ULTRA AUDIO -- Archived Article

October 1, 2003

Innersound ESL 800 Mono Amplifiers

Innersound burst onto the audiophile scene in 1996 with the introduction of the Eros, a hybrid electrostatic loudspeaker. Designed by Roger Sanders, the Eros mated a flat-paneled electrostatic driver to a 10" dynamic woofer housed in a transmission-line enclosure. Though credited with inventing the curved electrostatic panel, Sanders had forsaken his invention in favor of flat panels, with their narrower dispersion patterns and resultant reductions in room interactions and distortions.

The Eros caught the attention of many audiophiles and reviewers, who were captivated by its extraordinary soundstaging capabilities, its warm yet natural tonal balance, its holographic imaging, and its nearly seamless integration of electrostatic and dynamic drivers. The Eros was followed by the Isis, another hybrid design in a smaller package. Marketed as the Eros’s junior sibling, the Isis maintained many of the Eros’s virtues in a package that worked well in rooms of more modest dimensions, and at a price that promised to make the Innersound technology available to a wider audience of audiophiles and music lovers.

There are four amplifiers in the Innersound lineup, and the company has recently introduced a solid-state phono preamp to go with its balanced line stage. They also offer interconnects and speaker cables designed to optimize their other products.

The product reviewed here is the ESL 800 monoblock amplifier, priced at $7800 per pair. It falls between the ESL 300 stereo amplifier and the RS 1000 reference monoblock in Innersound’s lineup of solid-state amps.


If your electrostatic speakers require a heavy dose of current and voltage, the ESL 800 monoblock, capable of delivering a peak current of 135 amperes into an electrostatic load, is specified to meet your needs. The ESL 800 is rated to deliver 800W into an 8-ohm load and 1200W into 4 ohms.

All told, each amp contains a staggering 36 output transistors, all bipolar. Run at full output, the transistors would be capable of a combined 4500W, but unlike the rest of us, these bipolars are not required to work that hard. The Innersound strategy is to get a lot of transistors in the output stage, each doing only a small percentage of what it’s actually capable of, all working together in a way that does not overtax any one of them, thereby increasing the stability and life of all. Roger Sanders feels that, with a rated 135 amperes of current and 800Wpc available, there is little chance of an ESL 800’s current or voltage clipping into even the most demanding loads, and no need for complex protection circuits.

The output impedance is negligible, the distortion vanishingly small, Innersound claims. The circuit employs neither global feedback nor feedback in the output stage. The amp is biased in class-A/B, runs cool to the touch, and, according to Innersound, is extremely efficient -- no outrageous electric bills here. Then again, the ESL 800 won’t double as an auxiliary heating system on cold winter nights -- something on which owners of electrostatics, as driven by at least some output-transformerless (OTL) amps, have come to rely.

Each monoblock is of modest dimensions, weighing a mere 45 pounds apiece. While it’s not about to challenge the exterior design and fit’n’finish of a Jeff Rowland Design Group amp, the ESL 800’s basic chassis, silver faceplate, nicely engraved company logo, and blue Power On indicator have a somewhat understated, unassuming look that’s likely to work well with any décor -- largely by not calling undue attention to itself.

Each ESL 800 provides both balanced and single-ended inputs, and is fitted with two pairs of WBT speaker connectors. The latter are too close together, and weren’t easy to use with the spade connectors of my speaker cables. That’s my only quibble about the ESL 800’s style and configuration.

Why so much power?

Why so much available peak current and voltage? To answer that question, we have to take a closer look at the practical problems posed by the typical electrostatic speaker. The first problem is how to extract bass with extension, weight, dynamics, and slam -- the same problems many have with planar designs. But to the extent that the criticism applies to electrostatics and planars, it applies as well to most modern single-driver horn designs. In each case, the challenge is to find a way to reproduce the lower octaves in a way that’s consistent with the presentation of the rest of the speaker, and in a package that will fit, physically and aesthetically, in the user’s home.

If bass-reproduction problems are not unique to electrostatics, other problems are more nearly so. Whereas dynamic speakers present basically resistive loads, electrostatic speakers are mostly reactive. As the signal changes polarity, an electrostatic sends the signal back to the amplifier, inducing instability. In contrast to many dynamic loudspeakers that present essentially flat impedance loads to an amplifier, electrostatics act like capacitors in that their impedance varies with the frequency of the musical signal. The lower frequencies present very high impedance to an amplifier, while the higher frequencies present an impedance that can dip below 2 ohms. (Think of the original MartinLogan CLS. The problem, however, is not unique to electrostatics, as any owner of the original Apogee Scintilla knows.) As impedance drops, an amplifier must provide increasing amounts of current. An amp that’s not up to the task will leave an electrostatic sounding rolled off in the highs and, consequently, darker than life.


While the Innersound ESL 800 is designed specifically to realize the full potential of electrostatic speakers, Roger Sanders believes that they’ll work just as well with planar designs, as well as with conventional cone-and-dome speakers. As luck would have it, I had Sanders’ own Isis Mk I, Magnepan’s MG3.6/R, and Green Mountain Audio’s 1.5i speakers on hand at various times during the review process, so I was in a position to put Sanders’ claims to the test.

I installed the Innersound amps in my current reference system. The analog front end is a Well Tempered Classic with an improved Well Tempered Arm tracking the splendid and unfailingly musical Grado Reference II cartridge. Digital chores are handled by a Sony DVP-9000ES SACD player with Modwright Level 3 modifications. During the review process I used two preamps: a Counterpoint SA 5.0 (a 5.1 with a muting switch instead of the auto-muting 5.1), and, briefly, a JJ Tesla 243. Both the Counterpoint and the JJ are full-function tube preamps, the Counterpoint being both rectified and regulated with tubes. The Blue Circle Music Ring 1200 provided power conditioning; all cable and AC cord responsibilities were impeccably handled by Stealth.

The most important and least discussed audio component is the room, and I have a big one: 30’ x 18’ x 9’. Not by design, the room has always flattered good-quality audio systems of all varieties. There are big windows at either end, but also lots of bookcases, art on the walls, soft couches, and a baby grand piano. The room can be configured to allow me to listen to music when facing the long or the short wall. For the purposes of this review, I listened with the system arranged along the shorter wall. Speakers were about 9’ apart, with one ESL 800 behind each. Whereas the MG3.6/R calls for farfield listening, the other two speakers work best in the more traditional equilateral-triangle configuration. I was able to accommodate both preferred seating arrangements.


The ESL 800 is designed for electrostatic speakers, so the first question is: How did the ESL 800 fare with the Innersound Isis? The short answer: extremely well. Because the Isis is a passive design, the ESL 800 was required to handle both the transmission-line bass as well as the electrostatic panels. The Isis may be the baby in the Innersound family, but it has a ferocious thirst for power. With the ESL 800s in place, the Isis transmission line was fully under control, producing authoritative and extremely dynamic bass into the mid-30Hz region in my room.

David Holland’s bass on his fabulous Conference of the Birds CD [ECM 1007] was full, well-defined, and focused. This was the rule. The bottom end of every disc I played, from Maceo Parker’s Funk Overload [What Are 600 32] to Wilco’s Yankee Foxtrot Hotel [Nonesuch 79669-2], from the Art Pepper Quartet’s Modern Art [Blue Note CDP 7 46868 2] to the haunting performance of Aulis Sallinen’s Symphony No. 4 and Cello Concerto (Okko Kamu, Helsinki Philharmonic [Finlandia FACD 346]), was tightly yet fully rendered. The music had weight but was never heavy or leaden. The ESL 800s provided a foundation that was sufficient to carry the music, yet agile and fast enough to keep it moving. Nothing sluggish here.

In my experience, whereas all electrostatics possess a holographic midrange, some project images lacking adequate body and dynamic drive. Reviewers have tended to focus on the dynamics of speakers as a property that is important primarily for the adequacy of its reproduction of the lower registers, using such concepts as "bass slam" to describe what they have in mind. The truth is that the midrange must be dynamic and full-bodied if it is to convince the listener that the voice is coming from someone of flesh and blood as well as bones. Dynamics through the midrange are at least as important to a speaker’s ability to reproduce music faithfully and credibly as is "bass slam" -- more so, in my opinion, as live music rarely has the kind of bass slam that listeners crave from their stereo systems. It was in rendering a palpable, dynamic midrange that the Innersound speaker-amp combination shone.

I don’t think I could bring myself to listen to another audiophile recording of a female jazz singer whose head is rendered larger than planet Earth itself, and whose mouth seems large enough to swallow a Volkswagen Beetle. But I could listen to Norah Jones, Karrin Allyson, Isaac Hayes, and Al Green all day through the Innersound combo. The midrange was rendered with a see-through quality all right, but the images created were entirely three-dimensional. Heads were most definitely attached to throats, chests, and abdomens.

The Innersound combination projected the midrange in a way that charged even my large room. The sound was alive and vibrant, while remaining completely true to the traditional electrostatic "voice": subtle, nuanced, and seductive.

There was no hardness in the upper mids and no etch or edge in the highs. The sound was extended and lovely. Massed violins were sweet and soothing. Tasmin Little’s solo violin on George Lloyd’s quietly powerful Lament, Air, and Dance Sonata [Albany TROY029-2] was rendered with a natural beauty and sensuous tone gripping enough to make one’s heart stop. Jan Garbarek’s soprano saxophone on his rendition of Jim Pepper’s "Witchi-Tai-To," on the CD of the same name [ECM 1041], was atmospheric and sharp, almost piercing in its beauty. The Innersound combination was able to draw me into the music as much through its power and dynamics as through its ability to realize the more familiar electrostatic virtues.

The Innersound amp took control of its electrostatic stablemate while never overwhelming it. The amp gave the impression of being in complete control: an unfazed and unassuming presence that was up to the task but without any need to put a stamp on the proceedings. If Sanders’ goal was to create an amplifier capable of realizing the potential of an electrostatic loudspeaker, then the ESL 800 is a success. The sound was unfailingly musical, present, and coherent within the confines of a hybrid design, with musical details aplenty, and no thinness, bleached harmonics, or transistor artifacts. In short, if the Innersound combination is any indication, Sanders is right: The limitations often associated with playing music recordings through electrostatic speakers are artifacts of system matching and not inherent limitations of electrostatics.

My reviewing time with the Isis and ESL 800 was drawing to a close, so I brought home a pair of Magneplanar MG3.6/R speakers (full review in the works). I had listened to the Maggies several times, but never for an extended period. Often, they’d been mated with tube amps in the 100Wpc range. While the Maggies had always been unfailingly musical, I’d never been blown away by them. They struck me as a bit unfocused and veiled -- opaque, even -- with a slight smearing between the upper bass and lower midrange. Listening to the MG3.6/Rs, it wasn’t hard to understand why music lovers were drawn to them, but I didn’t get the hype.

Not until I biwired the Maggies and drove them with the ESL 800s. The Innersound amp may well be designed for electrostatics, but it was made for the MG3.6/R. Veils were lifted, the midbass snapped into focus, and the lower midrange became full and clear. The bass was remarkably extended and weighty, and the midrange . . . oh, that midrange. And the ribbon tweeter sounded like greased lightning, it was so airy. If anything, the Innersound ESL 800 was a better match for the MG3.6/R than for the Isis.

All the virtues the ESL 800 displayed with the Isis were present in spades in the Innersound-Magneplanar combination. If the former pairing warranted a long-term relationship, the latter called for holy matrimony. If the Innersound-Innersound combination was dynamic -- and it was -- it was nothing compared to the dynamics the Innersound-Magneplanars were capable of. I couldn’t resist playing Yello’s (Dieter Meier and Boris Blank) "S.A.X.," from their super-charged, high-energy Zebra [4th and Broadway 4065], at neighborhood-emptying volumes. Not just the room, but the house rocked. It was fun. I loved it, even if my wife and I were charged with disturbing the peace and, worse, reducing property values.

I was able to audition only briefly the Innersound amplifier with my dynamic speaker reference, the Green Mountain Audio Continuum 1.5i. I had run the GMA with PS Audio HCA-2, Mark Levinson No.29, and M-series Edge amps before settling on EL34-based monoblocks built by my friend Mark Pearson. The Innersound bested all the solid-state occupants of that link in the chain, but reminded me why I preferred the GMA with tubes. On the one hand, the Innersound produced grain-free highs and a clear window on the sound, while creating a bottom end that lifted and carried the music. On the other hand, leading edges were a bit less incisive, and the musical detail was not as fully realized with the Innersound as with the tube amp.

While the GMA did not trip up the Innersound, it showed the big monoblock to be a bit less than the last word in resolving inner detail and microdynamic shadings. Don’t get me wrong -- with the extremely revealing GMA, the Innersound continued to present musical detail while shunning hi-fi artifacts. Still, the powerful Innersound didn’t unravel every detail, layer by layer. I didn’t see as deeply into the music with the Innersound-GMA combination as I did with the tube amp-GMA pairing. The Innersound ESL 800 monoblock did not possess that last bit of resolution and harmonic completeness that characterize truly great amplifiers -- of which there are very few, in my experience.

Unlike many muscle amps with similar power ratings, the ESL 800 took charge without issuing a formal announcement that it was in charge, while confidently responding to everything the music asked of it. Much like the outer design of the chassis that houses it, the amp did nothing to call attention to itself. It excelled by shining the spotlight elsewhere in the system, and always in the service of music. To some, this might suggest that the amp lacked character or personality, but to my way of thinking, this was its character. The Innersound ESL 800 responded appropriately to every situation in which it found itself. It was never caught short, added nothing, and never drew undue attention to itself. It may well be a difficult load’s best friend. In my book, that’s character.


If you’re about to investigate the land of planar and electrostatic speakers, you’ll need an amp that won’t be tripped up by the loads and phase angles such speakers typically present; an amp that will let you realize the potential inherent in each speaker design; and, most of all, an amp that will let you enjoy the music in the distinctive ways in which planars and electrostatics present it. You might purchase different amps for each of these tasks, or you can save yourself a lot of trouble and money by purchasing a pair of Innersound ESL 800 monoblock amplifiers. That’s what I did. The Innersound ESL 800 is terrific, and a spectacular value at its price.

Jules Coleman

Innersound ESL 800 Mono Amplifiers
Price: $7800 per pair.
Warranty: Five years parts and labor.

2400 Central Ave., Suite L
Boulder, CO 80301
Phone: (720) 210-1925
Fax: (303) 413-1088

E-mail: info@innersound.net
Website: www.innersound.net


PART OF THE SOUNDSTAGE NETWORK -- www.soundstagenetwork.com
All contents copyright Schneider Publishing Inc., all rights reserved.
Any reproduction, without permission, is prohibited.

Ultra Audio is part of the SoundStage! Network.
A world of websites and publications for audio, video, music, and movie enthusiasts.