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A Review On:  STAX SIGMA


Rated # 125 in Over-Ear
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Audio Quality
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Price paid: $400.00
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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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