[ericj]

Many of you are aware of this phenominon - you get up one morning bright and early, stumble over to your computer, check your email, poke around ebay a bit, and before you know it you've bought something that maybe you should have left alone.

I bought a dog-eaten Stax SR84 kit. Yes, dog-eaten. At least, there sure seemed to be tooth marks on the earpads, and the earcups were pretty much shattered, if you can call them cups - the lambda-style frame is completely open-backed. And one side's cable had been shortened about 4 inches.

And a previous owner had replaced broken struts with some sort of aluminum contraption. I almost admired the tenacity, but, I still felt pretty stupid for paying as much as i did for what i got, which was basically a pair of SR-80 Pro drivers, SR-80 baffles, and an SRD-4 transformer box, along with a bunch of broken junk.

These are electret headphones. They are true electrostatics, but with no bias voltage. They are generally considered to be good, but not quite as good as full blown electrostats.

While Micro-Seiki and Toshiba both made excellent electret headphones back in the day (which were OEMed by the likes of Sony and Rotel), with few to no compromises made in the build quality, the Stax electrets are somewhat fragile.

Edit: I hate to admit it but Sony made their own electrets, and they were fairly good. Pioneer likely did as well in their SE-100 model.

Nearly everything is plastic, and the struts frequently break.

Initially i transplanted the drivers into the SR-30 that duderuud sold me. Since the SR-30 came with one dying driver, and both SR-80 drivers work fine, it made sense. It worked but really wasn't the fulfilling, so-called "lambda junior" experience I'd envisioned while making my bleary-eyed "buy it now" purchase.

Eventually i acquired a 2nd broken SR-30 from KurtW. This pair turned out to have a better headband than duderuud's pair, but with a broken strut, and missing the pins that hold the earcups together, and a bad cable, but what turned out to be two good drivers.

After some begging and pleading, wualta sent me some parts from his wrecked SR-80 Pro set, and i finally had enough pieces to build two complete sets. One SR-30, one SR-80 Pro.

Here's the basic parts of the earcup:



As you can see, the "cup" is just an open, acoustically-transparent frame, with a thin lining of an open-celled foam slightly less substantial than window screening.



The driver element is held in with double-sided tape. Under the white heatshrink is a pair of varistors that serve to protect the driver from over-voltage.

The white heatshrink is mine. Usually in an SR-80 the varistors would be open to the air, with the leads twisted around the posts in the lower left-hand corner of the baffle. What you see here is the SR-30 configuration, which works in the SR-80 frame anyway.



That pad in the middle of the back of the driver *looks like fiberglass, but it isn't. We don't know what it is. Only Stax, Fostex, and Audio-Technica seem to use the stuff.

It's more centered than it looks in the picture.



Here it is assembled, though i mistakenly used SR-30 pins on the SR-80.

These pins are all that hold the earcup assembly together, and hold the earcups in the struts. This is actually an upgrade over the earlier Stax design where the struts themselves were the only thing holding the earcup together, and a quick move could leave your earspeaker parts scattered all over the livingroom.



The other driver is missing the damping pad.

The SR-30 and SR-80 drivers are the same size, and some have speculated that they are the same driver, but the SR-30 has a bigger damping pad than the SR-80. Since i have four SR-30 drivers and only three of them work, I cut down an SR-30 damping pad to the SR-80 size.

I used double-sided sticky tape similar to the tape that holds the driver onto the baffle here. I could have used glue, but then i'd have to wait for it to set.



Like i said before, the SR-80 pads i have are dog-eaten. They're pretty flat anyway. So, here's one of the SR-30 pads i got from KurtW next to a foam lining from an HD-25, which i figure is vaguely similar to the original SR-80 foam lining - maybe a bit larger diameter.

I'm attempting to use the same doubletape to hold them onto the frames while i make new SR-80 earpads. I've picked one of the old SR-80 earpads apart to use as a pattern to cut out some of the same suede i used on my SE-700 headband. Should be more comfortable than the original earpads, when i finally get them stitched up and stuck on.



Here they are all together.

 

[Ross1]

Thats some amazing work, looks brand new in the last photo!

 

[wualta]

Ah, but do they work?

And what of the SR-30 (SR-60 outside the US market)?

 

[ericj]

Here's a more complete shot of the finished earspeakers. I just now realized that I've had this lexan cylinder in a closet here ever since i found it when i moved in. Not quite head-sized but beats the heck out of the cardboard box i photographed the SE-700 on:



So they don't look exactly new, the printing on the struts is a bit faded, but they look pretty good for the age and everything they've been through.

The earpads keep falling off. For some reason the doubletape won't stick to the residue that i can't quite completely clean off the back of the earpads, which seems to be left over from the defunct doubletape from their previous installation.

Quote:

Originally Posted by wualta Ah, but do they work?

Why yes, they do. There is a slight channel imbalance, but that's not unusual for 'trets of this age.

Quote:

Originally Posted by wualta And what of the SR-30 (SR-60 outside the US market)?

geeze . . . . So they made the SR-40 'trets, and then ages later upgraded it and called it an SR-30 in some markets and an SR-60 in other markets? *sigh*

But it's working too. I'll post pics of that in a few minutes here.

 

[ericj]

Not as many pics of the SR-30 build because it's much the same as the SR-80 Pro.

For those that may be confused, some years after the SR-30 and SR-80 were released, Stax refreshed the drivers slightly, giving them a thinner diaphragm, and then switched up to gold paint on the energizer and struts, and called them the "Pro" versions or "Gold" versions.

Since both my doner SR-30's have white paint on the struts, I presume that they are the older version.

From three pairs of earspeakers i ended up with one good cable, one cable that looks good but has a break in it somewhere, and one cable that looks like hell and has four inches chopped off of one end.

So i chopped a few more inches off that cable, giving it a uniform length on each side, and then removed the Y strain relief, split the cable further down, and installed some heatshrink where the new Y starts.

This means that the factory strain relief - which is identical between SR-30 and SR-80 - is history. I have these rubber grommets instead.

Edit: I should clarify; the cable that came with the SR-80 was already missing one strain relief, giving me five in total. I've determined that the break in the 2nd SR-30 cable is *within* the strain relief, so i may attempt a more ambitious cable fix with strain-relief transplant some day soon.

What's interesting about KurtW's SR-30 is that instead of two small plastic-cased varistors, it has one big silicone-coated varistor, and the damping pad is loose instead of glued on.



Yes, there's another driver partially attached to the cable. That's the working driver from duderuud's SR-30, which helped me determine that KurtW's cable is certainly bad. Sometimes it's just convenient to do the cable change one wire at a time and not have to think about what goes where.



Usually the varistor would be wrapped in heatshrink, but since I've put heatshrink on both solder joints, I didn't see the point.

The rubber grommet + zip-tie strain relief is maybe half as good as the factory version. If i were more concerned, I would have put a layer of heatshrink through the grommet, but I'm not real concerned.

And here's the final:



Works well. I can't decide if there's any channel imbalance yet, which is a good sign i suppose.

 

[wualta]

Ah! Good job.

Now-- how do they differ soundqualitywise? I've placed my bets.

 

[ericj]

Quote:

Originally Posted by wualta /img/forum/go_quote.gif Ah! Good job. Now-- how do they differ soundqualitywise? I've placed my bets.

Thanks!

Gimme a few days on the sound comparison, though.

 

[wualta]

Oh! forgot to mention-- to get that old gooey adhesive residue off, don't use what everybody tells you to use, Goo Gone. Try to find De-Solv-It. Smells good too. Failing that, risk a little lighter fluid on a tissue in an unexposed place.

.

 

[ericj]

Well the goo is long gone but for some reason the surface was still greasy-slimy and most cleaners weren't touching it.

I think rubbing at it with Fast Orange hand cleaner may have done the job. We'll see if they fall off again. The doubletape really should hold it, even just four small strips like that.

By the way, had a chat with my ochem/pchem/eet friend. He tells me that the chances of an electret "losing it's charge" are vanishingly small unless you heat them close to their melting point.

Also, varistors don't tend to fail unless they're taken way over their current rating, which just isn't going to happen in an earspeaker. At any rate, their failure mode is to revert to extremely high resistance, which certainly wouldn't affect the sound. They should also visibly appear burnt and/or cracked in this state. So, the varistors are not a likely cause of electret failure.

Way-over-driven 'trets might be damaged due to the membrane touching the stators or something, but aside from that, I'm leaning toward corrosion on the stator wires as a failure mode for old electrets.

But since there's no non-destructive way to disassemble a Stax electret element, i don't really have a way to investigate that. Poking through the protective stax 'condom' to get my ohm meter probe at the stator would let me measure resistance, I guess, but, for some reason i don't want to.

 

[wualta]

Quote:

Originally Posted by ericj I think rubbing at it with Fast Orange hand cleaner may have done the job. We'll see if they fall off again. The doubletape really should hold it, even just four small strips like that.

It should, now that you've put delicious dextro-limonene into play.

Quote:

Originally Posted by ericj By the way, had a chat with my ochem/pchem/eet friend. He tells me that the chances of an electret "losing it's charge" are vanishingly small unless you heat them close to their melting point.

Yes, but when things get old, they forget. An old electret might simply not remember where it put its charge last night before bed, thus "losing" its charge. Sadly, it might have left its charge on top of the gas pump during a fillup. Now there's a unit of electrostatic charge that electret will never see again. That's just common sense.

Then there's the phenomena of "loosing" its charge. When an electret is particularly angry at the style of "music" it's being forced to play, it can take its charge "off the leash", or "loose" (that is, unleash) its charge. Normally this merely results in a small harmless zap to the offender's ear, but over time the loss of energy to the abuser's body is fatally cumulative to the frustrated electret, and it often crawls into a nearby closet to die.

Or there's the Sennheiser Unipolar fiasco, a strange tale of sabotage from within by true-believer engineers devoted to magnets and coils..


Quote:

Originally Posted by ericj Also, varistors don't tend to fail unless they're taken way over their current rating.. At any rate, their failure mode is to revert to extremely high resistance, which certainly wouldn't affect the sound.

[serious for a moment, but only a moment] Could this possibly make one side of an electret 'phone weaker than the other?



Quote:

Originally Posted by ericj Way-over-driven 'trets might be damaged due to the membrane touching the stators or something, but aside from that, I'm leaning toward corrosion on the stator wires as a failure mode for old electrets.

[picks up protest sign, lights bonfire] That's just that crazy punctuated-statorism theory you science types have been kicking around for years! [leads angry crowd in chanting "Die, electrets, die!"]

Quote:

Originally Posted by ericj But since there's no non-destructive way to disassemble a Stax electret element, i don't really have a way to investigate that. Poking through the protective stax 'condom' to get my ohm meter probe at the stator would let me measure resistance, I guess, but, for some reason i don't want to.

Well, you could stick some AC on it and look for signs of an RC circuit... couldn'tcha?

 

[ericj]

Quote:

Originally Posted by wualta /img/forum/go_quote.gif [serious for a moment, but only a moment] Could this possibly make one side of an electret 'phone weaker than the other?

As i understand it, a varistor has a fairly high impedance to begin with, and a failed varistor just gets higher, so it stops doing it's job, which i suppose could make the driver more vulnerable to abuse.

I could be talking crap here, will do more research.

At any rate, I wasn't certain they were varistors until i got KurtW's SR-30. That deeliebob in there says "Silistor" on it - which is a brand name of silicon-carbide varistor.

Quote:

Well, you could stick some AC on it and look for signs of an RC circuit... couldn'tcha?

You mean like, push a sine wave through amplifier and energizer, hook up my o-scope to the leads?

Yeah i guess i could. Some day.

 

[wualta]

Quote:

Originally Posted by ericj You mean like, push a sine wave through amplifier and energizer, hook up my o-scope to the leads? Yeah i guess i could. Some day.

Yeah. [kicks empty tin can down the street] [tumbleweed joins urban litter blowing slowly across the scene] [promised shining future never arrives]

 

[krmathis]

Great work!

 

[Carl]

Next up, do the SR50.

 

[philodox]

Quote:

Originally Posted by wualta /img/forum/go_quote.gif Oh! forgot to mention-- to get that old gooey adhesive residue off, don't use what everybody tells you to use, Goo Gone.

Why not? Goo Gone works great... never had a problem with it dissolving anything and I am pretty convinced that is mostly urban myth in any case.