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!