Here is my comparison of the three Sigmas. The original thread is
http://www.head-fi.org/t/464873/stax-sigmas-compared-low-bias-sigma-pro-and-sigma-404
SIGMA COMPARISONS
I have had three different Sigma models, for a few years now, the original normal or low bias, the pro or high bias, and the Sigma/404 which is a Sigma rebuilt with contemporary Lambda 404 drivers and cable. However until recently I did not have an amp to allow me to directly compare the low and high bias models. A few months ago I obtained 2 SRM1 Mk2 Stax amps, which have both high and low bias sockets so this was the time to do a full comparison of these models. Normally these phones don’t even sit anywhere close to each other, the Sigma/404 is in the living room, the low bias Sigma in the tv room and the Sigma Pro 30 miles away in my office. So it was kind of a Sigma family reunion.
SIGMA HISTORY
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. 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 the Stax dealer. My Sigma pro is actually a modified low bias model, rebuilt by Stax Japan to the standard of the Sigma Pro. However the original factory-made Sigma pro is black rather than gray.
The Sigma, is unlike virtually any other headphone ever made. It is big and boxy and it places the drivers well ahead of the ears at right angles to the head. Because it is ahead of the ear, I call it 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. This feature it shares with only one other more or less contemporary phone, the AKG K1000. But the K1000 drivers sit in open space while the Sigmas are semi -enclosed with a partially sealed ear cup in which the walls are filled with what is, I believe, mineral wool.
DESIGN ISSUES
Interestingly, you can not run a Sigma/Lambda driver without its enclosure, the way you can an AKG K1000, something I tried to do some months ago. The Lambda driver gives no bass that way. This shows that it is inherently more difficult to get a full frequency response with an unsealed pre-aural design than with conventional designs.
The Sigma enclosure presumably provides a resonating chamber for the driver and it also blocks some of the back wave from the drivers. It is surprisingly successful in doing this since the Sigmas have pretty decent bass although not quite as much as the Lambdas which use the same drivers in a conventional configuration. The Lambdas 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.
It makes one wonder how the AKG has any bass at all given that it has no enclosure. I do not own a pair of these but I have spent some time listening to them on good systems and they are not seriously bass deficient, although they have less bass than I think the Sigmas have. They must either control the back wave or have very pronounced low frequency resonance and emphasis in order to have any decent bass. One review I read says the K1000 uses some form electronic equalization built into the cups to pump up the bass.
The only other headphones that I am aware of which use a pre-aural configuration are old 4-channel phones. Some such phones can be seen on the Koss history site although I believe there were other manufactures of similar phones. Of course, the surround sound phones also have two rear channel drivers behind the ear, i.e. in a “post-aural” position.
SIGMAS COMPARED
The Equipment
My low bias Sigma is basically unaltered from the factory design. My pro model was originally a low bias Sigma which was sent to the Stax factory in Japan to be rebuilt as a pro and given new drivers and cable. The Sigma/404 is similarly a modified low bias Sigma but rebuilt by the Stax US distributer with the 404 cable and drivers.
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 some comparisons between the Sigma Pro and Sigma /404 on a Stax 717 and also made some comparisons with the Stax 007A on the 717.
I do a certain amount of tweaking which I think adds to the sound quality. If you don’t believe these things work, you can skip this paragraph and frankly Scarlett I don’t give a damn. Contacts have Silclear silver paste and Progold applied. Cds are trimmed to be more circular using the Desk System lathe, and machine polished with Mapleshade polish. The cd’s have additional weights either using old Alsop sorbethane rings or stick-on covers. There are sorbethane feet under the amp and cd player and 2 PS Audio Noise Harvesters in the power strip.
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 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
How do you describe the general sound of the Sigmas for someone who has never heard them? The first words to come to mind would be “airy” and “ambient.”
Their soundfield is unlike virtually any other phones you will hear, except possibly the KGK1000 when listened with the drivers angled out. 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. I wouldn’t say this effect is perfect, but it may be about as much out-of-the-head sounds, as you can get from a headphone, without using something like Dolby Headphone processing.
At the last 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!” Now 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.
COMPARING THE SIGMAS
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 treble, bass and general definition. The Low Bias seems to have a broad peak between about 100- 500 Hz. The 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 low end peak.
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. .
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. With the 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. All the Sigmas 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. 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 and the Sigma/404 a competitor to the 007. The Sigma pro is intermediate between these two. 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 seem lacking in ambience.
At the last Canjam, I had some opportunity to listen to a number of the superamps, including the BHSE. The better amps scale up the Sigma pro and 404 quite well such that I felt the advantages of the 007A or Mk1over 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. The PS is a sort of parallel noise filter and adds some sparkle and sweetness the sound of this amp. Two points to note about this amp however: There are at least two versions of this amp the latter being sold as “improved”. I have both. I don’t know what all of the differences are but the earlier model has smaller capacitors around the power supply. Also the US versions have unpolarized power plugs. However it definitely sounds more powerful plugged in one way than another.
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 the low bias Sigma worked, especially with rock music. So get one if you can find one, and if you are really ambitious convert them with the 404 parts.
