General Information

Circumaural/"preaural" electrostatic

Latest reviews


New Head-Fier
Pros: Superb in every aspect except...
Cons: ...ever so slightly bass extension
I'm a rabid audiophile since the early '70s. I bought the first version sometime late '70s. Lost them in a move (I still think my roommate stole them...) Replaced them with the 2nd, better version which I've had since then. They are my reference for comparing all headphones and even when evaluating loudspeakers.
I've bought & sold probably 25 or more headphones since then (top-of-the-line BY, AKG, Koss, Senn, Grados, etc.) and have never found anything close to what the Stax Sigma can do.
There are some that have better/more bass but that's it. These beat on everything else and are absolutely unique even compared to Stax's more recent product line because of the ear speaker design/philosophy of front-aligned electrostatic drivers which use YOUR pinnae to channel & modify the sound.
I did my Masters thesis on "general systems theory applied to human psycho-acoustics" — most testers, reviewers & musicians don't realize that, above all other factors like equipment quality & recording techniques, the biggest factors by far in SQ is your room listening space, your position and YOUR Pinnae, Cochlea & Auditory Nerves!
No two people Ever HEAR the same signal in their brains!
The Sigmas are the Only headphones that make use of these principles and thus sound most realistic. The only thing lacking is lower bass extension...simple physics means you can't use a 2.5" diaphragm to substitute for a 20" kick drum! But the bass is Fast with the best rise & attack & decay of any headphone OR loudspeaker.
'nuff said!
  • Like
Reactions: stalepie
I agree with your assessment of these phones and would add something about my latest foray in headphone mods. There are serious issues of damping the vibrations in the earcups on most, if not all headphones and I and others have been getting major upgrades in sound by adding sorbothane pieces to dampen the earcups.
This thread shows what can be done to the Sigmas, as well as other Stax and other phones. 
As regards bass in these phones, you are not going to.miss much, especially if you do the sorb mod.  The problem, if it really is a problem, is that the enclosure is so open.  The Lambdas which followed the Sigmas, use the same size drivers and get extremely good bass as Till Hertsen's measurements show.
  • Like
Reactions: WHO23


Headphoneus Supremus
Pros: Outstanding mids, absence of sibilance, huge soundstage.
Cons: Need for powerful amplification; hard to find, increasing price.
After the great reviews by John and Ed, there is really not much to say.
The 009 is to my ears, a perfect headphone, although at times in the brighter side. In spite the 009's however, I frequently grab the Sigmas/404 for chamber music, vocals and chorales.
The bass can at times get a little boomy in complex orchestral passages, but the wonderful mids give piano, voices and small acoustic instruments groups a relaxed and realistic sound you can listen to for hours.
Certainly a specialty phone, and one I will never sell.
  • Like
Reactions: Premislaus

John Buchanan

500+ Head-Fier
Pros: Sound, comfort
Cons: Appearance
 The Stax Sigma series panoramic earspeakers.

1. Models and nomenclature

The Stax SR-Sigma panoramic earspeaker was introduced in 1977. There have been 3 versions officially released and 1 after-market version commissioned.

  • The original low bias, grey grille 1977 Sigma model (bias voltage of this model being 230V or “Normal”). The 2 um thick driver used was later recycled in the very successful Stax Lambda Semi-Panoramic earspeaker. The first release Sigma had a fabric coated, round bodied “kettle type” cord that connected the earspeaker to its amplifier/transformer and the grilles on this model are grey.
  • Later versions of the normal bias Sigma (1987) used a lower capacitance, flat black cable that was recycled back from the (then) current Stax SR-Lambda earspeaker. 
  • The later Sigma Professional (1987) version introduced the 580V Professional 1um driver then used in the Stax SR-Signature earspeaker. This earspeaker had black grilles, lower capacitance chocolate coloured cables (from the Stax lambda Signature), and a chocolate coloured headband, compared with the original black cables and headband.
  • A later version, the Sigma/404, is a Sigma rebuilt with high bias SR-404 1.35 um drivers and their corresponding very low-capacitance cables.
2. History and conception.

The Sigma earspeaker design was the result of a complete reassessment of how headphone sound is perceived. Up until that point, listening to headphones or speakers were considered completely different experiences. Headphones were designed to inject sound directly into the ears with as much sound isolation as possible between channels and also from the external environment, reflecting their communications genealogy. The drivers were parallel to the pinnae (= ear flaps) and either intra-aural, circum-aural or supra-aural.

Speaker listening has the drivers at a great distance from the ear canals and sound produced has to traverse a great number of direct, reflected and partially absorptive pathways before arriving at the ear canal, allowing much more modification of the sound as well as left and right channel blending. The drivers are also in front of the listener and roughly perpendicular to the plane of the pinnae.

Naotake Hayashi, the genius behind the original Stax company, decided that one of the differences between speaker and headphone listening was a result of that very isolation and direct aural injection inherent in the design of all prior headphones. He decided to make a headphone that would actually sound like listening to speakers in a partially reflective/absorptive room. The genius lay in his recreation of a room around each ear – a revolutionary concept that has never before or since been replicated. Each ear-cup was meant to approximate a partially absorptive and partially reflective series of surfaces for the headphone driver (now in front of each pinna and perpendicular to them, as per speaker listening) to bounce sound off and then into the ear canal. In other words, the direct injection principle was thrown out the window and now the drivers were only heard after firing sound into the ear canals via a reflection - and some absorption – from an internal lining of mineral wool. I am guessing that the ear speaker cages had to be constructed pervious to air, rather than designed with a solid body, for weight considerations (viz. a solid body construction would have been too heavy for comfortable wearing). Possibly there were also enclosed cavity effects to deal with if the headphones were sealed. Indeed, weight has been one of the main complaints levelled at the only enclosed Stax design, the Stax SR-4070 Monitor. The mineral wool lining of the cages, apart from reflecting and absorbing sound, much like a normal listening room, also provided some hermetic sealing of the cages allowing reduced front to back sound cancellation around the periphery of the drivers. In other words, the drivers could have bass (the lack of which has been a criticism of the relatively similar design AKG K1000), but not as much as if a solid body had been used. I am guessing that a solid body Sigma had been tried and discarded due to comfort and sound considerations, so a compromise between weight of the headphones and sound quality as well as bass extension was reached.

3. Comfort.

The Sigmas, although bulky and laughably unfashionable, have been engineered to be exceptionally comfortable on your head. It is literally easy to listen for hours without your pinnae contacting the metal inner grille of the drivers (Lambda series) or the earpads themselves (SR-007). They also seem to be much cooler in summer than either of the above. 

  • Sound. 
a. Low bass and bass.

The sound of the Sigma always has slightly reduced very low bass because of some residual front-to-back driver cancellation through the mineral wool earcup lining, but beyond that point, the earspeaker’s sound is very hard to fault compared to what one is used to. The bass that is present, until the very low bass roll-off, is of excellent quality. It also packs a wallop (in both pro versions of the Sigma being discussed), which is quite unusual for an electrostatic headphone. Oddly enough, that wallop isn’t quite as evident in the Lambda Nova Signature, which uses almost identical drivers.  Certainly there are no bass instruments that move back and forward in the sound-field, nor do they completely disappear as they descend the scale, as I heard with the Jecklin Float Electrostatics playing, for example, Tony Levin’s descending Stick run during Projekct One’s “Live At The Jazz Café” Track 3. On the Jecklins, Tony appears to walk out the studio door as the run descends to subterranean levels. On the Sigmas and the Lambda Nova Signature, he’s in the studio and hasn’t moved a muscle.

              b. Midrange.

The reproduction of vocals comes as close to free of sibilant emphasis as possible.  This is truly what you hear in live, unamplified music.  Particularly realistic are piano and voice – the smoothness of the sound is just as relaxing on replay as it is live. Indeed, after attending a piano concert in a relatively reverberant wooden hall, the Sigma/404s got the extreme dynamics of the piano without any of the brightness at higher volumes that the Lambda Nova Signature exhibited.  The LNS is supposed to be bereft of the treble “sting” of the Lambda 404, so the 404 would be far too bright for me, despite the same driver performing beautifully in the Sigma/404. This exquisite piano/vocal reproduction is unique to the Sigma series, in my opinion. 


             c. Treble

The later substitution of the 580V “Professional” bias drivers (either the Lambda Signature or the Lambda 404 headphone driver) to replace the original “Normal” bias driver, along with an upgrade of the original headphone cable, allows a reduction in the marked high frequency roll-off and a flatter extension to the very low bass reproduction compared with the original low bias Sigma. In my opinion, there are no drawbacks to this modification whatsoever and the top end sounds both smooth and evident, rather than smooth and rolled off as in the original Sigma

            d. Correct volume level.

 The Sigma/404 really shines a light on just how successful Mr Hayashi’s earspeaker design concept really is. Another bonus I have noted with these earspeakers is that it seems to be relatively easy to dial up the “correct” volume of sound – they just sound “right” at that point. Although this is not unique for these phones, I haven’t heard a headphone with such a pronounced “correct” volume level for a track. The bass and treble just seem to be in perfect balance with the midrange only at that volume. Peter Walker of Quad was a great proponent of the “correct” volume theory apparently. Strange that I also have Quad Electrostatic speakers.

e. 3-D sound.

Then there is the seemingly increased 3D space that these headphones portray – the sound stage seems to be actually in front of the head, with some front to back space, compared with the usual line-between-the-two-ears imaging. This is something I’m not as good at hearing, so I will leave it to others to give their impressions.

These differences allow greater appreciation of albums that were mixed for speakers in the standard control room, because that is exactly what the Sigmas replicate.  I would guess that apart from very low frequency roll-off, these earspeakers could be the greatest and most accurate magnifying glasses for mixing evaluation ever made.  They have in built room diffusion, diffraction and absorption effects without the left to right blending, so that each channel can be easily heard independently. 


5. Associated equipment needed.

I have found the Stax SRD-7 Pro, SRD-7 Mk 2, or SRD-7 Spritzer will do an admirable job of driving these very power hungry monsters, provided a good power amp is pushing them. Here, I use the Studer A68 power amp fed by an Apogee Mini-DAC and a Studer D730 CD player. If you wish to drive them with a direct drive electrostatic amp, I would suggest, at the very least, using any of Stax SRM-717/SRM-727/SRM-T2, Kevin Gilmore/Spritzer’s revamped T2 or the Blue Hawaii SE/Solid State Electrostatic amps. They are all powerful enough to drive these and the SR-007 Mk1/Mk2 series. The Lambda series are far less power hungry, despite using the same drivers as the Sigmas. Distance from the ears and absorption by the damping material may both account for these efficiency differences. The volume control levels for the Sigma/404 and SR-007 Mk1 I once owned were identical for the same reproduced volume – i.e. they seem to be equally inefficient - compared with a Lambda Nova Signature. 

6. Credits.

Finally, one has to admire a designer who actually truly said nay to any marketing considerations. These phones are laughably big and ugly, but if they were anything else, could they sound as good? Thank you, Mr Hayashi for not listening to the form-over-function naysayers, and I bet there were plenty of those in Stax board meetings in 1976/1977

Lastly, I would like to thank Edstrelow for the inspiration to upgrade my Sigmas to Sigma/404s – something I did on faith and have never regretted it for a minute, and Webbie64 for making me realise the error of my ways when I briefly thought about selling them.

7. Postscript - Stax Sigmas: high bias earspeakers compared – Pro vs Sigma/404. 

The 2 Sigma high bias phones essentially differ in 2 main physical ways.

  • The headphone cable used for the original Sigma Pro was the same as the one used on the Lambda Signature – not as wide as the one used for the Sigma/404 hybrid, which was first seen on the Lambda Nova Signature. 
  • The drivers are different also – the Sigma Pro uses the reported 1 um Lambda Signature driver, whilst the Sigma/404 uses the (2 generations) later 1.35 um drivers first seen in the Lambda 303/404.
Otherwise, the shells of the 2 earspeakers, apart from minor colour variations, are identical. The Sigma Pro driver appears to be very slightly more efficient than that of the Sigma/404.

In my set up so far, the two have been compared through 

  • Studer D730 -> AES-EBU digital outputs -> Apogee Mini-DAC ->  Studer A68 -> Spritzer Pro SRD-7 bias + transformer box. 
  • Studer D730 -> analog XLR outputs -> Stax SRM Monitor direct drive earspeaker amplifier. 

Both chains give repeatable results.

As seems to be the order of the day, the results are not what I expected.  The Sigma Pro is far better than its reputation suggested and even bests the Sigma/404 in a couple of areas.

Firstly, the deep bass is slightly more evident and the mid bass is much tighter and slightly less resonant. In the Sigma Pro, bass drum has slightly more punch rather than smeared thud. Deep bass has been one of my only criticisms of the Sigma series. This is the best I’ve heard it, but only by a hair’s breadth over the Sigma/404.

The midrange is about the same with both, but the treble is slightly more evident with the Sigma Pro. It’s close to a line call there, however.

As for dynamics – the Sigma Pro does dynamics somewhat better than the Sigma/404.  The Lambda Signature driver/cable just sounds a little faster than the 404 driver/cable. This tends to alleviate some of the complaints about a mushy low end of the Sigma series. 

The original Sigma low bias was far too rolled off at the top end in particular, despite the magical Sigma midrange being present there. It appears that either the Signature or 404 driver implants are a successful remedy to this, and give a phone that had huge promise a push into reference territory.

In summary, despite every single report to the contrary, I’m loving the Sigma Pro! Flame suit on, hearing aid batteries fully charged LOL. Stax SR-009? Who cares?


Awesome review!
Definitely agree with your praise of the Stax founder. He made a great phone as we say "by guess and by golly." Re the mineral wool and enclosure. The Stax drivers are unusable in the Sigma configuration without the enclosure and wool. They give virtually no bass when sitting in open air, unlike for example the AKG K1000. On the other hand if you seal the enclosures as I once did with duct tape, you get an impossibly boomy sound. Somehow Stax got it just right with the enclosure and mineral wool., decent bass without too much boom. Still a bit boomy though, probably why they make good rock phones.


There are no comments to display.