Circumaural/"preaural" electrostatic


  • Circumaural/"preaural" electrostatic

Recent Reviews

  1. KenLim
    best headphones (ahem..."Ear speakers") Ever! Period!
    Written by KenLim
    Published Nov 5, 2015
    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!
      stalepie likes this.
    1. edstrelow
      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.
      edstrelow, Nov 6, 2015
  2. gilency
    Natural sound, ideal for prolonged listening sessions without fatigue but with out most enjoyment.
    Written by gilency
    Published Apr 2, 2014
    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.
  3. John Buchanan
    Stax Sigma vs Sigma Pro vs Sigma/404
    Written by John Buchanan
    Published Nov 26, 2012
    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?


      strid3r likes this.
    1. gmcmage
      Awesome review!
      gmcmage, Nov 27, 2012
    2. edstrelow
      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.
      edstrelow, Mar 30, 2013
  4. galvy
    Vintage Sigmas still sounding great
    Written by galvy
    Published Nov 14, 2012
    Pros - Dimensional, detailed, smooth, rich sound; very comfortable to use for long periods
    Cons - Few options for amps due to normal bias
    Bought my Sigmas in the ''80s, after trying all the available Stax products and preferred the sound of the Sigmas by a wide margin.  Until recently have used them sparingly since I never really cared for the energizer/solid state amp combos I had available and thus spent far more time listening to a variety of speakers. Found a mint Stax SRM-T1W amp/preamp, and now I truly enjoy these 'phones. Plus the passive preamp section works quite well with the amps I now own when it's time to use speakers.
    1. edstrelow
      I generally prefer the high bias Sigmas but my old low bias set is still pretty impressive, especially in terms of creating a natural sound. For some reason these more often give a strong illusion of sound not coming from the phones themselves and I have often taken them off to make sure the speakers were not on. I don't seem to be able to find the SRT-1W amp/preamp. Are you sure that is its designation?
      edstrelow, Nov 14, 2012
  5. edstrelow
    Written by edstrelow
    Published Feb 13, 2011
    Pros - Unique Spatial Imaging, Electrostatic Clarity
    Cons - Rarity, Bulky, Looks,
    The Stax Sigmas are  pretty much unique among headphones, because they mount the drivers ahead of the ears and facing to the back of the head. Thus unlike virtually every other phone made there is no direct transmission of sound to the eardrum, but all sound reaching the eardrum is reflected one or more times by the external ear structures or ear canal.
    Stax did this to create a more lifelike presentation, since this type of reflected sound is what you would get from speakers or real instruments located ahead of you.  So in that sense virtually all other phones produce an unnatural presentation because they fire sound directly to the ear drum something that you would normally only get with a sound source beside you. I am sure this is something that other headphone designers do not want pointed out, but it is certainly true.   As far as I am aware, only the dynamic AKG K1000 allows the same type of sonic projection as the Sigmas, at least while its drivers are angled at 90 degrees to the head.    However the K1000 does not have the box enclosure of the Sigma and its drivers can be angled so that they resemble a normal headphone with drivers firing down the ear canal.
    I am covering 3 different Stax Sigma models in this review although Stax only released 2 models, virtually identical except that the later ran at the  pro high voltage settings.  
    Stax’ original Sigma is the low bias model which came out in 1977, 2 years before the first Lambda, which it resembles and which uses the same size and shaped drivers. The Sigma pro came out 10 years later as Stax' top-of-the-line headphone, using the Signature 1 micron driver membrane, which is the thinnest membrane Stax has used.
    The Sigma/404 never actually came out at all, since it is a modified Sigma design. But some folk have had these put together either by themselves or by Stax dealers. There are other Sigma combinations made by various persons, Sigma/303, Sigma/202, etc.  I wait for someone to upgrade a Sigma with the newest set of drivers used in the 407 and 507.  These drivers are going to be somewhat more difficult to install than the previous Lambda drivers  because the drivers are assembled differently than the previous Lambda drivers.
    I have read that  the original head of Stax regarded the Sigma as his masterwork and I am inclined to agree.  However ultimately the Lambda outsold it and the Sigmas were dropped. I suspect cost and the fuggliness of the Sigma were mostly responsible.  I personally don't care about looks as long as the sound quality is there.
    My Sigma pro is actually a modified low bias model, rebuilt by Stax in Japan to the standard of the factory-made Sigma Pro, although the original factory-made Sigma Pro is black rather than gray.    Similarly my Sigma/404 was an old broken low bias Sigma, repaired and rebuilt with 404 parts, this time by YAMASINC, the US Stax distributer.
     I call the Sigma a “pre-aural” design by comparison with the headphone design terms “circumaural,” which means basically going completely around the ear, “interaural” or between the ears, and “supra-aural” sitting over the outer ear. The Sigma is in a sense also circumaural because the earcups circle the ear, but the most significant feature is the forward “pre” position of the drivers. As I note above it this feature  with  the AKG K1000. But the K1000 drivers are in open space while the Sigmas have a semi -enclosed with a partially sealed ear cup in which the walls are filled with what is, I believe, mineral wool.
    Interestingly, you can not run a Sigma/Lambda driver without an enclosure, the way you can an AKG K1000.  When I tried to do this some months ago. The Lambda driver gave no bass. But when these  Lambda drivers, are enclosed in a Sigma enclosure they give rather decent bass almost as good as the Lambda.  The Sigma enclosure presumably provides a resonating chamber for the driver and it also blocks some of the back wave from the drivers.
    The Sigma does not have quite as deep bass as the Lambdas which have about as deep bass as you are going to get in a headphone. The story is told that the Lambda design was originally requested by Mercedes Benz to check the low frequency noise in their cars.
    The Sigmas are bit heavy but otherwise comfortable, more so for example than the Lambdas or most superaurals since nothing presses on the external ear. They also keep cool in hot, unairconditioned settings because of the large chamber around the ear.  However, because of their sheer size they do tend to slide a bit.

    The Equipment
    Most of my listening was done using a Stax SRM1Mk2 amplifier driven by a Sherwood Newcastle cd player, serving as a transport, and a Music Fidelity X-DAC V3,with a separate Little Pinky power supply. There is a Cardas digital line between the cd player and dac, Signal Cable silver IC’s , and power cord for the amp and a PS audio power cord for the DAC. Subsequently, I also made comparisons between the Sigma Pro and Sigma/404 on a Stax 717 and also made some comparisons with the Stax 007A on the 717.
    The Music
    I used 2 selections of rock/pop, an old Rod Stewart greatest hits collection and the Flashdance movie cd. My classical choices were Handel’s Messiah performed by the Toronto Symphony and Mendelssohn Choir and recorded by EMI and Handel’s Gottingen Te Deum performed by the mixed choir of Trinity College Cambridge and recorded by Hyperium. For pure orchestral music I listened to Berlioz’ Symphony Fantastique performed by the Philadelphia Orchestra and recorded by EMI.
    I always warm up the system and phones for at least an hour before listening. All Stax phones and amps need a warm-up to stabilize, but the Sigma warm-up seem to need this to increase their upper frequencies and reduce a certain low frequency boominess. When I set up the Sigma pro and Sigma/ 404 together with a 717 amp, at the LA Canjam I noticed that the Sigma/404 took a few minutes longer than the pro to get to its full volume.
    However I unplugged one phone when I was listening to the other. The SRM1Mk2 works fairly well with one Sigma but it seems to struggle running 2 at a time. The 717 has an easier time with 2 phones.
    The Overall Sigma Sound
    The first words to come to mind would be “airy”, “ambient” and "out-of-the-head."
    Their soundfield is unlike virtually any other phones you will hear. The pre-aural configuration creates somewhat of an out-of-the-head experience, i.e. sounds appear to be more in a real space, in front of the listener than stuck between your ears. I wouldn’t say this effect is perfect, but it may be about as much out-of-the-head sound, as you can get from a headphone, without using something like Dolby Headphone processing.
    At the last LA Canjam I set up the Sigma Pro and Sigma/404 together for comparison. One listener’s comment which I thought was particularly interesting was that the Sigma’s soundfield was “huge!”  I tend to hear the Sigma sounds coming from outside of the head with a room-like ambience. For other listeners evidently the soundfield doesn’t seem to project out of the head, it just seems big, somewhat like hearing an actual room or concert hall but still in your head. One way or another, it is unlike anything else you will hear from phones.
    As you go from the Sigma Low Bias, to Sigma Pro and then to the Sigma/404 you get a somewhat flatter frequency response, with more extended treble, bass and generally better definition. The Low Bias seems to have a broad peak between about 100- 500 Hz. The bass peak for the Pro seems more narrow with less peak towards the middle. The 404 has the least such peak but it still has a noticeable end peak.  It sounds very much more extended in the treble and bass frequencies than the Pro. 
    However the low bias Sigma puts it boominess to good use with the pop/rock music where this mid-bass peak gave the Stewart and Flashdance music a lot of rhythmic drive. This is somewhat surprising given that part of the reason Stax went to high bias was to get better dynamics. Nevertheless the Low bias phones can really rock. I think the explanation is that the mid-bass peak adds its own boost to the dynamics on this kind of music. I have noticed this type of effect before with some bookshelf speakers which may lack deep bass but still give a good rock sound because they have so much midbass.
    However, the low bias Sigma has a somewhat dull sound and lack of sparkle compared the other 2, possibly because of a more limited high frequency range and less air around instruments and voices. Nevertheless, with Handel's Messiah, the “rock” quality of these phones seemed to help the impact of the large ensembles, although detail was not what was obtained with the Pro and especially the 404. The loss of sparkle was more obvious in the Te Deum possibly because it was a more recent and better recording. The low bias Sigma performed better with the Berlioz orchestral music than I would have expected. Oddly I thought I heard an upper midrange peak with the low bias phones, even though they clearly lacked top end extension.
    The most noticeable problem with the low bias Sigma, for classical music, is the lack of deep bass. One is used to thinking of rock/pop music as needing good bass but while there is a lot of bass in rock music it does not go very deep. The drums and double basses used in classical music seem to go further down the frequency spectrum than the rock instruments. The Pro and especially the 404 are better in this regard although not quite as good as the Stax 404 and 007s.
    The Sigma Pro had less boom than the low bias Sigma, a more open and airier sound and more dynamic kick at the mid and upper frequencies.
    However, the Sigma/404 had clear advantages over the pro. It had more treble, and more clarity in the upper frequencies as well as better deep bass. Ambience was reproduced better than the other Sigmas. It did a better job than the other Sigmas on just about everything, except that the low bias was a pretty good rock phone. The Sigma/404 was the best of the three on symphonic music because it had both additional deep bass and treble. Generally I would prefer the frequency response of the 404 or 007A for orchestral music.
    All the Sigmas do well with all kinds of vocal music.  They are somewhat congested sounding with symphonic music probably because of the low frequency peaks and lack of treble, but the Sigma/404 was best of the three for this music. I have a fairly bright Tchaikovsky Swan lake and Nutcracker recording which the Sigma made come alive, even by comparison with the 007A. I have since found a number of such orchestral recordings which sound better with the Sigma/404 than the 007A.
    In Perspective.
    How do these phones compare with other available stats? I think that even the old low bias Sigma is worthy to be considered in the upper ranks of stat phones but the Sigma/404 is a competitor to the 007's.  Certainly the 007 has the better frequency extension, dynamics and detail. However the Sigma/404 has its unique soundstage, speaker or room-like and "out of the head." It is listenable on a very wide range of music and superb with certain kinds. For some kinds of music I find I prefer it to the 007A, especially choral and opera and even some recordings of orchestral music which are lacking in ambience. 
    At the last LA Canjam, I had some opportunity to listen to a number of the superamps, including the BHSE. The better amps scale up the performance of the Sigma pro and 404 quite well such that I felt the advantages of the 007A or 007Mk1 over the Sigma's were reduced.
    As regards the low bias Sigma, the Stax Srm1Mk2 is a very good amp for it, however I prefer my SRA12S since I plugged a PS Audio, Noise Harvester into its back power socket. I later obtained an old SRD6  Stax transformer which allows one to run the Low Bias Sigma from a regular power amp.  The sound is quite decent and seems better balanced tonally than with the dedicated headphone amps.  However, compared to the amps, you do lose detail and airiness which is a deal breaker with many people.
    Unfortunately, the price of these phones keeps going up but considering that you may be geting, with a Pro or Sigma/404 a unique product, I rate the value high. Currently the going rate for a low bias Sigma is about $400.00 which is about what I paid new for mine back in the day when they were still being made.  The Pro's are hitting about $1,000.00.  When I had the Sigma/404 assembled it cost me almost $500.00 in upgrade parts and labor, to which you would have to add the cost of the basic Sigma.
    For anyone wanting to get into Stax phones cheaply, the Stax low bias transformers are available second hand for anywhere from $20.00 and up. However high bias Stax transformers are fairly rare and are going to cost almost as much as some dedicated amps.  There are a number of aftermarket transformers for high bias, which are also pretty costly,   I have heard that with with a top class power amp these could be quite effective.
    It was instructive to me to get these three phones together. I was quite familiar with them all, but when I plugged them into the same set-ups I was a bit surprised by what I heard. I was pretty sure the Sigma/404 was a great phone, and I had no surprise there, but I was surprised a how well even the low bias Sigma was, especially with rock music.  So get one if you can find one, and if you are really ambitious, convert them with the 404 or other parts from the more recent  Lambdas. But I would still prefer a Sigma pro over the low bias model.   And I hope someone figures out how to adapt the 507 drivers soon because that could result in an even better phone that the Sigma/404.
    More Detailed Review
    Comparison with STAX 007A
    First Review of Sigma/404


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