John Buchanan

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 grilled 1977 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 first release Sigma had a fabric coated, round bodied cord that connected the earspeaker to its amplifier/transformer. 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 driver then used in the Stax SR-Signature earspeaker. This earspeaker had black grilles, lower capacitance chocolate coloured cables, and a chocolate coloured headband, compared with the original black cables and headband. A later version, the Sigma/404, was a Sigma rebuilt with high bias SR-404 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 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 before or 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. 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). The small listening cavities of those 2 phones also leads to sweaty ears, which is far less apparent with the Sigma series. 4. 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. Certainly there are no bass instruments that move back and forward in the soundfield, 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, 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. This exquisite piano reproduction is unique to this headphone, 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 in the very bottom-end reproduction compared with the original low bias Sigma. In my opinion, there are no drawbacks from 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. 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. 5. Associated equipment needed. 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 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 were 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 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 the Studer D730 -> Apogee Mini-DAC -> Audio Research LS5 Mk2 -> Studer A68 -> Spritzer Pro SRD-7 bias + transformer box. 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. This has been one of my only criticisms of 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 it! Flame suit on, hearing aid batteries fully charged LOL. Stax SR-009? Who cares?

In my opinion, the Studer run balanced out to balanced in to the SRM Monitor sounds smoother than the unbalanced connections to an SRM 1 Mk 2. It really is a lovely piece. Sold the SRM-717 and...gulp....the SR-007 Mk 1.

LOL - glad I saved AUS$14,000 by buying a demo 989 pair instead of the 2905s.

Holy shit! I've got 989s at home, but a double quad of Quads!!! How high are those ceilings?

CharlieX - whoa. Very nice combo there - listen first. Big $$$ to better

I kinda wanted to see if it was usable on a Sigma - the dimensions seem right TIA Duggeh - what's the weather like over there? 40C here yesterday - had a lovely day down by the marina chatting to a lovely woman most of the afternoon. All good.

WTF!!! I don't care about the re-capping - there may have been other reasons for that - but removing the wool from the Sigma Pros is just wrong headed. 2 obvious reasons - firstly reduced front to back isolation around the drivers will cause an even more severe deep bass roll-off than these phones already have. Secondly, the wool provides the equivalent of room type diffractive, reflective and absorptive surfaces. Remove it at your peril, my friends.

Duggeh, when you get that 4070 headband, could you please post pics of it?

Agreed - this is a real time saver.

Apart from colour differences and quality of cable differences, yes - the same.

I was very lucky. It is in absolutely new condition! Not cheap, but worth it. I've only ever seen one for sale before and it had seen far better days. Not this one - looks like it came straight out of the wrapper. Go for an SRM 717 - it's excellent also, and scales up well balanced......or......gulp......Spritzer's SRM-T2. You know you need to get that one LOL.

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Never heard the Sigma Pro, but Birgir's opinion of the other two is correct. IMHO, the Sigma/404 is the best as it has the best treble response, and a slightly smoother and deeper bottom end. I think Naotake Hayashi was a very smart man to have invented these phones. He really had a good lateral view, and came up with a different way of creating the illusion of listening to speakers in a room. He apparently liked the Sigma best, but at that time, there was nothing else in that line apart from the Sigma Pro. Now we have the Sigma/404 option.

No schematic, no manual, no box.

I'm not gonna pull any boards out - these are the best I can do in situ. There is a vertical daughter board and a horizontal mother board. The other side of the metal shield is the SRM-1 Mk2 board. Pictures by buchanan67 - Photobucket

The following is what I can understand of the theory behind the diffuse field equalisers Stax produced and an assessment of the sound of one of them. Diffuse Field Equalisation vs Free Field Equalisation. Headphone frequency response measurements, conducted with a microphone in front of the headphone driver, much as speaker measurements were conducted, observed that headphones with a measured flat frequency response did not sound as flat as they measured. Comparing the sound of a flat measured speaker with a flat measured headphone revealed extreme tonal differences that started off a whole lot of investigation into why they sounded different and how a headphone’s frequency response could be altered to make it sound like flat measured speakers. An experiment was set up as follows: 1. A loudspeaker playing a frequency sweep was recorded by a high quality, miniature microphone in one of two types of room – either an anechoic chamber or an approximation of an ordinary room (see later) – and the frequency response was charted. 2. The same microphone was inserted into a subject’s ear canal and the same speaker replayed frequency sweep was charted again. 3. It turned out there was quite a difference between the charted frequency response of the two recordings. 4. It was postulated that if the frequency response of the recording made by the microphone in the ear (see 2) could be altered by pre-equalization to ultimately match the shape of the frequency response of the recording of the same microphone when not in the ear (see 1), and further, that replay of that in-ear microphone recording could be made to sound the same as that of replay of the recording made by the same microphone in a room if that pre-equalisation was applied. 5. This gave rise to a target measured frequency response for a headphone to sound like a flat measured speaker i.e. if the headphone had a measured frequency response that looked like the target response, it should sound flat when reproducing a recording that had been mixed with speakers in front of the mixer, and sound as if one was listening to speakers in front of him/her, rather than via headphones. 6. The concept of pre-equalisation of headphones was thus born. Pre-equalisation could either be mechanical (i.e the driver frequency response was manufactured to behave that way e.g. the AKG 240DF – not so easy) or electrical (which should be cheaper, easier and field-adjustable), and meant that although the headphones now had a frequency response that had been altered to something that looked decidedly non-flat when measured, it reproduced the sounds coming from a sound source with the same frequency response at the ear canal as if recording and replay over headphones had not been introduced into the chain i.e. the headphone replay should now sound the same as sitting in the room and listening to the speakers. Two main theories of the correct pre-equalisation curve were forwarded. The first, called free-field equalization, suggested that the above experiment be conducted in an anechoic chamber (like a field, free of reflective, asbsorptive and refractory surfaces). So, to reiterate, a free field equalized headphone is designed to sound like the reproduction of speakers as if a listener is sitting in an anechoic chamber. Although an anechoic chamber is more reproducible as a standard, it was argued that nobody listens in an anechoic chamber (and indeed, most listeners find even speaking in an anechoic chamber uncomfortable) and a reasonable approximation of a standard listening area be used to conduct the above experiments. This was called diffuse field equalization. There are many things that alter sound between the release from the sound source and arrival at the ear canal. Reflections, diffraction and absorption from objects in the listening environment, reflection, diffraction and absorption by the head, hair and ears all contribute to alteration of sound before it reaches the ear canal. Diffuse field equalization, as mentioned before, is an attempt to make the replay of a recording on headphones sound like you are listening to the same recording through speakers in a non-anechoic room. Experiments were also done so that headphone users were asked to equalise various sharply limited frequency bands’ playback on headphones until they had matched the loudness of the same playback through speakers and with headphones removed. A good correlation was obtained between this method and the probe microphone recording method. The direction of sound (from the front in a reverberant field) with speakers is far removed from actually injecting the sound directly into the ear canal. Stax, pondering this problem, possibly because they couldn’t successfully mechanically create a diffuse field equalized headphone, and any electrical equalizer would have to be a custom unit, first decided to create a new headphone that coupled its own reproducible miniature room (complete with uneven diffractive, reflective and absorptive surfaces) called the Stax Sigma. It had headphone drivers that fired from anterior to posterior instead of laterally into the ear canals. The sound was bounced off irregular “wool” into the canals, creating a mechanical diffuse field room for each ear as well as having "speakers" that fired sound from the front, rather than straight into the canals. It was partially successful, but listeners either hate it or absolutely love it. Personally I love it, but they were inefficient headphones, they sounded quite rolled off at both ends of the frequency spectrum. They were also huge and very odd looking. Better drivers than the original Sigma drivers (which were the same as the then current Lambda) improve the frequency extremes, allowing the merit of the theory to finally shine through (e.g. the very rare Sigma/404 hybrid). Stax later decided (around 1988), instead, to bite the bullet and build custom equalisers to electrically equalize their latest headphone range to provide individual target diffuse field responses for each of its various then current headphones (the ED-5 for the SR5 normal bias headphone, the ED-1 and SRM-Monitor for the Lambda Professional high bias phone and the ED-Signature for the high bias Lambda Signature). The headphones could then be less bulky than the Sigma and more fashionable (see my avatar for what the Sigma looked like – it definitely had a style only a mother could love). Again, reactions to Stax engineers’ diffuse field equalized headphones literally polarized listeners into “hate it” or “love it” camps. I would guess that economically, this proved to be a dead end, and any research into diffuse field equalization has never been publicly mentioned again by Stax. As mentioned above, the three ED diffuse field equalisers were designed for three different Stax phones (the ED-5 for the SR-5, the ED-1/SRM Monitor for the Lambda Pro and the ED-Signature for the Lambda Signature). The ED-5, ED-1 and ED-Signature were placed between the source and the headphone driver and are connected by way of RCA cables. The ED-1 matched the construction and size of the SRM1 Mk2 and was finished, like those units, in either black or silver. The ED Signature matched the chocolate brown of SRM-T1/S/W. The SRM-Monitor incorporated an ED-1 and an SRM1Mk2 Professional into one large package and was finished in either black or silver, and had switchable RCA and XLR inputs. The ED-Signature would most likely also match the 404 and Lambda Nova Signature. The ED-1 equalisation (in my case, provided by a very rare SRM Monitor) sounds rather nice with the Lambda Nova Signature and surprisingly good on the Omega 2 Mk 1, despite being the wrong equalisation for the latter. The upper midrange/lower treble, in particular, sounds quite a bit flatter and the low end remains in good balance with the mid and high. As Bill Sommerweck said in his review of the ED-1 in the April 1989 issue of Stereophile, track 9 on Stax' own “Space Sound” CD changes from objectionable (without the equalizer switched in) to quite listenable with the equalisation switched in. In my opinion, there is no magical out of the head experience, except when listening to the aforementioned CD, or the Ultrasone binaural tracks (i.e. binaural recordings). These are seriously spooky. Try them with someone who is not used to listening to headphones and see what happens when you cue up track 1 or 2 of the former, or the fireworks track of the latter. Sabine whispering in your ear - Oh yes! Shower spraying on your shower cap - OMG! Now, here is where things start getting weird. I had a listen to the Sigma/404 and the SR-007 phones with the equaliser on and they both sound great - it may just be happenstance, but I've never heard "Kind Of Blue" sound so wonderful and with plenty of lower bass (which even the SRM-717 doesn't seem to match). This is strange, as the above frequency response for the ED-1 is flat in the bass, and should not be suited to the Sigma/404 or SR-007 anyway. Maybe it's a de-emphasised treble spike? I don't know, but whatever, this pre-equalisation is not just scientific theory and sounds really good. To me, the sound has gone from lots of good hi-fi parts and moved to an organic whole. To my ears, the Stax SRM Monitor is the single best piece of equipment I have ever purchased.

Thanx Duggeh.....

The Sigma/404s are definitely singing their siren song to you......listening to them now.

That is just not true.

LOL - I suspect that what you are looking at is the DAC portion of the Phillips LHH-2000. BIG dollar machine even now.

It's pretty cheap in Western Australia too! Beautiful dark red brown colour. I had some side panels made for the CD player from this wood - they look better than the original factory supplied option.

I was incorrect - the coloured wires, because they are so short, have to be connected to the back of an amplifier. The other terminals are for the speaker cables to connect to. Sorry about the senior moment. To repeat (correctly) for a normal amp with speaker cable screw down terminals, the coloured wires connect to the speaker terminals at the back of the amp, the screw terminals are used to attach the speaker wires. Sorry about that!

[url=http://earsp.web.fc2.com/kako-d/srd-5/np-srd5-3_jpg_view.htm]STAX

Presumably these are the plugs attached to the ends of the left and right speaker outputs from the SRD-5. They are of no use unless you want to connect those outputs to a speaker with the equivalent female terminals, and as these are nearly unheard of, I would suggest removing the connectors. The inputs (from the amplifier) should be bare screw down terminals also at the back of the unit from memory. You should run your amp output to the bare screw terminal inputs of the SRD-5 and the cabled outputs on the unit to the speakers + and - terminals. The speaker/headphone switch will then work correctly.

I would have a go at inserting microphones into the canals, but leave the microphones poking out of the canals with their diaphragms at the same position as the diaphragm of the replay headphone. It seems odd that most dummy heads have the microphones nestled into an ear canal, where the only correct replay for this recording will be with an in-ear-canal headphone, because if you are using an around ear headphone, the sound will be modified by the dummy head pinna on recording and your own pinna on replay. It would be interesting to try a dummy head with the microphones in several positions for different headphones. 1. buried in the ear canals for IEM replay 2. with the microphones poking out of the canal slightly for on ear phones 3. with the microphones as mentioned above for around pinna headphones 4. at the side of the head and facing forward for something like a Stax Sigma or an AKG K1000 (with the microphones placed at the centre of a prospective headphone transducer)