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Stax SR-009 Electrostatic Earspeaker (Playback 54)

Stax SR-009 Electrostatic Earspeaker (Playback 54)

For many decades the name Stax has been synonymous with top quality electrostatic headphones, and many discerning listeners have regarded their top models as among the best, if not the best, headphones presently available. Last year, however, Stax changed the game by releasing a new ultra-rare and ultra-expensive flagship model—the SR-009 electrostatic earspeaker ($5250). To appreciate what a quantum leap into the price/performance stratosphere this represents, consider the fact that Stax’s previous flagship model, the SR-007 Mk2, sells for $2650 (the SR-007 Mk2 will continue in the product line and be sold alongside the SR-009).

The SR-009 is, to the best of my knowledge, the most costly headphone on today’s market, and there is frankly only one thing that can possibly justify its lofty price: performance, performance, and more performance. Does the SR-009 deliver the sonic goods? That’s the question this review will address, but the unequivocal one-word answer is, yes! If any headphone can ever make a $5250 price tag seem warranted, this is the one. Read on to find out why.


I’ve been around electrostatic speakers (and headphones) for most of my adult life, so that I’m very familiar with the technology, but for those of you are new to electrostatic drivers allow me to provide a brief bit of background information that will help explain what makes them special.

Unlike conventional dynamic drivers, electrostats do not use rigid cone or dome-shaped diaphragms, nor do they use voice coil/magnet-driven motor assemblies. Instead, electrostatic drivers use exceedingly light, thin, flexible, conductive, membrane-like diaphragms that are suspended between two fixed, perforated, mesh-like metal panels that Stax simply calls “electrodes” (some manufacturers call them “stators”—hence the term “electrostat”). The only moving part of the driver is the conductive diaphragm, itself, which is incredibly light: in fact, Stax diaphragms are less than 2 microns thick—meaning they have vanishingly low moving mass.

When the earphone is in operation, a continuous bias voltage (580V in Stax designs) is applied to the diaphragm (the voltage is supplied by a purpose-built electrostatic headphone amplifier or “energizer,” as Stax calls its electrostatic amps). In turn, balanced audio signals are applied to the driver’s electrodes, so that one electrode takes on a positive charge while the other takes on a negative charge. As audio signals are applied, the conductive diaphragm is pushed away from one electrode panel and simultaneously attracted toward the other, and vice versa, moving back and forth in precise response to the ebb and flow of the audio signal.

To see a well-illustrated explanation of electrostatic technology in action, click on this link to the Stax web site: http://staxusa.com/Technology.html.

In principle, electrostatic drivers provide several key benefits that no other driver type can fully equal:

Simplicity: the only moving part in the driver is the diaphragm.
Precision & Control: the entire surface of the driver is driven, rather than just the center of a cone or the rim of a dome, as in conventional dynamic drivers. This means the whole moving surface of the driver is under the precise control of the audio signal at all times. (The same is also true of planar magnetic drivers.).
Ultra Low Mass: The very low mass of the electrostatic driver allows terrific transient speed and the ability “turn on a dime,” so to speak. (While planar magnetic drivers are also low in mass, their diaphragms do incorporate conductive metal traces arranged as planar “voice coils,” which do add a little bit of extra mass).

There are really only two perceived drawbacks to electrostatic headphones:

Specialized Amps Required: Conventional headphone amplifiers are not capable of driving electrostatic headphones. Instead, electrostatic ‘phones require dedicated, purpose built electrostatic headphones amps (or energizers), sourced either from Stax or other amp manufacturers. As a rule, electrostatic amps (or at least ones good enough to be used with the SR-009, tend to be quite expensive).
Caution – High Voltages Within: By their very nature, electrostatic headphones are high-voltage devices and therefore require amps capable of supplying the requisite bias voltage (again, 580V for Stax headphones), and of supplying high-voltage, balanced-output audio signals. To my knowledge there is absolutely no rational reason to feel anxious or fearful about wearing or using electrostatic ‘phones. Even so, I would concede that some listeners simply do not like the idea of having drivers with hundreds of volts coursing through them positioned just fractions of an inch from their ears.

Technical highlights of the Stax SR-009

Diaphragm: The SR-009 features a diaphragm less than 2 microns thick that is made from what Stax calls a “High Polymer Ultra-Thin Film” material called “Super Engineering Plastics”). According to Stax, this is a “much thinner material than conventional engineered plastics.” Stax promises the new diaphragm results, “in a vast dynamic range, excellent tone quality and frequency characteristics from low to super-high (frequencies)!”
Electrodes: The SR-009 features all-new MLER electrodes (the acronym stands for Multi-Layer ElectRodes,” that are made through a new process that uses both “ultra-precision photo etch” and “high tech heat diffusion” techniques to join three layers of material to form the thin, flat electrode panels. Specifically, Stax sought to balance three performance variables—the thinness of the electrode, the stiffness of the electrode, and the non-resonant qualities of the electrode—with an eye toward maximizing all three at once. At least one authorized Stax dealer claims that the manufacturing process for SR-009 electrodes is three times more difficult to implement, and takes six times longer to execute, than comparable processes for other Stax electrostatic headphones.
Enclosures: The SR-009 is as open-back design, and features extremely rigid driver housings “precision machined from lightweight aluminum” (giving new meaning to the term, “carved from billet.”).
Cables: The distinctive signal/bias voltage cable for the SR-009 is made from “silver plated, high purity copper wire 99.9999%.”
Headband: The SR-009 headband uses a, “newly designed 10-click slider mechanism for optimal wear and comfort.” The same mechanism is also used on the STAX SR-507 Lambda Open-Back Headphones.
Ear pads: The SR-009 features thick, comfortable, asymmetrical (as in, thicker toward the rear and thinner toward the front) ear pads covered in “hand crafted Lamb leather.”
The Box: Even the box the SR-009 comes in is special; it’s a padded case made of paulownia wood, whose properties are said to be optimal for “long-term preservations of the product.” Stax includes a special insert in the SR-009 shipping carton to explain that that pawlonia wood is lightweight, heat resistant, insect proof, and offers “moderate cushioning properties.” Moreover, the wood also helps regulate humidity to help keep the ‘phones from becoming overly damp while in storage.



Let me come right out and say it: the Stax SR-009 is the most accurate, neutrally balanced, responsive, nuanced, and detailed audio transducer I’ve heard. It is also—unlike some electrostatic headphones I’ve tried—capable of very robust dynamics and powerful (though ultra-clean) low bass (areas where some electrostats seem to run out of steam earlier than would be desirable). In short, the SR-009 is the best headphone I’ve yet heard.

With this said, however, let’s also note that today’s best dynamic driver-equipped and planar magnetic headphones offer performance that, while not entirely on a par with the SR-009, falls not terribly far behind, while selling for about 60% – 80% less that the Stax ‘phones do. My point is the there is true greatness in the SR-009 design, but greatness that comes at quite a dear price. Still, there are (at least) two ways to look at the issue of pricing. One might be to say, “$5250 is just too much to spend for any headphone; I can get close enough to the sonic mountaintop for a fraction of the SR-009’s price,” which is true. However, another way to look at the situation might be to ask, “In what other class of luxury goods—be it wristwatches, cameras, or high-end audio gear—can you get the very best there is for as little as the SR-009’s cost?” Depending on your frame of reference, then, the SR-009 could be considered an unattainable luxury, or (dare I say it?) a bit of a bargain as best-you-can-get material objects go.

What is the “sonic character” of the SR-009? Well, that’s a tough question to answer because these ‘phones are so neutral and so faithful to the sources that drive them, that they really don’t seem to have a signature sound of their own, though certain sonic common denominators do always seem to be present. For instance, no matter what ancillary components you use, you can count on the SR-009s to sound very accurately balanced, extended on both ends of the audio spectrum, consistently fast, extraordinarily transparent, and capable of delivering lavish amounts of detail (provided the rest of your system is up to the task). But the tricky part is that the underlying sonic “persona” of the SR-009 can and does shift in both subtle and profound ways as you connect these headphones to different amplifiers and source components.

What, then, is the sound of the SR-009? The answer is that the sound of the Stax will always be an uncannily revealing and nuanced rendition of the signature sound of whatever components you happen use to drive them. Make a change—any change—in your playback system and the SR-009 will immediately report the sonic results (whether for good or ill). No nuance is too small for these ‘phones: they detect and report on every single element in the signal path with almost disarming ease and candor.

During my time with the SR-009, I had the privilege of powering the headphones with three different world-class electrostatic headphones amps: the Woo Audio WES ($5000 – $6000 plus, depending on configuration), the Kevin Gilmore-designed Head Amp Blue Hawaii SE ($5000), and a pre-production prototype of the upcoming Cavalli Audio Liquid Lightning (projected price, $4250). As I listened through the three amps, I was struck by the fact that the apparent “core sound” of the SR-009 changed in distinctive ways every time an amp switch was made. Each of amps offered a compelling, though distinctive take on “state of the art” sound, and what floored me was the almost shocking extent to which the Stax headphone took on the sonic personality of each of the amps (almost as if it became a whole new headphone every time it was fed by a different amp). If ever an audio product were a true sonic “chameleon,” the SR-009 is the one.

Similarly, I tried some very subtle tuning changes in the Playback reference headphone system with the SR-009, such as switching between two slightly difference versions of the same brand of balanced audio interconnect cables I had on hand. Instantly, the SR-009 reported (and in no uncertain terms) the exact sonic differences between the similar but not identical cables, or any other components I substituted within the test system. Where some headphones reveal sonic differences between ancillary components in a general way, the SR-009 reveals the impact of system-level changes in much more detailed, explicit, and precise ways. For this reason, prospective buyers will want to be very careful in selecting components for use with SR-009s. The good news is that these ‘phones will tell you precisely what your system components are doing, but the potentially not-so-good news is that they may expose performance shortcomings you’d prefer not to know about.

Some electrostatic ‘phones I’ve heard exhibit audible limitations in terms of bass power, definition, and depth, and also show a certain reluctance to tackle large-scale dynamic swells with appropriate gusto (both of which are, by the way, areas where today’s finest dynamic and planar magnetic models tend to perform very well). The SR-009, however, shows no such limitations. It can do killer bass with the best of them, and can do the full spectrum of dynamic events—from the subtlest microdynamic shifts in emphasis right on up to full-on moments of orchestral bombast—all in a very convincing way. Indeed, I would say the SR-009 offers state of the art bass performance and dynamic agility.

After careful listening, I have concluded—as has my colleague Tom Martin—that Stax has delivered a milestone headphone in the SR-009. That’s because the SR-009 is qualitatively better than every other headphone we have heard thus face, and this qualitative difference enables the headphone to cross a meaningful threshold in terms of fostering musical involvement. Interestingly, every member of the Playback team who heard the SR-009 independently arrived at the same conclusion: the SR-009 simply draws listeners closer to the heart of the music than any other headphone we have tried.

I won’t tell you the SR-009 “trounces” the other top-tier headphones on the market, because that would be untrue and misleading. It’s just that the SR-009 is a small but worthwhile increment ahead of its competitors, not just in one or two performance areas, but across almost all of the most important areas of performance. This is what sets the Stax headphone apart. In a separate but related Playback review of Woo Audio’s terrific WES electrostatic headphone amp, Tom Martin observed that the benefits of the SR-009 (when powered by a top-shelf amp) include its ability to sound “more transparent than other headphones, … lower in distortion, … and more dynamically accurate.” His comments are spot on; the SR-009 is a true benchmark design.

No other headphone we’ve tried can tell you more about your favorite recordings or about the components in your system, though it is important to bear in mind that today’s best dynamic and planar magnetic ‘phones can come fairly close to the Stax and for considerably less money. Still, when push comes to shove, the Stax SR-009 seems—as some sports writers might put it—like a man competing among boys. The only caveat here, if there is a caveat, is that the SR-009, for obvious reasons, can only sound as good as the amp that drives it.



Some of my industry friends who happen to be manufacturers of subwoofers recommend using the track “Hotel California” from the Eagle’s live recording Hell Freezes Over [Geffen] as a test of bass resolution and pitch definition. What attracts them, I know, is a syncopated, low-pitched drum pattern (a sustained “booom” followed by two shorter notes, “boom-boom”) that appears near the beginning of the song. What’s interesting is that after the pattern is introduced and repeated roughly twice, the listener discovers that, as one of my subwoofer buddies put it, there are “dual bass drums at different pitches to discern” (because within the short two-note pattern, the second note seems to played at a slightly higher pitch than the first). But when I played this track through the SR-009, I learned something I had never realized before (not even with the best of subwoofers), which is that what’s really happening is that the second of the notes is played on a large conga where the player strikes the drum with one hand while pressing against the head of the drum on the second note (perhaps with his elbow or free hand) to delicately bend the second note upward in pitch. As I listened, the Stax ‘phones afforded me one of those “Aha” moments where I thought to myself, “So that’s how they get the higher note in that sequence; sweet!” My point is that this is the sort of small yet richly rewarding discovery you can expect to make—over and over again—when listening through the SR-009s.

To appreciate the SR-009’s dynamic prowess, try listening to the track “Lil’ Victa” from SMV’s (that is, Stanley Clarke, Marcus Miller, and Victor Wooten’s) Thunder [Heads Up]. The track feature three master bass guitarists taking turns soloing, and part of what is thrilling is to hear how very, very different the three identically tuned instruments sound in the hands of these masters. But what I would draw your attention to is the sheer variety of fingering and plucking techniques the musicians employ, including hammer-ons (where the player presses the string against a fret with such vigor that mere fretting the string causes a note to sound) and other slap’n’pop techniques (where the player might, say, hook a finger under a string, pull it upward and then release it, allowing the string to bounce fiercely off the fingerboard, unleashing an explosive burst of transient energy). What I found gripping—and at times almost frightening—was the sheer speed and power of the SR-009’s on those intricate and vigorous bass runs. What I soon discerned is that most other headphones—even some very fine ones—tend to round off some of those bass guitar transients to some degree, whereas the Stax beautifully captures the raw intensity and blindingly fast attacks of the notes without so much as a flinch or a whimper. This, incidentally, is something no previous Stax model in my experience could do near as well as the SR-009 does.

Another impressive aspect of the SR-009 is its ability to keep very small low-level sonic details in focus, even in the midst of passages where much larger dynamic swings are in play. As an example, I put on the concluding two sections of Part 2 of Mahler’s Symphony No. 8 [Michael Tilson Thomas/San Francisco Symphony, SFS Media, SACD], and discovered that I could pick out very delicate individual vocal lines, even during choral swells, or could discern extremely subtle changes in the attack and tonality of brass section instruments, even as huge dynamic swells were unfolding. In short, the SR-009 is one of those oh-so-rare transducers that can do many things at once, yet without ever stumbling or ever putting a sonic foot wrong. This is something that’s very hard for even the largest and most capable of high-end loudspeakers to manage, yet it’s a feat the Stax ‘phones pull off with almost casual, offhand ease.

Finally, the SR-009 always seems able to dig deep (deeper than any other headphones can) to tease out the innermost elements of tonal purity and, yes, tonal beauty in any given recording. I was floored by both the breathtaking intricacy and coherency of the Stax ‘phones as they reproduced the Clockwise performance of Robert Paterson’s Freya’s Tears [American Modern Recordings]. Freya’s Tears is a short piece in three movements that was written for Clockwise, a duet consisting of a harp and violin. The middle movement of the piece, “Freya’s Tear” features soaring and at times very intricate harp passages set against beautiful yet at times quite angular and penetrating violin passages. I was struck by the fact that the SR-009’s were able to show the round, fast-paced, and percussive quality of the harp, complete with luscious tonalities that seemed almost illuminated from within, while at the same time perfectly capturing the knife-edge balance struck between sweetness and incisiveness as the violin explored lines written in its upper registers. The eerie thing is experiencing how rich, full, complete, and lavishly detailed the SR-009 can make two instruments sound, even when playing intricate material simultaneously. No other headphone I know of can match the SR-009 is this respect.


Consider this headphone if:

• You want a neutral, accurate sound with wide bandwidth, and you prize clarity and low distortion.

• You’re the sort of person who really wants to know precisely what’s going in his or her audio system and/or favorite recordings. The SR-009 will give you “the truth and nothing but the truth,” and will do so with disarming ease and grace.

• You believe, as does Mercedes-Benz, in a philosophy of “the best or nothing.” At this moment in time, we think the SR-009 is the finest headphone in the world. Period.

Look further if:

• Down deep, you think only a “nutball” would consider spending $5250 on a headphone. But caution: Once you hear the SR-009 in action, you may discover that you are, in fact, just such a “nutball.”

• You find the overall cost of an SR-009 system daunting (recognizing that SR-009-based systems can easily set you back $10k or more, once you factor in the price of an appropriate amp). Also look further if you aren’t prepared to spend some time finding your ideal amplifier match.

Ratings (compared to cost-no-object headphones):

• Tonal Balance: 9.5 (some might discern a very slight region of emphasis in the upper midrange, though I am not convinced that this is anything other than the Stax “telling it like it is” regarding flaws commonly found in modern recordings).
• Clarity: 10
• Dynamics: 10
• Comfort/Fit: 9.5
• Sensitivity: 9
• Noise Isolation: 3 (Frankly, the SR-009’s do virtually nothing to block out external noise—perhaps the price we pay for their terrific transparency?)
• Value: 8 – 10 (Depends on your frame of reference. Yes, it’s the most expensive headphone on the planet, but no, $5250 is not too much to pay for the finest headphone there is—and one that puts many five- and even six-figure loudspeakers to shame.)


Stax’s SR-009 is a reference headphone that sets a new benchmark for transparency, low distortion and dynamic accuracy while delivering a wide and balanced frequency response. This is the standard against which all other top-tier headphones must be compared.


Stax SR-009 Electrostatic Earspeaker
Driver: push-pull circular electrostatic element, diaphragm is less than 2 microns thick.
Housing: open-back, machined aluminum
Frequency Response: 5 Hz – 42 kHz
Required Bias Voltage: 580V
Sensitivity: 101 dB/ 100V
Ear Pads: handmade, Lambskin leather
Signal Cable: High-purity, “six nines” copper, sliver plated.
Weight: 21.3 oz.
Warranty: 1 year, parts and labor
Price: $5250

U.S. Distributor:
Yama’s Enterprises, Inc.
(310) 327-3913

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