Investigation into possible mods for the Stax SR-404

[spritzer]

As most of you know, the new Stax Lambdas suck compared to the older models. This "fact" has puzzled me for quite a while since the same drivers mounted in either the SR-Sigma or the 4070 housings just sing. In the case of the Sigmas, the Sigma 404 out paces the other two Sigma models in every area I can think of by a large margin. To make a long story short, I picked up an old, beaten up SR-404 in Japan to test out some theories. I could have bought a new one but half the fun is to take some old, neglected banger and make them shine again.

The first thing I noticed about this set was that the drivers didn't align like they should. They were only a couple of millimeters off but still, they shouldn't be like that. Here are the drivers, mounted to the aluminum baffles, and as can be seen they are a bit crooked. Click the pictures to view larger ones:

Next I tried to pull on the driver and see if it would budge. This is the result with only moderate force:

The drivers shouldn't move a millimeter in any direction so this wasn't good news. It is clear that Stax have broken off from the old formula (more on that later) so earlier today I ripped the drivers apart and was greeted with this:

What you can see here are the remnants of the double sided adhesive that Stax used to use to mount the earpads and the drivers to the baffle. They have now switched to a clear substance but it is even worse as anybody that has had the earpads on a new Lambda come loose can attest to. If the new sets made today are also like this one then these are grave news indeed.

Now there is some back story here. The older Stax Lambda/Sigma drivers were fully glued together (as in back stator, the brass rings which hold the diaphragm, the front stator and the metal grill) by a glue similar to polyurethane which is very strong but takes some hours to set. Now Stax have gone through a few variations of these drivers over the last 32 years (SR-Sigma was released in 1977) but at some point they stopped fixing the metal protection grill to the drivers with the slow setting glue and used double sided adhesive pads instead. This is a big deal since this glue is much, much weaker and the driver hangs on the metal grill which is in turn mounted to the baffle. That means any changes to the bond between the metal grill and the driver will change how the drivers dissipate unwanted energy and thus alter the sound quite a bit. The possible solution to all of this is to glue the drivers to the metal grill with some polyurethane and see if it makes any difference. That's exactly what I'm doing now but the process will take at least 24 hours so stay tuned.

To finish off for now, here are all the parts cleaned and ready to be glued again:

[faust3d]

Nice write up with cool pictures. Let us know how the sound changes once the drivers are re-glued.

[swt61]

Hmmm. All this time when everyone was saying you're a real Stax guy, I thought they were talking about your pickup truck...

Now it's all starting to make sense.

Really though, great work! I love these kinds of threads, where you really get a chance to learn something. Very interesting stuff, and it will be even cooler if this ends up altering the sound of these headphones for the better.

[grawk]

Lots of my neighbors have diesels with stacks

[spritzer]

The odds are that the sound will change for the better with the re glued drivers and remove some of the dreaded etch. If that is the case then I might even take it a step further and attach the drivers to the baffle with polyurethane.

[The Monkey]

Well done, Birgir, I look forward to the results. What did you use to clean the various parts above?

[spritzer]

That glue is nasty to say the least so I try to pick off what pieces that I can with some needle nose pliers, then I use a razorblade to clean up the larger chunks and finish off with some cooking ethanol.

[Hopstretch]

... finish off with some cooking ethanol.

Sounds like one of my nights on the town.

This is great work.

[derekbmn]

Birgir, have you thought about experimenting with having some custom baffles made out of some different material(s) ? That may yield some nice improvements.

[The Monkey]

That glue is nasty to say the least so I try to pick off what pieces that I can with some needle nose pliers, then I use a razorblade to clean up the larger chunks and finish off with some cooking ethanol.

I used rubbing alcohol, I believe at your suggestion, to get the tape residue off when I changed the pads on my LPs and Signatures. Worked well. I ind it difficult to clean out the bits of acoustic foam when it disintegrates.

Do you think the 404 would sound better with "fluffier" pads (i.e., putting the driver farther from the ear)?

[Duggeh]

Strange things can start to happen when you move drivers further away from the ears. Or change the angle. Things can sound more out of the head, but never come back in close when they are supposed to. Just a couple of mm can make a really big difference. I'll bet Stax eventually settled on and then continued on with, the distance and angle they've had with the Lambda design for so long with good reason.

This is from my experience playing with angles and distances on the AMT (and the SFI drivers, to a lesser extent) but I expect that the same principles would apply to the 404 driver.

[spritzer]

Birgir, have you thought about experimenting with having some custom baffles made out of some different material(s) ? That may yield some nice improvements.

I've never thought much about changing the baffle but I'd very much like a new Lambda chassis which isn't made out of cheap plastic and would clamp the drivers in place.

I ind it difficult to clean out the bits of acoustic foam when it disintegrates.

That is almost impossible unless you strip the drivers like I just did. It was a good move for Stax to switch over to the new dust guards.

Strange things can start to happen when you move drivers further away from the ears. Or change the angle. Things can sound more out of the head, but never come back in close when they are supposed to. Just a couple of mm can make a really big difference. I'll bet Stax eventually settled on and then continued on with, the distance and angle they've had with the Lambda design for so long with good reason.

This is from my experience playing with angles and distances on the AMT (and the SFI drivers, to a lesser extent) but I expect that the same principles would apply to the 404 driver.

That's exactly one of the issues with the SR-007A/Mk2, the drivers are too far away from the ear.

[spritzer]

An update. I've got the drivers fitted now with my regular double sided adhesive to the baffles and it is definitely an improvement compared to how they sounded before. The upper midrange is still too prominent but the etch is mostly gone..

I've encountered an issue though as the drivers are airtight now and pushing the phones towards the head results in a momentary loss of output. I have to do more testing later today but this shouldn't be a problem.

Next step should be fixing the drivers to the baffle with some urethane and then even some... ... damping. I'm certainly willing to loose some of the airy sound to make the sound more linear.

[wrecked_porsche]

I've encountered an issue though as the drivers are airtight now and pushing the phones towards the head results in a momentary loss of output. I have to do more testing later today but this shouldn't be a problem.

This happens with the Lambda Signature too right? Whenever I push on with reasonable force and speed while music is playing, a momentary loss of output. If, so, then not much of a problem because how often does one push the phones towards one's own head while listening? Of course, that is if I understand correctly what you're saying here.

[faust3d]

I've encountered an issue though as the drivers are airtight now and pushing the phones towards the head results in a momentary loss of output. I have to do more testing later today but this shouldn't be a problem.

I always had this issue as well as slight fart with my SR-303 and people where laughing at me.

[TheSloth]

I had 303's for a while and the sound had an impossible peak somewhere in the upper mids/treble that caused a horrible sharp colouration on flutes with fundamentals around F/G 2 8ve's above middle C (I think the fundamentals would be around 1.2k?). Yeah, it was that specific, but it drove me crazy.

Is that what you mean when you say the 'etch'?

[spritzer]

This happens with the Lambda Signature too right? Whenever I push on with reasonable force and speed while music is playing, a momentary loss of output. If, so, then not much of a problem because how often does one push the phones towards one's own head while listening? Of course, that is if I understand correctly what you're saying here.

Yup, it's the same thing. It's not a big deal but I wonder if it may become an issue once the drivers are permanently glued to the baffle. \

I had 303's for a while and the sound had an impossible peak somewhere in the upper mids/treble that caused a horrible sharp colouration on flutes with fundamentals around F/G 2 8ve's above middle C (I think the fundamentals would be around 1.2k?). Yeah, it was that specific, but it drove me crazy.

Is that what you mean when you say the 'etch'?

That's exactly it and it drives me mad too. The SR-007A/Mk2, HE90 and SR-Omega all have it to some degree as well. What makes it even worse for the SR-303/404 is the forward nature of the phones.

[faust3d]

Could it all be attributed to poor energy dissipation? If this is the case that I bet the intensity of this etch would change from pair to pair depending on how well the glue set and is holding the driver elements.

[Dusty Chalk]

That's exactly it and it drives me mad too. The SR-007A/Mk2, HE90 and SR-Omega all have it to some degree as well. What makes it even worse for the SR-303/404 is the forward nature of the phones.

Is it something that a little notch filter could take care of?

[spritzer]

Could it all be attributed to poor energy dissipation? If this is the case that I bet the intensity of this etch would change from pair to pair depending on how well the glue set and is holding the driver elements.

There could be some variation from unit to unit but this is a fit and forget glue so there aren't many ways to do differently. It's probably a combination of unwanted energy inside the drivers and just how damn bright these drivers are. That works in their favor in the Sigma and 4070 chassis so some damping might be in order to tame the upper midrange but it has to be done without lifting the upper bass too much. I'll probably "borrow" the cups from a Lambda Pro to see how the mineral wool affects the drivers but I'd really like to use some material that is easy to find almost anywhere.

Is it something that a little notch filter could take care of?

Certainly to some degree but since it is caused by a number of things which have to happen at the same time I simply don't know how effective the filter would be. Take the SR-Omega for instance, you have to play them relatively loud and then hit the right notes for the etch to appear.

[spritzer]

I've held off fixing the drivers permanently to the baffles so that I could play with damping a bit. I have a pretty good idea what the better glue will give me (deeper more controlled bass) so that can wait but the biggest issue is the upper midrange/lower treble peak. The etch involved has mostly gone but the drivers were obviously made to sound like this and the only way I can change it is to crack them open and change the diaphragm. Not going to happen though as that would make this a one off and I don't want all the PM's asking me to do this "mod" to some other sets.

What I can do is to try and damp it away but that's not easy either as damping always affects some other parts of the spectrum, most notably the bass. I've never been happy with the Lambda bass (overblown and lacks precession) so I really don't want to boost that area at all. I could also put in a port to raise the midbass which should balance out the sound a bit but that requires precision drilling at an odd angle into the baffle and that would be a last resort.

I had two types of felt at hand (which I got from Eric with the 120ohm SFI drivers) and they do boost the bass a bit and also tame the bump but not enough. Looks like I will have to go to the arts and crafts store after the weekend and buy samples of everything they have.

[Torpedo]

Looks like you're going through an interesting path. Good luck finding an acceptable compromise.

[spritzer]

Thanks and I need all the luck I can get since this won't be easy. Back to the salt mines...

[faust3d]

Try a combination of open cell foam and felt. Can't wait to see what sound you will get out of them. I bet anything of that stuff will reduce the soundstage depth and width while improving precision.

[spritzer]

That's exactly what I'm after. The soundstage on the Lambdas is huge when called for so it can afford to be trimmed down a bit. We'll see if some extra foam does the trick.