Analogue revived !

Modifying the Fostex 126

The stock Fostex 126:


And a rough baseline frequency plot of the stock, unaltered Fostex 126. These are real world, in room measurements taken at low volume, inches from the dust cap…  just to show the difference in response of various dust caps. We are looking at 5Khz and up. See the rising response and the well known peak at 7Khz? Some call this a shout, others say it adds presence. My opinion is that it is a distraction if your source material is high resolution.


Next, I replaced the stock paper dustcap. Not wanting to be outdone by the Accuton ceramic coned drivers, I used an ultra high-tech fractal lattice of crystalline calcium carbonate, also known as eggshell. 😉


The eggshell dustcap frequency response: Notice the sharp drop off after 12Khz. But the 7K peak is completely cured.


With a phase plug: While flattening the response a bit, it does nothing for the rising response curve. And things are dead above 12Khz. It sounds unnatural.





Open, with no phase plug or dust cap: Peaky and dead on top. Bass drops off dramatically because the former is vented with 4 small holes. You can see one former perforation in the photo below. To maintain bass, especially in a horn enclosure, the former must be covered.





And finally my favorite: A piece of mylar packing tape glued to the former, and then shrunk tight with heat. You have to hear this to believe it! There is more going on here than just the flatter frequency response. Vocals have such detail that you can hear the spit gurgling in Adele’s raspy throat! Ottmar Liebert’s plucked strings have timing and presence that I have never heard before!


And the mylar dustcap frequency plot. The rising response is gone, and yet there is the impression that sensitivity has increased. Dispersion is more natural with a noticeably larger sweet spot. Also, we now have useable response to 18K.


I was very surprised that the dust cap on the Fostex 126 has such an influence on its sound. I hope this information helps bring great music to your life.


By request here is exactly how to do the Greenvalve Dustcap Mod on the Fostex 126 speakers. Be forewarned, it is ugly but if done correctly the improvement in sound is substantial.

You need these items:

-Mylar packing tape. The kind that makes a crinkly sound when you ball it up. Not the softer stretchy kind. (I used ‘Duck’ brand, distributed by ‘ShurTech’) I think it came from Walmart.

– A short piece of 3/4″ sched 120 PVC pipe.

– A utility knife or razor blade

– A lighter

– A ‘Q tip’ (or toothpick)

First: Remove the stock paper dust cap… be careful not to cut the black adhesive at all. The voice coil wires are easily cut, as they are wound around the former. I make a slit down the middle of the dust cap and pull with two fingers while working my way around the former with the razor blade. With the dust cap removed it looks like this:

Next: We need a round piece of packing tape. Cut off a piece of packing tape and lay it on a hard, flat surface, sticky side up. Hold the 3/4″ PVC pipe firmly down on the tape while cutting around the outside diameter of the pipe. Carefully remove the circular piece of packing tape from the bottom of the piece of PVC pipe.

Getting closer: Using a ‘Q tip’ that has one end cut to a point, take the circular piece of tape and center it on the former, sticky side down. Use the ‘Q Tip’ to tuck the excess tape around the former. Rub around the diameter of the former to make sure the tape does not come loose. I have used glue, but don’t think it is necessary as I have not had the tape ever come off by itself. Glueing does make the mylar lay flatter, but I noticed no difference in the sound.

Finally, the most difficult part. Using the lighter to heat the packing tape. The tape should be tight around the former. You don’t want it to flop around as the speaker moves in and out. The best way to do this is to hold the speaker face down. In a circular motion move the flame around the diameter of the cone, shrinking the tape from the outside towards the middle.

You will know if the tape is too loose if you hear harshness, distortion in vocals or any buzzing during sustained notes. You want the tape as tight as possible without melting a hole in it. It can look this ugly, but no holes are allowed.

Add a drop of hot glue, if you think you will be able to hear a difference.

Mike Thornton’s idea of using a blow dryer instead of an open flame to shrink the Mylar tape works very well. Also, using the soft foam gasket tape around the inside of the former allows the modded 126 speaker to play at higher volume without the tape starting to make noise.


Thats all thats too it. Enjoy!


13 responses

  1. Pingback: Experiments with the Fostex 126 « GREENVALVE AUDIO

  2. Mike Thornton

    I used a blow dryer on a blown 126-worked really well-can’t listen to it but at least i know it works!

    February 24, 2017 at 8:34 pm

    • That’s a much better idea than using a lighter! I will use a hair dryer for shrinking the Mylar tape from now on. If you try this mod, also use the foam speaker gasket tape to circle the inside of the voice coil (as shown in the last photo.) It lowers distortion at high volume. Thanks Mike!

      February 25, 2017 at 12:20 am

  3. Mike Thornton

    I didnt see the gasket tape inside the voice coil-i’ll probably do a working 126 today-thx for the tip!!!!

    February 25, 2017 at 6:31 am

    • Mike Thornton

      I also spoke to planet 10-well actually owners wife since he is in hospital atm.I told her to make him aware of your mod.He uses EnABLE mod to fostex cones and sounds like a great mix with your mod.He gets amazing reviews for his mods to Fostex and other drivers.None the less an interesting site to take a look at…..Mike

      February 25, 2017 at 6:58 am

      • Mike Thornton

        He also adds ductseal to top and bottom of magnet-i added this also before i replaced 126.Perfect for adding stability to cast basket.Results in a cleaner sound and a worthwhile mod.Only have one side done since getting 126 off with whatever Ed uses in his attempt to damp is a chore.I’ll probably have to use a razor blade to loosen up the seal.But the one i duct sealed is a positive improvement.Nice full range stock but plenty of room to improve….Mike

        February 25, 2017 at 7:24 am

      • I have heard the damped basket mod, also eNable treatment, and several other mods that ‘refine’ the Fostex 126. They are not as drastic a change in sound as replacement of the dust cap with Mylar tape. I like the sound of the modified 126 more than using the Heil tweeter and the stock 126 with a crossover.

        February 25, 2017 at 8:19 am

  4. Mike Thornton

    I’ll take your word for that-but i just installed ductseal on good right 126.Dramatic difference!!Before only left was treated and i could hear improvements in left so had do do right 126.Everything about 126 improved.Now i need to do your mylar mod and i think 126’s are good to go!!!!Not bad for $2 investment in duct seal…..Mike

    February 25, 2017 at 10:24 am

    • Mike Thornton

      Haven’t done mylar yet-but did use duckseal to damp the ringing basket on 126.Results are truly amazing for the $2 cost at home depot for 1lb of it.Do it and you’ll ge wider and deeper soundstage-pinpoint imaging-perfect tone-transient attack substancially improved.Sounds like a component upgrade-but in fact only $2 and maybe 30 min per speaker to apply.Believe me this isn’t magic dots or other bs-this really works.Try it and let me know what you think.Open baffle should be even more beneficial but i’ve only used it on horns so far …. mike

      March 1, 2017 at 1:57 pm

      • Mike Thornton

        Even finally talked hard headed Ed into trying it but he went ot Lowes instesd of Home Depot and says he won’t try it on his main horns.Because he says it will thro off his voicing of the truth.How in the hell could improving your speakers do that.If anything it will make the truth sound better.If not the truth is not really the truth lol…..Mike

        March 1, 2017 at 2:06 pm

  5. Mike Thornton

    i auditioned the truth a year or so ago and was really underwhelmed.It was very pedestrian in my system but i listen to 90% vinyl.No gain a no go.But i tried to hear on cds what the hoopla was all about and just didn’t hear it.And the fact that my truth had rca’s and they were all off center.Just looked like a quick slap together.And sounded not special at all.I have 3 active tube pre’s in my house and it couldn’t touch any of them.It just sounded off.Then today Ed told me he voices his truth with the song radar love.I about busted a gut laughing!!And he was fine with his horns and would never mod them because thats what he voices his system.Think in the meantime i’m getting into the open baffle landscape.The horns are fine but limited and i’m looking for a new buzz!!!Horns will probably end up in the basement used as a video only system.My living room will be where i listen to music only……Mike

    March 2, 2017 at 12:04 am

  6. Gumi

    i have fostex up163 with aluminum dustcap, could improving the sound also with this mods? thanks – Gumi

    July 6, 2017 at 7:17 am

  7. FireTriode

    Very interesting modification. “I like the sound of the modified 126 more than using the Heil tweeter and the stock 126 with a crossover.” Now that’s a strong statement.
    I just happen to have a pair of 126’s here, one with a dented in dustcap, so they would be perfect to try this modification on.
    Has your tape stayed on so far (It’s been a few months now)? Just wondering about longer term permanency of the mod.
    I can vouch for both the duct seal and the eNaBl mods having great, positive improvements with these. I’ve heard both, individually and the results are cumulative. Now, if I can do all 3 at once…

    October 8, 2017 at 8:11 pm

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