I revamped the Sigma/404 review slightly and want to repost:
The Stax Sigma series panoramic earspeakers.
The Stax SR-Sigma panoramic earspeaker was introduced in 1977 with the original low bias Sigma model (Bias voltage of this model being 230V or “Normal” – the driver used was later recycled in the very successful Stax Lambda Semi-Panoramic earspeaker).
The Sigma earspeaker design was the result of a complete reassessment of how headphone sound is perceived. Up until that point, listening to headphones and 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 and were 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 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 actual recreation of a room around each ear – a revolutionary concept that has never since been replicated. Each earcup 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.
The sound of the Sigma always has reduced very low bass because of some residual front to back driver cancellation through the mineral wool lining, but beyond that point, the earspeaker’s sound is very hard to fault compared to what one is used to.
Firstly, the sound doesn’t have the bright edge of any other Stax headphone and secondly the reproduction of vocals comes as close to free of sibilant emphasis as possible. Both are what you hear in live, unamplified music – in particular piano and voice – and the smoothness of the sound is just as relaxing reproduced as it is live. Indeed, after attending a piano concert in a relatively live hall, the Sigma/404s got the extreme dynamics of the piano without the brightness at higher volumes that the Lambda Nova Signature exhibited. This exquisite piano reproduction is unique, in my opinion. The later substitution of the 580V “Professional” bias 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 high-end frequency loss and an increase in the very bottom-end reproduction compared with the original low bias Sigma. The Sigma/404 really shines a light on just how successful Mr Hayashi’s earspeaker design concept really is. The bass that is present, until the very low bass roll-off, is of excellent quality. Certainly there are no bass instruments that move back and forward in the soundfield, nor do they disappear as they descend the scale, as I heard with the Jecklin Float Electrostatics (for example, Tony Levin’s descending Stick run in Projekct One “Live At The Jazz Café” Track 3).
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” when you hit it. Although this is not unique, I haven’t heard one with such a pronounced “correct” volume level for a track. The bass and treble just seem to be in perfect balance with the midrange at that volume. I have found the SRD-7 Pro or SRD-7 Mk 2 will do an admirable job of driving these very power hungry monsters with a good power amp pushing them. If you are going for a direct drive amp, I would suggest at the very least using any of Stax SRM-717/SRM-727/SRM-T2; Kevin Gilmore/Spritzer’s diy T2; or Kevin Gilmore’s 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. Distance from the ears and absorption by the damping material may account for these differences. The volume control levels for the Sigma/404 and SR-007 Mk1 I used to own were identical – i.e. they seem to be equally as inefficient - compared with a Lambda Nova Signature.
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.
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.
