and upon first blush, I didn't hear incredible differences between the two amps.  

The instruction book is also sage in the advice it gives on getting the best sound out of the system, and we followed the advice, almost to the proverbial T. 

InnerSound wants the speakers angled inward, at a rather steep rake, so that the listener is at the apex of an equilateral triangle.  You might picture HP with a big flashlight on his head, trying to line up the center strip of both panels in order to best focus the sound.  And indeed, this focused positioning did give, contrary to my expectations, not only the best recreation of orchestral and vocal images, but the best sense of a real space in which those performers were making music  including that dimension we refer to as stage width.  

We dug out some of the house favorites, including a few new to Markwell.  Out came Casino Royale, and out it stayed well past the time we listened to the EROS.  So did the Fifth Dimensions' Magic Garden, particularly for Carpet Man and for Jimmy Webb's infectious, inventive, and beguiling arrangement of the Beatles' classic, Ticket to Ride.  This last disc, as I learned to my consternation, can sound mostly horrid with the wrong speaker system (e.g., the high-efficiency Alon Lotus).  But the EROS (and A. Nudell's APM), perhaps because of their speed, smoothed, indeed ironed out the amusicalities and let the room dance.

Out came the Mercury/Philips CD remaster of Howard Hanson's classic The Composer and his Orchestra, the new and most unTelarcian sounding version of Rodrigo's Concerto de Aranjuez, delicately and masterfully conducted by of all people, Erich Kunzel, in a version that sounds much like the old Argenta/Yepes version on Decca/London.  And where would I have been without the Mehta reading of Holst's Planets on London (well, actually, for me, one planet  Saturn  the most challenging cut on the disk and the best played), the Alexander Gibson reading of the Carmen Suite on an RCA/Classic reissue.  And that inimitable first cut on Bella Fleck's (and friends') Uncommon Ritual album, as exciting a demonstration of what CD can do at its very best as I know of, with some sonically stunning riffs on the double-bass mid-piece.

Those double-bass descending chords will illustrate, as happily as anything that presently pops to mind, just what a superb blend Roger Sanders has achieved between the woofer and the main panels.  The woofer's attack is tight, quick, and most low in distortion (all of which puts another feather in the mythic cap of good, if stylized transmission line design).  It would seem to me that he has done something exceedingly clever here; he has designed a woofer system that is like unto the strong points of the electrostatic and then run the woofer up to such a high frequency that the woofer lays the foundation for most of the orchestral fundamentals while letting the 'stats take over the highest fundamentals and overtone system, thus getting a more life-like cohesion than he could have achieved at a lower frequency.

A further result of this is a warm, most romantic-sounding speaker (reminiscent of the Aerius without the drawbacks of the frequency extremes and without a perceptible loudness ceiling)  warmer and more romantic than you'd expect an electrostatic design to be, and with considerable dynamic snap and gradation.  All of which would be as nothing if the thing didn't sound so alive.  Indeed, there seems to be only a narrow range within which the speaker comes alive.  Once that ideal level setting is found, the speaker will handle all of the dynamics thrown at it with aplomb.  Set it a bit below the magic point (sorry 'bout that), and the speaker doesn't get up and kick in its reproduction of the music, above the magic point and it simply is too loud, even to the point of overloading the room.

I suppose the long-range test will be how well it wears over time.  Will long-term listening reveal its character superimposed with too heavy a touch on the music, thus robbing the sound of its infinite variety and discrimination?  Like many another electrostat, I can't say that I know just yet.

We certainly could hear amplifier differences through the speaker, although, like the Magnepans, not to the degree that you'd get from a VR-8 or an APM.  But given the expert fiddling a great High End dealer could do  these things would seem to be relatively room-forgiving (the speaker sounded good despite the acoustic horrors that spell Alexis Park); they certainly did not sound all that different in the more stunning acoustic of Room 2  I believe a hands-on demonstration would beguile you and be worthy of traveling some miles to hear.  Given their price and the music they make, I wonder if you could do multiples better at multiple their price.  This one, kiddos, is for hard-core music lovers, of whatever stripe and hidden poetry.



THE DESIGN OF THE EROS
The InnerSound Eros, designed by Roger Sanders, is the result of his 26 years of work in electrostatic speaker and transmission line woofer research and development.  Sanders began designing ESL systems in 1972, and from then on, electrostatics, in one form or another, have been his passion.  In 1990, his thoughts took form as a compact/integrated ESL/transmission line design that saw the light of day in a series of articles in Speaker Builder magazine.  These articles, his book, The Electrostatic Loudspeaker Design Cookbook and his continuing  R & D, prompted the British company Omni Digital to commission him in 1996 to design the finest speaker he knew how, with no restrictions save overall size; the speaker had to be relatively compact.  This challenge appealed to his sensibilities, he accepted the work.

Although Sanders developed the idea of a curved ESL diaphragm in order to gain wider overall dispersion in the listening field, he ended up using a flat panel in the finished speaker because he found that the high-frequency response and imaging characteristics of a planar dipole radiator were superior to the curved panel.  A bit of off-axis air was traded for superior overall performance within the nominal listening window of the Eros.  Using a flat panel also results in much less reflected sound, since the bulk of the energy is radiated directly at the listener, minimizing interference from the room's boundaries.

Sanders chose to use an active electronic crossover in the Eros, rather than a passive system.  As he says, active low-level crossovers can be made with much steeper crossover slopes and the crossover 'Q' (damping) can be optimized.  And unlike high-level, passive crossovers, a low-level crossover cannot be overloaded  The Eros uses a 24 dB/octave slope at 450 Hz, which allows the bass driver to cease its contribution within one octave, versus two or more for passive, 12 dB/octave (or less) types.  Extensive critical listening was the overriding developmental tool used by the Omni Digital team responsible for the final design of the crossover, which took some eight months to realize.

Sanders chose Class A operation for the integrated circuits, which means that the semiconductors in the ICs are always turned on electrically so they do not have to wait for a music signal to tell them to begin operation.  This gives greater purity and sweetness of tone, and fewer time/phase distortions than the more conventional Class AB design, which depends on an incoming signal to trigger the ICs (or other semiconductor devices) into operation.  Class A operation is quite inefficient: much of the current employed to keep the devices on is dissipated as heat, but the sonic benefits, Sanders believes, are worthwhile.

The most critical part of Sanders' overall design was the choice and tuning of the bass system.  ESLs are notoriously difficult to match well with dynamic, moving-coil woofers.  Sanders chose a transmission line-loaded woofer, an enclosure system that directs the rear wave down a non-resonant, highly damped and controlled pathway, terminating in a vent that allows the deepest bass to exit the enclosure in-phase with the main radiated sound of the bass driver itself.  The impedance of such a system can easily be controlled; the lack of resonances in the line lends itself to decreased variations in impedance. His two most important sonic considerations were seamlessness and transient agility.  Transient performance is perhaps the single strongest attribute of electrostatic designs.  Sanders rejected a sealed box (acoustic suspension) because these designs are somewhat non-linear and low in efficiency.  Conventional ported (bass reflex) systems, despite acceptable efficiency, focus on frequency response rather than transient performance, and the radiated energy from a port is difficult to control in the phase domain, often causing boominess and smearing.  A simple labyrinth, an undamped transmission line, creates resonances and phase shift problems at the lowest frequencies.

Sanders' finished transmission line is eight feet long and carefully folded to fit inside a compact box.  The damping material is a synthetic fiber rather than wool, because it has more even packing characteristics and does not settle and bunch as much.

The result of these decisions is a bass system that can operate up to 450 Hz, virtually unheard of in a hybrid ESL design, with minimal phase shift in the bass response compared to the rest of the frequency spectrum, and a transient quickness that allows the listener to forget that there is more than one driver in the system.  The bass response, particularly the midbass, he says, is controlled and even, eliminating peaks and dips in the response of the driver, which would spoil the transient speed and articulation.

Although Sanders chose Class A operation for sonic reasons in the electronic crossover, he defends his choice of Class AB for the stereo bass amplifier because, after extensive listening, he could hear little improvement with the less efficient Class A.  Also, the amp, despite its 200 watt-per-channel rating, runs cool and draws relatively little current.

Overall system efficiency is rated at 90 dB/watt at 1 meter, although, as HP points out, the things seem to play much louder than this figure indicates.  The nominal impedance of the woofer is an almost flat 4 ohms, while the impedance for the electrostatic cell starts at 113 ohms at the 450 Hz crossover point, and continues down, in an almost straight line, to about 2 ohms at 20 KHz.  Although this impedance curve may seem odd to those accustomed to dynamic speakers and their often erratic impedance fluctuations with frequency, it is important to note that, because it is basically just a capacitor and extremely efficient in voltage driving requirements, an ESL panel does not dissipate power, in the conventional sense.  Therefore, any high-quality 8 ohm-rated amplifier should be able to drive these speakers well, as HP experienced with the 23 watt Viva Auroras.  Sanders rates the overall frequency response of the Eros at 24 Hz to 27 KHz. (He prefers not to give a ñ figure for deviation from this spec, since the final sound, especially in the bass, is somewhat dependent on room influences.  He says that the lower limit of useable in-room response is generally somewhat below 30 Hz, and the highs, if listened to on-axis, are even and extended and not particularly room-dependent.)

The Eros is Sanders' first true commercial offering, but there is much more on the way, from a smaller, passively crossed-over system at about half the price of the Eros, to a state-of-the-art attempt consisting of a pair of 2 x 6 foot panels mated with a Hartley 24 inch woofer (transmission line, of course) that can be switched out at will, crossing over the woofer system at 85 Hz (the panels are to go down to 40 Hz).  Shades of Servo Statik!  We hope to have much more to say about this company's products.SLM


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