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Thoughts on Upgrading Stax Sigma Normal Bias to Pro - SR-303 drivers or SR-404?

post #1 of 16
Thread Starter 

I'm in the beginning process of converting my Normal bias Sigmas to Pro, and am looking for thoughts on what to expect between either the 303 and 404 drivers in the Sigma frames. There are a few threads about the 404 versus the Normal bias Sigmas, but not much that I could find on using the SR-303 as a donor. Anyone done a conversion with 303 drivers, who has also heard the 404s?


Initially, I had hoped to find someone to do the conversion for me. I emailed around a bit, and found no one was apparently willing to do the conversion any more, so I figured I would look into doing it myself. After reading through a few threads, I decided to look for a suitable donor, rather than just tear into my SR-404s blindly. I eventually found a broken set of SR-303s pretty cheap on eBay, that had various issues with broken and missing pieces on the frame, but with good drivers. So I started with removing the SR-303s drivers, as I figured if I messed-up the removal of the 303s, I woudn't be too upset. That went pretty well, so I carefully removed the normal drivers from the Sigmas, which also went fine. I took lots of photos, if anyone is interested. So now I have one SR-303 driver installed and will soon have the other installed. I'm using a light-coat of rubber cement to attach the driver. Assuming all goes well, my next thoughts will be on whether to leave the SR-303 drivers in-place or whether to open-up the SR-404s and install those drivers. Also, what to do with the normal bias drivers that were in the Sigmas originally? Anyone know what other models Stax used them in? I guess I could put them in the SR-303 (or 404) frames and add some new parts to fix the broken pieces and then have some Normal Bias Lambdas in 303 (or 404) frames, or maybe just sell the original drivers to someone who wants spares. They would be unique with SR-303 (404) frames and "kettle-cord" cables, thats for sure. I'll certainly hang onto the drivers until I am sure I don't hate the conversion and want to return them to normal bias.


Another other question for all you Stax experts out there: One of the drivers of the 303 had it's black metal protection mesh pushed-in. From what I read from other threads, this is just cosmetic right? Can anyone confirm this is indeed just cosmetic, and won't affect the sound in any way? The drivers seemed to be balanced before I removed them from the original 303 frames, but I just want to be sure I don't waste my time on a set of drivers that won't be matched acoustically. Here is a photo of the one driver that has the pushed-in protective mesh:




I'll post more photos of the process if anyone is interested. Thanks in advance to those that might offer input or helpful advice in the conversion!



post #2 of 16

The 303 and 404 drivers are exactly the same. The differences lie in the earspeaker cable quality and the colour of the housing.  The 303 cable isn't twisted ala the 404 at the cable entry point, so it lets the cable sit better.

Edited by John Buchanan - 10/20/13 at 2:16am
post #3 of 16
Thread Starter 

Thanks John, that's just the sort of info I was hoping to learn. I've enjoyed reading your many posts regarding the Sigmas. In fact, your experiences were a big motivator in my wanting to do this conversion. So again, thank you very much!




The 303 and 404 drivers are exactly the same. The differences lie in the earspeaker cable quality and the colour of the housing.  The 303 cable isn't twisted ala the 404 at the cable entry point, so it lets the cable sit better. 
post #4 of 16

Thanks for the kind comments, but I would like to give a shout out to Edstrelow for sending me down the 404/Sigma path - one I never regretted for an instant. It expanded on the limited good points of the Sigma. The Sigma Pro is just as good IMHO.

post #5 of 16

Did almost the same a few days ago.

Have a pair of Sigma NB with kettle cable that I took apart.

Diassemble those old Sigma's did go pretty well.

1) lifted the earpad corners a bit to be able to remove the small screws

2) removed the two screws that is holding the headband attached to the housing

then it pretty much opened up by itself


removing the cables from the drivers was a bit delicate since the glue holding the drivers together has lost a significant part of its strength, so pulling the cable needs to very gentle while ensuring that the driver is not diassembling itself :)

I decided to cut the cable when removing it from the housing. It is possible to cut the rubber stuff that fixes the cable to the house in such a way that it is possible to remove the cable.

Since I didn't need the cable, I decided to just cut it clean.


cutting the driver free from the baffle was easy since the glue almost has no strenght left (being 35 years old by now).

cleaned the surface with a very sharp razor blade, and then some isopropanol.


Taking the 303's apart was almost as easy, but only requires the earpads to be lifted in the corners to be able to remove the small screws, and then it is ready to open.

the cables are soldered to the driver. The middle connector (the foile) has a spade-connector that is connected using a nut/blot.


I am a bit worried that the soldering heath might cause the membrane to be damaged.

So I decided to loosen the nut/blot, and use an iron to de-solder the two stator cables.


Cutting the 303 drivers loose was a bit more tricky as the glue was rather strong ... took me 10-15 min for each driver ... the 303 has some sort of filter inside the grill frame that one needs to be careful not cutting into.


I decided to glue the 303 driver onto the Sigma baffle as-is; meaning I did not reverse the driver (the visibil red ring was cleaned with isopropanol before the gluing operation).


I put glue onto the Sigma frame (approx 2mm thick and approx 1mm from the edge of the hole).

Then I applied the driver onto the baffle very gentle,

took a look from the other side to check how the driver was centered (I guess one needs to be rather accurate placing the driver and only small adjustments can be done)

then a firm pressure was applied to the driver and left it to dry for about an hour or so.

One also needs to take care that the driver connectors are not getting too close to the bottom edge

I had a cable from a 404 set that I decided to use instead of the 303 cable. Although the grey color of the 303 does match the housing much better than the delisious brown Stax color :-S


Attaching the wires to the drivers is rather tricky as there not much space to work in when the cable is attached to the housing and the individual wires are rather short.


I soldered the lower (nearst to the baffle) first.

Then I attached the foile using the nut/bolt.

Last I soldered the upper stator wire.

(I have never been soldering Stax wires before - the soldering starts turning shiny and then it turns grey, which is kind of worrying to me; according to Spritzer that is how they turn out. I am pretty sure they are not underheated).



I have reversed the stator wires in the Sigma

I don't know if it is the right thing to do.

Anyway , my thinking is that the way I have mounted the 303 driver onto the Sigma baffle will result in the same side facing towards the ear, and since the Sigma drivers are reversed in phase due relying on reflected sound rather than direct, my conclusion is that I need to switch the stator wires.


The filter inside the grill frame needs to be fixed since it is no longer fixed in between the driver and the baffle. I used some strong tape.


Getting the mineral wool back into place is also a bit tricky, but if one is carefull when opening up the Sigma, most of it can be left in place apart from around the driver and around the cable/wires.



I have never had a pair of Sigma Pro, so I can only compare the Sigma/404 with my Sigma NB (I have both flat cable and another kettle one).

The Sigma/404 definetly shares the same signature as Sigma NB,

I have much the same impression as John Buchanan:

Gilency, the Sigma/404 sound is more accurate than the Normal, but it's still too fat on the bottom end to be neutral, and has a very low bass roll-off. But those Sigma/404 phones......while the Lambda Nova Signatures are as cool as a cucumber, the Sigma/404 is sweaty, smiling and happy. A very involving phone



Thanks to John Buchanan and Edstrelow for sending me down that road.


Big, big thanks to Spritzer for giving advices.



post #6 of 16
Thread Starter 

Thanks for the detailed post Soren_Brix. 

It sounds like you took a slightly different approach, but still arrived at positive results; nice work!


After installation of the second driver on my set, I noticed I had an imbalance one one side. Upon close inspection, I determined the metal protective mesh was in contact with much of the driver (as I noted above had concerned me earlier), but it definitely was negatively affecting the sound. Once I removed it, and re-installed it, I had good balance again between the drivers. I'm still using the 303's cable and there does seem to be a significant difference between the low-end of the stock SR-404s and the SR-303 drivers transplanted to the Sigmas. I'm sure some of it is the difference in the case-style between the 404 and Sigmas, but it's much less low-end than I had with the Normal bias drivers. I double-checked I didn't reverse the phasing of a driver, so that's not it. So I guess I will have to hunt-down a set of the 404 cables and install them into the Sigmas now. I am certainly enjoying the mid-range as before and can definitely hear much better high-frequency extension than before. It has been a very worth-while adventure so far, but its looks like I have some more work to be done before I call this project completed.


At some point, I need to get busy on a new amp for the Sigmas too. I bought an "Egmont" amp from Spritzer which blows away my SRM-T1S, but I was hoping to get into something like KG's Tube-amp #1 with 6S4As, to really light-up the Sigmas. I have a nice Sansui Q-3535 stereo tube amp which works great with a SRB-7B on the low-bias sets, but I want to get the detail from a direct-drive tube-amp AND the dynamics I get from the Sansui and transformer-box. I can't swing a BHSE anytime soon, so I'm just trying to get as close as I can "for now". What are you using to drive your Sigmas? I know John favors his Class-A SS amp and transformer setup. Maybe I can get a nice SE 300B jobbie-built and try that with a transformer setup: I dunno.... around and around we go eh?



post #7 of 16

Hi Cameron


I am glad to hear that you succeeded in getting yours to work as well.

One thing that I did different from Spritzer is that I took the driver including the protetive mesh and mounted it on the Sigma baffle; afaik Spritzer reverse the driver, and therefore needs to remove the protective mesh and mount it on the other side.

I have just reversed the stator wires instead.


AFAIK the housing determines much of the upper bass lower midrange boost the Sigma's are having.

To my ear the NB Sigma and Sigma/404 does have similar sound signature, although the 404 does seem to have greater detail in the midrange and much more high end than the NB has.

The 404 sounds to have better low end than the Sigma/404's as you yourself have noticed.


I am using a KGSS (bought from Spritzer) to drive my Sigma/404's.

I also have a SS amp/transfomer: Lyngdorf TDAI 2200 + SRD7mkII and this combi do sound very good and very engaging (I rate that very high) - actually I have bought a couple of Lundahl 1630PP transformers and a Spritzer BIAS board to see how good it can become when everything is best-in-class-sort-of ;-)

If you browse through some of the Stax threads you'll see a lot of people using the SRD7 along with even 007's


Right now I consider doing another conversion of my 404's to another Sigma/404's and put the third Sigma NB I have up for sale :-)

post #8 of 16
Thread Starter 

Lord help me: I just bought a pair of minty SR-007 Mk2s with a SRM1 Mk2!

Both come with factory-fresh looking boxes and the 007's case.

I haven't even finished the Sigmas yet! Well, at least I can compare them all pretty soon eh?

Hey, can anyone point me to a thread about Spritzer's mods to make the Mk2s more Mk1-ish (or is that on the "other" site)?


Okay, now what do I need to sell-off to pay for this new lot.....




post #9 of 16
Thread Starter 

So John, 

Did I read somewhere that you sold you Sigma-Pro/404s and bought a Sigma-Pro proper? Care to expound on the virtues of the "real" Pro verses the Sigma/404 "hybrid"? Of course inquiring minds want to know: do I sell the Sigma/404s I have now and pay-up to get some Sigma Pros? I have the 303's cable on currently, but was thinking of cutting-up a Stax extension cable so it would match the grey and black on the old Sigmas. I did sell-off the SR-007s and SRM Mk2 amp, they were really nice, but I much preferred the open and natural sound of the Sigmas. 



post #10 of 16
Thread Starter 

Hi Soren_Brix, 

I revised my set and followed your approach with reversing the mesh and drivers / polarity. If I could find a partially built KGSS, I've got a power supply board from one, but it looks like no one has the amp PCBs any more. I've been using my Egmont from Spritzer as my "daily" amp, as it works really well. However, on a lark I tried messing around with my Sansui Q-3535 (push-pull EL34s) as a drive-amp and it actually works really nicely! Its not a BHSE, but it does work about as well as the Egmont. I am sure if I were to optimize the Sansui to be more Stax-friendly, it would be worthwhile. Did you ever build-up that box with the Lundahls? I've wanted to try the same thing and would like to head your thoughts on that approach.



post #11 of 16

Hi Cameron


I have not yet done anything to build the Lundahl transformer box, although I have all neccessary hardware - my life is a mess at the moment :-S    ... I might give it a shot this weekend however, and let you know afterwards :-)



post #12 of 16
Thread Starter 

Hi Soren_brix, well that certainly doesn't sound too good; I hope the "mess" gets better for you soon!!


Yes, please do let me know how you like it and what amp you end-up powering it with. It sounds like Class A power is the way to go: but tubes or sand.. that is the question!



post #13 of 16
Thread Starter 

Can any owners out there recommend where to get ear-pads to use with Sigmas? I've emailed Electromod and Audiocubes, and both say "no longer made / not available". They look to be the same size as SR-303 / SR-404 ear-pads, but I do recall reading in a thread that the Sigmas ear-pads were shaped differently. Can anyone chime-in with recommendations?




post #14 of 16

Hi Cameron


According to Sprtizer you can replace the Sigma pads with 202 pads ( http://www.head-fi.org/t/648803/stax-sr-sigma-spares#post_9111679 ).


Looking at the 303/404 pads (that I have) they seem to be angled a bit which the Sigma pad's are not, so I guess the 202 pads are not angled.



post #15 of 16


I just aquiered a Stax SR 303. 

I'm really interested to get the pictures you take dismantling yours.

One driver (cell) still glued, but partially only and it need to be re-centered and re-glued.

As all the case is in plastic, I'm quiet affraid to brake the frame dismountling it.

If you have suggestions......

I may also need / wish to replace the cable that is now still ok, but may not last long.

The unit was unused for almost 10 years.


Thanks in advance.



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