Sanders Sound Systems ESL Amplifier
Form the heart of a fine reference audio system.
Review By Ron Nagle
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  Let me premise what follows by saying I am impressed by what I have learned so far. These ESL amplifiers spring from the mind of an audiophile and not just a numbers-crunching paradigm driven techno type. Having said that Mr. Sanders is an interesting story unto himself. Back in 1974, he authored an article in Speaker Builder magazine about electrostatic speakers. Then in 1976, this was followed with an article on amplifiers designed to drive them. In 1980, he wrote about the construction of an electrostatic speaker with a curved diaphragm and he was the first to develop this curved profile. You might remember that a similar Curvilinear electrostatic radiating surface is the hallmark of all of the loudspeakers made by the MartinLogan Company. He is best known for his contribution to the state of the art in his book, "The Electrostatic Loudspeakers Design Cookbook" published by The Audio Amateur in 1993.

In 1997 Roger Sanders joined Raj Varma to form the Innersound Company. Their products were a hybrid loudspeaker with a cone woofer and a flat electrostatic panel and the Innersound ESL amplifier designed to drive their loudspeakers. Both products received a lot of good press in the audiophile articles written at that time. Then in 2003, Innersound moved from Georgia to Boulder, Colorado. The year 2004 Roger Sanders left and in 2007 he formed Sanders Sound Systems now located in Conifer, CO. It is important to note that Mr. Sanders has managed to not only continue to make and improve his ESL amplifiers but after a gap of three years, he has resumed production of his innovative electrostatic loudspeakers. The now discontinued Innersound loudspeakers and the new Sanders sound systems loudspeakers abandoned the curved electrostatic panels he developed explaining that the curved surface introduces a new set of problems not inherent with flat electrostatic panels.

Facts Of Life
It just may be that you need a very special amplifier to get the best performance from an electrostatic loudspeaker. Electrostatic loudspeakers present an amplifier with a very varied and difficult load more complex than conventional cone driven loudspeakers.

The designer of a dedicated ESL (Electrostatic Loudspeaker) amplifier has to consider many factors, not only the crazy impedance shifts but also the fact that as frequency increases electrostatic speakers morph into something that looks like a giant capacitor. Basic electronics textbooks will tell you that at high frequencies capacitive current leads voltage by 90 degrees. To make an obvious point such an amplifier must be able to supply enough voltage and current to more than match any demands made by frequency changes and dynamic swings in music. Most importantly, it is necessary to supply this power in an evenhanded manner (low distortion) at all frequencies. All of these factors mandate that the power source be absolutely rock solid stable under any conditions. Normally I would mention only a few salient amplifier stats and leave the rest to the specifications I list at the end of an article. However, some of the specifications that describe the Sanders ESL Amplifier are damn amazing. I wouldn't want these languishing as an unread end of the article after thought. All the same I would like you to understand like must audiophiles I am really not into measurements because experience tells us they usually do not predict how an amplifier will sound, having said that good specs are generally not a bad thing.

Amplifier Nomenclature
It is important to understand that the Sanders Sound Systems amplifier under review is a different and much improved version of the earlier Innersound design. The chassis is now aluminum and is of a heavier gauge construction. The balanced XLR connectors are Neutrik, and the RCA connectors are Cardas. The more powerful-shielded toroidal power transformer is massive and fills about 75 percent of the chassis interior. This larger transformer has increased the power rating by 60 watts it is now rated at 360 watts per channel into 8 Ohms. A redesigned power supply circuit board now eliminates all wire except for a 2-inch piece connecting it to the speaker binding posts. The low-level input circuitry is redesigned and uses higher-grade transistors. More powerful and more linear bipolar output transistors are now used; this in combination with improved circuitry lowers harmonic distortion from 0.03% in the Innersound version to a vanishing 0.008% for the revised Sanders amplifier.

In this newer ESL amplifier there are 18 bipolar hi-power transistors in each channel connected in parallel they have a combined power rating of 4500 watts at 10 MHz. The peak current rating is 135 Amperes. The term damping factor refers to a ratio of amplifier output impedance to loudspeaker load impedance, this computation gives you some idea of the control exerted at bass frequencies. These select hi-capacity power transistors present a total parallel output resistance of only 0.01 Ohm. This translates into a very high damping factor of 800 into an 8-Ohm load. The power rating listed is 360 watts per channel into 8 Ohms and 600 watts per channel into 4 Ohms and the amplifier is stable down to 1 Ohm. The rated power is 1700 VA per channel into capacitive loads with both channels driven from 20Hz to 20kHz. The distortion is listed as 0.03% from 20Hz > 20kHz. Noise less than 100dB below rated output. Last but not least the specifications are for a linear class A/B amplifier. The definition of a linear amplifier is: an amplifier where the output signal is a replica of the input signal and the output is directly proportional to the input. There are many more specifications I could list but you get the idea.

There are two ergonomic difficulties that I encountered. The owner's manual suggests that you could leave the amplifier powered on and this would eliminate stress by not cycling the amplifiers power on and off. Naturally, this makes a lot of common sense but I have a tube preamplifier and it needs to be powered down when not in use. In addition, it is not a very good idea to turn on my tube preamplifier while it is plugged into this power amplifier. There should be a way to mute the amplifier output and some indication that it is muted. My last concern is that the power switch is located on the back panel and if you have the amplifier stacked on a shelf in an equipment rack (like I have) than it would be hard to reach.

Raison D'etre
Hooked on air, the kind that surrounds me in sound. It is the something that makes music appear on a gossamer fabric of film that energizes the air around. I have owned Quad ESL 63 electrostatic speakers for many years. I love the way they make music that flows effortlessly and naturally in continuous wavelets free from mechanical artifact. While having never stopped trying to squeeze the very last drop of music out of my Quads, over the years I replaced all the panels and upgraded the grills than installed a mod squad resistor capacitor network plus mounted them on dedicated Gradient woofers. So you may understand my abiding interest in Roger Sanders ESL amplifier made specifically for electrostatic loudspeakers. The ESL amplifier arrived just as I was in the process of evaluating a pair of MartinLogan's brand new Vista loudspeakers. This $3995 electrostatic hybrid is the only loudspeaker in the extensive MartinLogan line that does not have a built in 'Class D' woofer amplifier. It stands 57 inches high 40 inches of that is a narrow line source electrostatic panel and below in a ported enclosure is an 8inch metal cone woofer. Common sense will tell you if you want to use these loudspeakers to evaluate audiophile electronics then you best had better not use their built in amplifier to complicate the musical mix. At this point I would love to go off on a tangent and tell you what a mess most switching amplifiers make of high frequencies but that is best left for another time.

Amplifier Trials
Using my Marantz DV8400 Universal disc player feeding an Audio Research SP 9 MK3 preamplifier signal from this then routed to my power amplifiers via three meters of Wire World Eclipse 2 unbalanced cables, this was the unchanging reference front end. My in house loudspeakers are called, Strata Mini by AV123.com. These 4 way loudspeakers have a built in woofer amplifier so even though I listened to them they are not part of this report.

Initially I powered the ML Vista loudspeakers with a PrimaLuna Prologue 2 this is a 40wpc KT88 integrated amplifier. The bass was less defined and not as agile when compared to the mid range and treble speed but overall it was not at all hard to listen to. Next up was my feature filled OutLaw RR2150 receiver. This 100-wpc receiver has bass and treble controls and a separate EQ control to select bass turnover frequencies. With this EQ control, I was able to limit the amount of bass at a frequency of 55 Hz confirming that without this bass cut the bass sound was, as the British would say a bit "plumy". Next in line was my much-modified 250 wpc Hafler 500 amplifier. The main advantage here was that it has the ability to pump a lot more power into the Vista loudspeakers.

At this point what I heard was a lot closer to the musical truth. The Hafler seemed to be able to supply the power necessary to inject life into the performance by way of dynamic contrasts. The Sanders SEL Amplifier arrived at this time and in a manner of speaking it placed all the other amplifiers into a different musical context. What I had been doing was not just switching amplifiers but sampling music through a variety of tinted lenses. My analogy is that in retrospect some of the amplifier characteristics I noted were subtle colorations not inherent to the music. None of these was unpleasant but none of these was quite accurate. In retrospect, there was a suggestion from the tube amplifier of a subdued shade of mauve. The Outlaw amplifier tended more toward a shade of pale blue and the Mosfet Hafler was a pleasant slightly fuzzy light beige. Forgive my imaginative rainbow of a description but that is the best way to convey my subjective impressions.

Aural Aspects
What do you hear when you have all the clean music power you need and limitations have been pushed to a place that is practically unapproachable. Metaphorically speaking, we can experience this (borrowing a worn phrase) as a clearer window on the performance. With the Sanders ESL amplifier driving the MartinLogan Vista loudspeakers the cone bass drivers were now held in a tight grip and became nearly as articulate as the electrostatic panels. (Note: 90 percent of the audio power produced is used to drive the loudspeaker at frequencies below 200Hz) At a manufacturers specified frequency of 20 kHz the Vista loudspeakers become more capacitive and the resistive load drops down too a very difficult 1.3 Ohms.

Right now I can't think of a better recording to illustrate the articulation and speed of the ESL amplifier than a wonderful version of "You Were Always On My Mind" by Willy Nelson. I found it on a Sony (CD A21562) it is from an album titled Willie Nelson Yours Always. This is a wonderfully engineered and detailed studio mix with transient speed and studio ambience that highlights the ESL amplifiers resolving ability. At two minutes and forty seconds into this cut there is a brief faint sound of a person in the distant background whistling along with the melody. Meanwhile Willy's vocal resides front and center in all his nasal glory amidst the glisten of steel guitar strings. Backup vocals are etched and clearly delineated and recede in layers from the immediate left center stage. In a strange and unexpected way the metallic reverberation of the guitar strings complements the country twang of Willy's voice.

If you want to test low frequency tonal integration than I can think of no better way than to play "Adagio d' Albinoni" as performed by Gary Karr and Harmon Lewis. This was originally on a Japanese Firebird label but it may be available via the Cisco Music catalog (GCD8003). Recorded in a cavernous Japanese cathedral it is a duet of a large sonorous pipe organ and Karr's centuries old Amati bass fiddle. It is not enough to just rattle low bass it is another thing to get all the wooden warmth and rich sad sounding harmonic overtones of this bass instrument correct.

The reverberations and echo of the low register organ pipes holds the woofer for a moment on a deep sustained rumble. The resinous bass bowing sighs and breathes a mournful moan that tugs at your heart. The sound is absolutely organic and for a time you forget that it's not flesh and blood calling out to you. Every contributing element of my system and music selections was clear to hear. If I had to pick my best CD it would more than likely be The Look Of Love by "Diana Krall"; it is the Verve [589-597-2] SACD release. She appears on this sound stage warm and made of flesh and blood with a subtle pause and a moist intake of air she seems as real as life. Moreover, if I wasn't happily married and I could just get my; but that's another story. I would like to mention one last recording even though it's not strictly a reference but rather a fun recording I enjoy very much. It is a gathering of famous artists brought together to celebrate the Queen of England on her Golden Jubilee. Recorded live out doors in front of Buckingham Palace it is ever so clean spacious and dynamic with ESL power applied to the Martin Logan Vistas. You can find it on Virgin [7243 8 12833 25] and it is called Party At The Palace. Did you ever hope to hear Brian Wilson performing "GodOnly Knows" backed by The Royal Academy Of Music Symphony Orchestra? Of course there is much more to commend it, like Rod Stewart, Elton John, Paul McCartney and Eric Clapton to name only some of the talent on this disc, it is wonderful music that is wonder filled!

Coda
What can I say, practically all the clean power one could ever want, capable of driving any audiophile loudspeaker, surely the last amplifier you will ever buy? Understand that I believe the ML Vista electrostatic hybrid loudspeaker serves as an excellent test for an amplifiers ability to drive any type of loudspeaker. Most certainly, this amplifier represents the current here and now edge of technology applied with precision without becoming hard or sterile sounding. Having said that I have a confession to make, there are some euphonic tube colorations that I really like. One side of my reviewer's brain craves exactitude while the other half soaks in a soft bed of rose petals. What to do?

Logic tells me accuracy is undeniably essential to what I need to know. The Sanders ESL amplifier can form the heart of a fine reference audio system. It could serve as a tool to evaluate every type of loudspeaker or even the performance of each separate driver. Our you might use it's purity to hear deeper into a recording and dissect the way it was recorded and mixed down. However, there is a way to have your cake and eat it too. Using its neutrality there are many options that were not available before. You could go upstream and swap in softer sounding interconnects or feed it from a tubed preamplifier, like my Audio research SP9 MK3. Oh! Lest I forget, Mr. Sanders makes a dual mono version of this amplifier and a (what the hell could he be thinking) 1000 watts per channel stereo amplifier! With the permission of the Editor, the boys down at the IRS and my spousal partner I would like to provide a new home for this much deserving amplifier. Semper Hi-Fi.

Specifications
Type: Stereo power amplifier
Frequency Response: DC through 100 kHz
Output Power: 360 watts RMS/channel @ 8 Ohm resistive load
Slew Rate 500 Volts per microsecond
Impedance: 50 kOhms @ balanced and unbalanced inputs
Gain: 26 dB
Damping Factor: greater than 600 into 8 Ohm load.
THD: Less than 0.003% @ 20Hz to 20kHz
IMD: Less than 0.002% @ 20Hz to 20kHz
Dimensions: 17 x 5.5 x 16 (WxHxD in inches)
Weight is 52 pounds.
Serial Number: 00112
Price: $3995

Company Information
Sanders Sound Systems
12054 Deer Trail Road
Conifer, CO 80433 

Voice: (303) 838-8130
E-mail: support@sanderssoundsystems.com
Website: www.sanderssoundsystems.com