Innersound Electrostatic Amplifier

Innersound Electrostatic Amplifier
Frank Alles & Martin Appel
1 June 2000

DoubleTakes! is a column that attempts to apply a Siskel & Ebert twist to audio reviewing. In this edition my colleague Martin and I will report on an intriguing new amplifier, designed specifically to drive the difficult capacitive and resistive loads that many electrostatic speakers present.

In keeping with the spirit of the whole S&E bucket of worms, I’ve established a rating system based on zero–5 GooseBumps. The system may be interpreted as follows:

GooseEgg = Omelet-time, an underdeveloped concept!
1 GooseBump = Gosling, largely sophomoric performance
2 GooseBumps = Blue-Collar-Goose, good mean-level performance
3 GooseBumps = White-Collar-Goose, above average, by a neck
4 GooseBumps = Goose hierarchy, among the best in its class
5 GooseBumps = Midas territory, the top of the heap!

silver_black.jpg (15803 bytes)Specifications

Rated power
2000 VA/channel, 20Hz to 20 kHz, both channels driven into an electrostatic speaker of up to 4nF capacity
300 Watts/channel, 20Hz to 20kHz, both channels driven into a resistive 8 ohm load
600 Watts/channel, 20Hz to 20kHz, both channels driven into a resistive 4 ohm load
–3dB at 5Hz through 100kHz with single-ended inputs
DC through 100kHz with balanced inputs
Distortion (THD+Noise)
Less than 0.08% from 10Hz to 30kHz at full output, both channels driven
Slew Rate
50 Volts/microsecond
Input Impedance
50 kohms unbalanced or 100 kohms balanced
Greater than 100dB below rated output
Output Impedance
0.4 ohms from 20Hz to 20kHz
17″W × 5.5″H × 14″D (43cm W × 14cm H. × 36cm D)
41 pounds (18.5 kg)
InnerSound Inc.
1700 Hwy 16
Whitesburg, GA 30185
Phone: (770) 838-1400
Fax: (770) 838-0111
Web site:
Price: $2995. USD
Warranty: 5 years, parts/labor to original purchaser.

“Relaxed and dynamic—what a combination. Listening to the man, Frank Sinatra, on Sinatra Sings For Only The Lonely on Capitol reissued to CD, Frank’s voice had nuance, definition, substance, and that magic presence that was strikingly real.”

Martin begins:

InnerSound is a relatively new company that has already made a significant splash on the audio scene with their hybrid electrostatic speakers, “Eros”, and “Isis”, both of which received very favorable reviews in the audiophile press. I myself, have had the pleasure of reviewing the Isis speakers (visit the archives section at or In my review I alluded to the promise of a new amplifier from this speaker manufacturer, that was in its prototype stage of design by Roger Sanders, chief honcho at InnerSound. Well, several months later I received a call from Roger that the amp was being shipped to my abode for review.

Roger, very cordially, took the time to discuss with me the basic concepts behind the design of his amplifier, which he calls “The Electrostatic Amplifier.” I’m not an electrical engineer and therefore will try to impart to you his ideas in a more simplified way; those of you who require a more technical treatise can refer to his white paper at

Basically, Roger Sanders has designed this amplifier to handle the very demanding and taxing loads that electrostatic speakers place on them (as low as 2 ohms) and consequently work extremely well with the less demanding loads of more conventional magnetic speakers.

If I can sum up his philosophy in a nutshell it would be that adequate, clean power is required to truly convey all the musical information through your system by eliminating the harsh effects of clipping and that this should not come at the cost of a new car! He does this by providing a massive output stage. Says Roger, “Each output transistor is capable of delivering 250 watts-and there are eighteen of these per channel. As a result, it could deliver a staggering 135 amps of current with a combined power rating of 4,500 watts per channel.” I called Roger to clarify this statement. He said that one would need a power supply four times the size of the amp to truly produce this amount of power. The reason for using this number of output transistors was to provide a very large safety factor without the use of protection circuitry, which could have deleterious affects on the sound quality. All of this is done to produce music, effortlessly, clearly and accurately without that “transistor-sound.”

So, you figure this amp must be the size of a steamer trunk weighing about 250 pounds and have enormous gargoyle like heat sinks, right—wrong. Finned rows of heat sinks do flank the sides of the amp, but their profile is modest, looking more as if they belong on a 30-watt amplifier.

On top of all this, he recommends leaving the amp on all the time. “What” you say, “my electric bill will bankrupt me!” Part of the genius of his design is that the amp only uses a few watts at idle and runs cool to the touch.

Physical Description

Visually, the amp is subdued and elegant with an unobtrusive blue indicator LED per channel and the InnerSound logo with the same tasteful blue treatment. This amp comes in both black and silver. The amplifier is laid out beautifully. By that, I mean the left and right gold plated binding posts are large and are adequately spaced far enough apart for easy access by the largest of cable connections. Each pair is placed on the corresponding left and right sides of the rear panel. Centrally located on the rear panel are two sets of inputs, one pair of gold plated RCA’s and one pair of balanced XLR inputs. The power cord IEC connector is also conveniently located in the lower center of the rear panel to provide easy access to any after-market power cord experimentation. The front panel is a substantial 3/8″ aluminum with a slight rounding of the corners, for ease of handling, and a rocker type on/off switch.

System Setup

I placed the amp in my system which consists of the Sony DVP7000 DVD player as transport, the Sunfire Theater Grand Processor II as DAC (review upcoming), the Total Media Systems Adiabat 8.5s, latest mod, all connected with AnalysisPlus Silver Ovals and Harmonic Technology’ s PRO-11 AC power cords. I look forward to trying HT’s latest power cord after reading my associate, Marshall Nack’s fine review. All my equipment was plugged into Monster’s HTS 2000 power strip. Vibrapods and Black Diamond Racing Cones were used for vibration control and Tecknasonic anti-resonance units were used with the speakers.

Listening Impressions

I let the amp burn in for about 100 hours with a variety of CDs running 24 hours a day and was now ready to start the listening process. From the first moment of listening, I knew something very different and very special was happening. The impression of refinement with musical accuracy going hand in hand with all the benefits of power, clarity, and a 3-dimensional soundstage to die for was incredible. This was not your ordinary solid-state amplifier. Large orchestral works were fully illuminated with articulated instrumentation and dynamics. Listening to the RCA Living Stereo CD, Stokowski: Rhapsodies (RCA 09026-61503-2), cut 2, Enesco’s Roumanian Rhapsody No. 1in A, Op.II, one of my favorite recordings, the grin stayed on my face long after the music stopped playing. I’ve never had a sound in my listening room that brought me this close to a live event

Of course, I had to share this with my audiophile friends. I invited two of my associates over for a listening session. After playing just a few cuts the first comments were something to the effect that the sound reminded one of the feeling he gets at a live classical concert, not a hi-fi system! It had all the definition, dynamics and 3-dimensional space that live performances evoke. This came from an audiophile that uses an analogue source and tube amplification—and I was playing digital, on solid-state! Comments came pouring forth like, “relaxed, detailed, accurate, dynamic and spacious.” Relaxed and dynamic—what a combination. Listening to the man, Frank Sinatra, on Sinatra Sings For Only The Lonely on Capitol reissued to CD, Frank’s voice had nuance, definition, substance, and that magic presence that was strikingly real. His orchestral back up, provided by Nelson Riddle, was deep in the soundstage, precisely placed, accurate and lush without any edginess.

This amp is as neutral and revealing as one could want. It will reproduce the source accurately, without addition, coloration, or euphony. Unfortunately, some CDs could use a little help, but will not get it from this amp or any other pieces in my audio chain. For this review, the Sunfire was used only in straight stereo mode without any digital sound or holographic processing that the unit is capable of.

When one use terms like revealing, detailed, accurate, one might get the idea that this adds up to an amplifier that could have a dry analytical sound. Well let me dispose you of that notion right away. When I talk about accuracy, I’m speaking about reproducing the instrumental timbres correctly, with the attack, full body, and decay that the instrument produces. The image specificity of instruments in the 3-dimensional space within the recorded venue (and the recreation of that space) was simply superb.

Comparisons-Berning ZH 270 OTL

The Berning OTL, an excellent amplifier in its own right, made for a very interesting comparison. This amp had been in my system for about eight months and I was very familiar with its virtues (see my review May ’99). We A/B’d the two amps, and while the Berning presented a fine audio picture, the consensus was that the InnerSound amp was further along in portraying a lifelike quality and a more encompassing soundstage. On some CDs, the Berning’s tubes gave a slight softening that one of my associates preferred. Overall, the InnerSound proved to be the preferred choice.

Fine Tuning

In setting up the amp, I placed it on a shelf supported by Vibrapods, on the base of my rack unit, a serious tweak in it’s own right. After several hours of listening, one of my “buds” suggested I could get even more out of my system by trying some Black Diamond Racing Cones directly under the amp. We tried it and the result was clearly worth it. We perceived greater smoothness and clarity coming out of a quieter background.

Just a Few Nits

Since nothing is perfect, I have a few suggestions. While trying to keep the price within reason ,certain accommodations had to be made. The front panel rocker switch just doesn’t have that real solid feel that you want on a truly high-end piece. Since Sanders recommends leaving the amp on all the time, it’s not a major concern. In addition, I would like to see a little heavier gauge metal used on the chassis for superior vibration resistance. I realize that some concessions had to be made to keep costs down, but perhaps a “special edition” might be created addressing these concerns.

Goosebump Rating

My associate at Stereo Times, Frank Alles, is simultaneously and independently doing a review of this product. Before going on record with my GooseBump rating, I just want to add a few more comments. This amp retails for $2,995. There are amps selling for two, three, even ten times this price. I have not heard them all but I have heard a respectable number and I have to say that Roger Sanders has thrown down the gauntlet to those manufacturers. He has designed an exceptional product, one that offers a level of performance that will place it soundly in the upper strata of today’s finest amplifiers.

This amp gets 4¾ GooseBumps from me because I haven’t heard every amp out there, and because of the few minor nits I had to pick. Bottom line—the InnerSound amp has clearly become my new reference. Need I say more? Keep listening!

–Martin Appel

Frank chimes in:

“I preferred the expansive soundstaging and virtually unbridled dynamics that the InnerSound delivers without breaking a sweat.”

My colleague, Martin, has heaped much praise upon the InnerSound Electrostatic Amplifier and I have to concur with many of his observations. However, I disagree with him with respect to his comment that the on/off switch felt less than solid—it actually felt more solid than most other like-switches in my experience. Also, with regard to the gauge of the metalwork used in the chassis, I believe that Roger Sanders has struck a very appropriate balance, neither overdoing nor skimping on it. Besides which, there is no reliable way to predict sonic performance based on the gauge of the metal in the chassis. The circuit topology and the quality of the component parts are far more reliable indicators of audio quality and Sanders has paid close attention to optimizing those aspects of the amp’s design.

It should be noted that this amp was perhaps the quietest, best-behaved amp that I’ve ever used in my system, both electrically and mechanically. After about 6 months of use, there are no operational anomalies to report.

To put the InnerSound amp through its paces I used it in two different systems. My source components were the same for either system. I used the Townshend Audio Mk III Rock turntable with a modified Rega RB-300 tonearm and a Transfiguration low-output MC cartridge, which fed an AHT/P DM phono stage. My digital source was the Parasound C/BD-2000 transport coupled to a Parasound D/AC-2000 processor via a Harmonic Technology digital cable. The sources were routed through an AHT tube line stage. In this system, the InnerSound amp was substituted for Monarchy Audio’s SM-70s, which were used as monoblocks to feed the electrostatic panels of my InnerSound Eros speakers. The InnerSound bass amp drove the woofer sections. Additionally a Paradigm Reference Servo-15 subwoofer was employed. So in this bi-amped system (tri-amped if you count the subwoofer amp) the InnerSound Electrostatic amp was used only to drive the ESL panels in the midrange and treble.

In my alternate reference system, the Parasound digital-duo fed the AHT line-stage, and a Sonogy Black Knight amplifier was relieved of duty by the InnerSound, which drove a pair of Carver AL-III ribbon hybrid speakers full range.

It should be noted that this amp was perhaps the quietest, best-behaved amp that I’ve ever used, both electrically and mechanically. After about 4 months of use, there are no operational anomalies to report.


When I replaced the sweet-sounding Monarchy SM-70s with the InnerSound to drive the Eros’ ESL panels, I wasn’t quite sure what to expect. As it happened, I was pleasantly surprised to find that like the SM-70s, the InnerSound amp was pleasant sounding, with no rough irritating edges. This beast was smooth and it became apparent early on that the high-frequency response was extended, airy, and delicate. In fact, the treble appeared more extended than it was with the Monarchy amps.

At least partly because of the treble extension, the soundstage appeared to grow a bit wider, and deeper as well. The midrange presentation was slightly more reticent than that of the Monarchy amps. It was more akin to the perspective of the Sonogy Black Knight, which also lent to the illusion of greater soundstage depth. Though the Sonogy amp and the InnerSound had a similar sonic signature (in the midrange) the InnerSound amp, while sounding every bit as extended as the Sonogy, seemed more relaxed in its presentation.

Yet despite the InnerSound’s smooth, relaxed character, it was strikingly incisive in its lightning delivery of fast transients. Hand claps, snappy percussive strikes and plucked strings were as sharp and clear as one could hope for, and could even be startling on certain effects like gun shots and popping corks.

Ultimately, I believe that the InnerSound amp surpassed the Sonogy in terms of providing a slightly more spacious soundstage and a macro-dynamic envelope that showed no signs of strain or compression even when played at VERY loud levels. I was not really aware of compression taking place in my other amplifiers until I had the InnerSound to use for comparison. Suddenly, certain familiar passages became more dramatic and impactful with the extra juice the InnerSound could supply. Try a raucous cut like Rusted Root’s “Drum Trip” from When I Woke (Mercury 314522713-2) with its manic percussion and tribal ambiance—I felt as if I was about to be sacrificed at a sacred ritual—which I believe was the artist’s intent. Big amplifier make powerful magic!

On the other hand, going to the Carver AL-IIIs and running the InnerSound full-range proved to be an interesting contrast, and provided some additional insight. On the Carver speakers, which are admittedly much less efficient than the Eros, the amp’s heat sinks became warm to the touch as it clearly had to work harder to sustain the same dB levels (in addition to being run full-range). Still it ran cooler than either the Sonogy or the Monarchy amps, and it maintained its generous soundstaging and dynamic capabilities, showing no signs of strain or compression.

If I could single out any area where the InnerSound gave up ground, I’d have to say it was the area of bass control. While the extension of the bass and amount of bass energy seemed excellent, bass lines seemed slightly soft, i.e., not quite as taut, when rendered via the InnerSound. In this regard, it was outmaneuvered by the Sonogy and even by the much less powerful Monarchy SM-70; both these amps imposed a tighter grip on both electric and acoustic string bass, and were easier to follow. This can be witnessed on any number of tunes and one that pops into my head is Cake’s “Sheep Go To Heaven,” from Prolonging the Magic (Capricorn 314 538 092-2). I find myself forced to grin and sing along to this song where the lead singer is audacious enough to repeatedly entreat his audience to “Go to Hell!” As I said, there are many cuts with strong runs on electric or acoustic bass, so if you don’t like my selection you are free to choose your own. I believe you can understand my point. When I informed Roger Sanders of my impressions of the amp’s bass, he offered the following comment:

“Looking at this from an engineering standpoint, it would seem that our amp should have the best possible bass performance. After all, bass is generally the best from amps with high damping factors and gobs of power. Damping factor is determined by dividing the load impedance by the output impedance of the amp. The ESL amp has probably the lowest output impedance of any amp in existence (0.04 ohm), so has a very high damping factor as well. And of course, it has tremendous power. So our amp should sound as good or better than the others.”

In fairness to Sanders, it should be noted that my assessment of the InnerSound’s bass performance may not be conclusive. It is an area that would require further testing and scrutiny before any definitive statements could be made. The fact that my colleague Martin had nothing negative to note regarding the amp’s bass supports Sanders’ position. And it is always possible, for whatever reason, that the InnerSound amp performed somewhat atypically with the Carver AL-IIIs. As always, I recommend that potential buyers try to arrange for an in-home demonstration before committing to any major purchase.


Aside from the slightly soft-sounding bass I referred to with the Carvers, I have almost nothing to complain about regarding the amp’s performance. At times, and on certain cuts, I preferred the slightly sweeter and more forward midrange presentation of the Monarchy amps. However with the majority of my recordings, and especially in a bi-amped system where the bass performance of the mid/top amp is moot, I preferred the expansive soundstaging and virtually unbridled dynamics that the InnerSound delivers without breaking a sweat. Also, with classical music, I found the amp’s multi-layered presentation of a symphony orchestra to be right on target.

As for my GooseBump rating, I believe I have to go along with Martin on his award of 4¾ GooseBumps, but for different reasons. I have heard much of the competition and I feel the high rating is entirely justified. Roger Sanders has done a very commendable job of designing an amplifier that excels in a particular application and he deserves high praise for his achievement. Owners of many dynamic speakers are sure to love this amplifier, but owners of electrostatic speakers may find it simply irresistible. I did!

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