AOM Logo November 2002

The InnerSound ESL Stereo Power Amplifier Mark II

Martin Appel

Roger Sanders, the affable headman at InnerSound, informed me that he'd come out with a Mark II version of his much-heralded amplifier and asked if I'd be interested in reviewing it. I responded, 'How fast can you get it here?'

Let me back track a little. I did the first review of Sanders' original ESL amplifier in 1999 for my former magazine, Stereotimes. I was so taken with the amp that I purchased it for my reference. Many of my colleagues had similar reactions and even hp of The Absolute Sound, gave it his 'Editor's Choice' four star rating, in Issue 125. High praise, indeed.

Now, Roger Sanders, who's been designing speakers and amplifiers for over 30 years, was not about to sit on his laurels. Never satisfied, he sought ways to improve an already excellent product.

Before I start the review, a few facts about the amplifier. It runs in AB mode and comes in a clear anodized aluminum or black finish. I found that it needed more than my usual 100 hour burn in, closer to 200 hours to achieve its full capabilities. Even now, it keeps improving. It comes with balanced and single ended inputs and a detachable power cord. It retails for USD$2,995.00.

Let me summarize Sanders' concept of amplifier design. Given enough clean power and eliminating the use of protection circuitry, which can also contribute mightily to that 'transistor sound', one avoids clipping and all the musical information comes through in its entirety. To do this, Sanders gives us an amplifier that produces 300 watts/channel at 8 ohms, 600 watts/channel at 4 ohms into conventional loads and 2000 volt-amps/channel into electrostatic loads. This is surely enough power to drive any type of speaker. He does this by using 18 output transistors per/channel, each one capable of 250 watts, totaling an astounding 4,500 watts/channel, and a bandwidth of 10 Mhz.(To actually achieve 4,500 watts, according to Sanders, it would be necessary to have a power supply four times the size of the amp.). What makes this even more amazing is that the amplifier only weighs 42 lbs, runs cool, and I do mean cool, with midget sized heat sinks (Further technical description may be found at

InnerSound's ESL power amp Mk. II

The following was Sanders' response to my query as to what the differences were between the original ESL amplifier and the Mark II:

'Hi Martin. Turning to your questions about the difference between our original and Mk II ESL amp, there are three differences between the original ESL amp and the Mk II version. 1) We replace the original Mitchell speaker binding posts with German, WBT binding posts. The WBT posts have the advantage of being keyed to the chassis so that they will not rotate. Unlike most posts that have round shafts, the WBTs have a square shaft so 1/4" spades don't rotate on them. If a spade rotates when being tightened, its gold plating can become loose, leaving a contact that can oxidize over time. The WBTs also have an anti-rotation section above the spade so that rotating the knob to tighten it will not cause the knob to rotate against the spade. Again, this prevents damage to the spade's gold plating. Finally, the WBTs are simply better looking and have higher quality gold plating than the Mitchell posts -- in other words, they are the finest German quality. 2) The Mk II amp uses global feedback. The original ESL amp had a small amount of local feedback and no global feedback. This was done to satisfy the demands of some audiophiles who believe that global feedback is bad and must be avoided (this is not true, but their perceptions are our reality). However, the lack of global feedback meant that there was no feedback used in the output stage of the amplifier. So to keep distortion low, we had to use more bias than would otherwise be necessary and the amp's damping factor was higher than what it would have been with global feedback. The result was that the amp ran warm and didn't control the bass quite as tightly as one would expect from such a powerful, solid-state amplifier. Despite the audiophile opinions regarding feedback, the fact is that a modest amount of global feedback, which is compensated properly at high frequencies, improves amplifier performance in every way. It does this without any adverse effects of any type. So we decided to add a small amount of global feedback to improve performance and take our chances with audiophile opinions. As expected, this significantly improved bass control and "tightness". It also dropped distortion to immeasurable levels. This allowed us to turn down the bias even further with the result that the Mk II amp now idles completely cold. 3) Because waste heat is now virtually non-existent, there is simply no need to turn amp on and off. Therefore, the front panel power switch was moved to the rear panel. This ''cleans up" the front panel and improves the appearance of the unit. Great listening, Roger Sanders, InnerSound, Inc.'

This reviewer has to report very happily that the design changes have taken an amplifier from excellent performance levels to startling. If I could briefly sum it up in one word it would be 'clarity'. This amp is as neutral and accurate to the source as one could hope. 'Garbage in, garbage out', they say. In this amp's case, never more true. If you want euphony, or what some audiophiles mislabel 'musical', this is not the amp for you. I'm not implying that it sounds hard, dry or etched. But, if you really want to hear what's on the recording, then this amplifier is one to be taken very seriously.

How did the Mark II sound vs. the original?

Starting out at the frequency extremes is always interesting and usually tells an interesting sonic tale. High frequencies, as produced by Louis Nashe's cymbal strikes, Ali Ryerson's flute, or Dizzie Gillespie's trumpet, were very informative. The Mk. II didn't appear to extend further than the original, but there was more information. Notes had more texture and inner detail -- there seemed to be just a little more air in the shimmer of the cymbals, as well as more instrumental resolution.

And the bass? It was clear from the first notes that Roger's efforts had born fruit. All the comments I made about the high frequencies were mirrored by the bass frequencies. There was improvement in impact and palpability in Ray Brown's bass as well as more air around the instrument than before. The detail of fingering was more apparent. The same held true for electronic bass and drums. Kettledrums, at the orchestral rear, had visceral impact and definition. Mallet strikes were clearly defined with accompanying reverberation and natural decay. You clearly heard that kettle 'cookin''. It was more believable. Ah, there goes that clarity thing again. When those double basses play in the opening few minutes of Liszt's Hungarian Rhapsody No.2 on RCA's Living Stereo conducted by Stokowski, their 'woody' quality, resonates through the floor, up through your feet and into your gut.

As far as the middle frequencies, there's nothing that tells the story better than listening to the human voice. Whether listening to Frank Sinatra, Diana Krall, or Shirley Horn, the textures of notes and individual characteristics and nuances of each note was so clearly expressed that it became easily apparent what each singer was trying to convey. The emotions were right out there for you to feel, and these artists were communicating them to you.

All the typical audiophile parameters were handled with aplomb by the original, but slightly bettered by the Mark II. Sound staging was wide and deep with plenty of air. Dynamics, both macro and micro, were also there in abundance. The new amp seemed just a bit more explosive and more delicate at the same time, a remarkable achievement.

Orchestral works, in general, were very convincing, taking me one step closer to the concert hall. Again, if the recording had it, you heard it: chairs squeaking, musicians moving, the musicians breathing, sheet music being turned, etc. What this detail retrieval also meant was that I heard all the instruments in the orchestra making music corporately, not the all-too-familiar blending of musical noise. This was not done at the expense of musical enjoyment but gave one a heightened appreciation of the music and musicianship. Musicality, anyone?

Amplifiers do not exist in a vacuum but are part or a system in the audio chain. I've recently added new pieces into my system that will generate upcoming reviews. You've already heard from me about some of the outstanding cable products by Acoustic Zen. I've been using their latest digital cable, Silver Byte into a new DAC by Birdland Audio, the Odeon-Ag, from their Silver Series. The signal is traveling into a new preamplifier by KORA Electronic Concept, a remote controlled, triode tube, full function unit that will get the full review treatment. All of these new pieces were in place when the Mark II came along. This proved to be an ideal situation since I could listen and evaluate the sound with only changing one variable. It is often difficult, if not impossible, for a reviewer or serious hobbyist to make judgements about pieces of equipment while simultaneously introducing two, three or more variables into a system.

The Audiophilia Star Component AwardOur Editor just gave a rave review to the Burmester 911 Mark III power amplifier in the previous issue, anointing it with an Audiophilia Star Component Award. Part of the problem for reviewers is that one is unlikely to be able to compare all the fine equipment out there head to head and we can only go by our own experiences in making recommendations. The InnerSound ESL Mark II, at a little less than one-sixth the price of the Burmester, would receive a star rating for extraordinary performance and value. For the power hungry out there, Roger also produces a monoblock version of this amp, doubling all power ratings, and price, as well as his statement Kilowatt monoblock at $14,000US per pair. Take that, Burmester and Krell! I would love to be able to insert the Burmester in my system and invite my audiophile buds over for a good shoot out. A chance to dream… One last little tidbit: Roger Sanders has just recently come out with his 'statement' 150 watt/channel tube amplifier. I wonder what this will sound like? The man never sleeps.

Keep Listening.

[It is with great pleasure that we award The Audiophilia Star Component Award to the InnerSound ESL Power Amplifier Mark II. Congratulations! - Ed]

Associated Equipment
Digital Front End -- Sony DVP7000 (modified)/transport, Sunfire Theater Grand Processor II/DAC
Amplification -- InnerSound ESL amplifier
Loudspeakers -- TMS ADIABAT 8.5 speakers (modified) discontinued
Cabling -- Acoustic Zen Cables: Silver Reference interconnects (XLR), Matrix Reference Interconnects (XLR), Krakatoa power cords, MC2= ZEN digital cable
Accessories -- Black Diamond Racing Cones, Vibrapods, Monster HTS 2000 power strip

Associated Recordings
Reference Recordings RR-96CD Rachmaninoff: Symphonic dances Oue/Minnesota Orchestra Telarc CD-83373
Ray Brown: Some of My Best FriendsThe Piano Players
Pablo OJCCD-744-2 Clark Terry/Freddie Hubbard/Dizzy Gillespie/Oscar Peterson: The Alternate Blues RCA Living Stereo: Stokowski Rhapsodies Capitol: Frank Sinatra sings for Only the Lonely
AM Records SP 4280 Cat Stevens: Tea for the Tillerman

The InnerSound ESL Power Amplifier Mark II

InnerSound, Inc. 1700 Hwy 16,Whitesburg, GA 30185
tel. 770 838 1400 fax 770 838 0011
web: e mail:

Source of review sample: Manufacturer loan
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