Ultimate Audio, Spring 1999, Rufus Smith, Reviewer

Because of their all-important midrange purity, electrostatic speakers have always held a special place in the hearts of audiophiles.  You can't, however, cheat Mother Nature and the laws of physics when it comes to low frequency extension.  In this case, there's a direct correlation between electrostatic panel size and low frequency extension.  The lower the bass response, the bigger the panel required not to mention the attendant speaker impedance problems and dipole cancellation effects.
In order to extend the frequency response of an electrostatic speaker, several speaker designers have attempted to pair it with a conventional dynamic driver with decidedly mixed results.  In the end, either the slow, ponderous woofer's inability to keep up with the speed of the electrostatic panel or the different driver colorations almost always prove to be the Achille's heel of hybrid electrostatic designs.  Once the ear identifies this discontinuity between drivers, so ends the infatuation with the hybrid electrostatic design.

The $4,500 Eros speaker from InnerSound represents the latest in a long series of attempts by speaker designers to marry an electrostatic panel with a dynamic woofer.  But Roger Sanders, the designer of the Eros, appears to have hit on a solution to the driver integration problem, where so many before him have failed.

After fiddling with the speaker's low frequency controls, it's clear that Sanders has done an excellent job of mating and blending the Eros' woofer with its electrostatic panel.  The blending of the woofer and panel is seamless, and at no point in time did the transition between drivers draw attention to itself.  Part of the secret of Sanders' design appears to be directly linked to his use of an exceptionally quick transmission line woofer to reproduce the low frequencies.  Here is one low frequency driver that has the prerequisite speed to keep pace with the essentially massless, lighting-fast electrostatic panel.

So, how does the Eros sound?  Its low frequencies are quick, clean and well-defined.  While the speaker will not reach into the depths plumbed by a 32-foot stop on an organ, it still has enough extension to provide the foundation that gives the bass its power and glory. (Manufacturer's note:  Revisions to the Eros have resolved this issue.  Low frequency extension now approaches 20 Hz in most listening rooms.)  

Neatly illustrating the seamless blend of drivers is Charles Mingus' bass cutting a rich path through the musical tapestry on Goodbye Pork Pie Hat from Mingus Ah Um (Columbia/Classic CS8171).  While the first-order harmonics of the bass fall below the 450 Hz crossover point, the second- and third-order harmonics created extend well beyond the crossover point.  And the transition between drivers is extremely smooth.

Another excellent example of the Eros' bass performance is found in its reproduction of the Bodhran drum featured on the Braveheart soundtrack (London 448 295-2).  Through the Eros, the drum is extremely well-reproduced without any smearing of the details.  The bass from the Sanders-designed transmission line woofer is tight and extremely low in distortion.  The initial transient attack that occurs when the mallet strikes the drum head is followed by a very natural decay.  The drum literally rolled effortlessly through my listening room.  At no time was the Eros' bass lumpy or ill-defined.

Ascending the frequency scale, I found that the midrange, when properly set up (in other words, after getting the woofer/panel balance right), was silky smooth, detailed like you'd come to expect from an electrostatic driver and remarkably free from boxy colorations.  The Eros especially stood out when it came to reproducing the qualities of the female voice.  Holly Cole's voice is warm and silky, with a very seductive quality to it.  She also has the wonderful ability to let her voice convey a wealth of emotions that the Eros easily captured and conveyed.  This is particularly noticeable on Don't Let the Teardrops Ruin Your Shining Heart from the Holly Cole Trio's Don't Smoke in Bed (Alert Records CZZ 81020).  On this cut, Cole sings through the pain of a former affair and the alcohol abuse it spawned.  There's no mistaking her pain and agony through the Eros.

Electrostatics also often run into problems in the upper octaves.  Beaming, for example.  A drop in impedance in the 20 KHz region to below 2 ohms can cause problems for tube amplifiers and even solid-state amplifiers with less than optimal power supplies.  Not with the Eros.  The Eros' upper octaves extend up into the stratosphere.  Violins are silky smooth with no sense of hardness.  The carillon on Tchaikovsky's 1812 Overture as performed by Antal Dorati and the Minneapolis Symphony Orchestra (Mercury Living Presence D111714) rings with a clarity that I have never heard before.  The Eros allows the sound of the bells to flow through the musical landscape.

The 1812 also serves as a demanding test of a speaker's ability to handle macro- and microdynamics.  In the finale of the Dorati 1812, the orchestra builds to a crescendo while adding cannons and bells to the mix.  The Eros does an exceptional job of keeping everything separate and in its proper place.  Nothing overpowers anything else.

The Eros' other outstanding quality is an ability to convey and reproduce a beautifully detailed soundstage.  Images within the stage are extremely well-delineated.  Take, for instance, Track 10 from Phantom of the Opera (Polydor, 831 273-2 Y-2), which features six cast members singing simultaneously.  Each performer's voice is easily discerned within the depths of the stage.  An even better example of the Eros' resolving power is the Turtle Creek Corale's performance of Pie Jesu from John Rutter's Requiem (Reference Recordings, RR-57CD).  The Eros brings Dallas' Morton H. Meyerson Symphony Center into your room, and allows you to appreciate the sound of a real chorus singing in real space.

The Eros' openness and clarity gives the listener a clear window into the depths of the soundstage.  Sara K. performing If I could Sing Your Blues from the album Play on Words (Chesky JG90105) is an excellent test of a component's ability to reproduce depth and the Eros passes this test with flying colors.  The trumpet that opens the piece was at least ten feet from the microphone on the right hand side, and the Eros places it in precisely that spot.  Sara K. appears closely centered between the speakers.  Yet, at the same time, her voice fills the room with a natural reverberation.  The guitar appears slightly behind Sara and has a warm, natural tone.  This speaker is a real champ at reproducing depth and unraveling the layering of instruments and musicians.

The Eros, unlike many other electrostatics, can certainly play loudly.  Its sensitivity, the use of a separate amplifier to drive the woofers, and the fact that the panel does not require a lot of power, makes the speaker an ideal choice for members of the low-power, single-ended triode crowd.

I spent several thoroughly enjoyable weeks using the Eros with the 30 watt per channel VAC Renaissance 30/30 Mk.2 recently reviewed in the pages of Ultimate Audio by Myles Astor.  Since the electrostatic panels' impedance drop to 2 ohms at 20 KHz, you really need an amplifier capable of handling a capacitive load.  Well, let me tell you:  the Eros and the little VAC were a match made in heaven.  Music flowed into the room without the slightest bit of grain.  Notes started and stopped on a dime, here was an amplifier that brought out the best in the electrostatic panel.

The Eros isn't a perfect speaker.  I guess we'll have to wait for Sanders' ultimate speaker design for audio nirvana!

First and foremost, the Eros has a tendency to beam the high frequencies directly at whatever they are facing.  If you can't see your reflection in the electrostatic panel (that is, with the speakers pointed straight at you), kiss the high frequencies goodbye.  This trait makes the Eros a one-person speaker  unless you are willing to line your listening buddies' chairs directly in front and behind you.  Now, don't get me wrong  the off-axis performance of the Eros is not that bad.  It's just that rolled-off highs rob the music of some of the qualities that make this speaker so wonderful.

Soundstage width also proves problematic with the Eros.  No matter where the speakers were placed in the room, the soundstage never extended beyond their outer edge.  This made for some rather skinny orchestras in large-scale symphonic works.  By contrast, my reference Wilson WATT/Puppy 5.1s will throw a soundstage that is wide as all outdoors when the occasion calls for it.  The Eros was just unable to duplicate the feat in my listening room.  But this was not really distracting, given the strength of the Eros in the areas of soundstage depth and clarity.  (Manufacturer's comment:  The Eros has superior soundstage imaging, depth, and clarity because it projects a phase-coherent wave-front.  This correctly handles the image and does NOT allow it to get out of control and extend beyond the edges of the speakers.  If a wider image is desired, simply place the speakers more widely apart  the Eros will never develop the dreaded hole-in-middle that limits how far apart you can space ordinary speakers.)  

Finally the speakers proved sensitive to certain components  especially those of the balanced variety.  When I first received the speakers, I placed them in my reference systems using the BAT VK-500 amplifier, BAT VK-51 line stage and BAT VK-P10 phono stage.  No matter what I tried (including several calls to Sanders), I was unable to prevent transients from going through the speakers when the preamplifier was muted.  It wasn't clear whether the problem was the result of the single-ended crossover being inserted between the balanced preamplifier and amplifier.  Substituting a passive preamplifier and the VAC Renaissance 30/30 tube triode amplifier into the system seemed to solve the problem.  In fairness, Sanders is currently working on a balanced crossover that will solve this problem (this crossover will be made available to current owners as an upgrade).

Shortcomings aside, the Eros is an exceptional first effort from a new speaker company.  Combined with the right electronics and careful attention to setup, the Eros is a speaker that remains true to the music.  InnerSound has a contender on its hands.  This is one speaker I can recommend to those who value the resolution of an electrostatic speaker, but at the same time want a full-range loudspeaker.  Now, gimme that balanced version!

Roger Sanders, the designer of this unique speaker, is considered to be one of the world's leading authorities on electrostatic speaker design.  Sanders is the author of The Electrostatic Loudspeaker Design Cookbook, considered by many to be the definitive treatise on the subject.

The Eros is a hybrid bi-amplified system that mates a 15" x 42" (W x H) flat electrostatic element with a dynamic, transmission line woofer.  It is interesting to note that Sanders opted for a flat panel for the ESL, in contrast to current conventional wisdom that use curved panels to increase dispersion.  Although Sanders invented the curved electrostatic panel in 1978 (first published in Speaker Builder Magazine in 1980), his research showed that flat panels exhibited  better high frequency response, greater efficiency, and imaged much better than curved panels.  

A unique aspect of the Eros' design is the use of a 10-inch transmission line woofer to reproduce low frequencies.  One of the advantages of the transmission line design is that the rear of the low frequency wave is directed down a highly damped, non-resonant path that ends in a vent.  As a result, the deep bass exits in-phase with the radiated sound of the main driver.  The Eros' transmission line is over eight feet long and is folded so that it fits into the compact box that forms the base for the electrostatic panel. 

Another somewhat unorthodox design approach in the Eros is the crossing over of the woofer to the electrostatic at an unusually high frequency of 450 Hz, using a 24 dB per octave slope.  According to Sanders, this arrangement is in large part responsible for the speaker's tight, well-defined and articulate bass  yet at the same time, it maintains the midrange purity.

Sanders also designed the Eros with an active, outboard electronic crossover, instead of the more common passive systems used in other hybrid electrostatic designs.  This active, low-level crossover allows Sanders to use steeper crossover slopes and better damping.  The Eros uses a 24 dB per octave slope at 450 Hz, which allows the ESL panel to take over within an octave, versus the two or more octaves normally required by passive designs with a gentler 12 dB or less per octave slope.

The front panel of the crossover sports switches for the active crossover and the bass amplifier, in addition to a knob for adjusting bass level.  This knob shouldn't be used to adjust the amount of bass, but rather to correctly set low frequency level for a seamless integration between the panel and the woofer.  When setting the low frequency level, InnerSound recommends listening to the midrange and finding the point where it's clean, clear, and detailed.  If the midrange appears muddy and undefined, reduce the bass.  On the other hand, if the midrange sounds thin or distant, the bass level is too low.  Finally, the owner can easily replace the InnerSound-supplied bass amplifier with one of his or her choice.


Obtaining the best sound from the Eros demands careful setup!  Do not expect to plop them down in the room and get good music.  InnerSound provides detailed setup instructions  and you'll be rewarded if they're followed to the letter.  Initially, I placed the speakers in the spot in my listening room where the Wilson WATT/Puppies resided.  The speakers were toed-in so that you could see your reflection in the center of the panel, but something was amiss.  Images were fuzzy and undefined.

A visit from Sanders helped remedy the situation.  He moved the speakers so the outside edge of each speaker was about three inches from the side wall, equidistant from my listening location, and toed-in as before.  He emphasized that the speakers must be equidistant from the listening position.  Now the soundstage opened up with a great sense of detail and three dimensionality.  Gone was the recessed and sucked-out midrange.  So, with careful placement, these speakers can really sing.

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