Fostex FE207E - Magnet Cutting - diyAudio
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Old 31st March 2009, 01:40 PM   #1
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Default Fostex FE207E - Magnet Cutting

I'm interested in the FE207E on OB at the moment (after hearing Mssr. Phil Townsend's Feastrex D9nf on OB which is hauntingly sweet and unforgettable.)

Rather than go for the supporting woofer (which is what Phil did), I thought I might try to raise the Q of the FE207E. Series resistance was okay but only gets me part of the way (I have a 2-watt Bottlehead).

So I'm wondering if anyone has (*gulp*) modified the magnet assembly. The FE207E has a thick metal casing on the back, which encloses two magnets arranged in a bucking configuration. I have no need for shielding.

Ignoring the sacrilege of hacksawing a nice driver, has anyone been brave or crazy enough to cut through the metal encasing to remove one of the magnets? Or if not, why not? And what might be the expected results of removing one of the bucking magnets?

I'm assuming the result would be sort of like an FE206E with a radically undersized magnet, with higher Q, less of a rising response and worse transient response, and perhaps a lower HF rolloff?
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Old 31st March 2009, 02:11 PM   #2
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I do not believe that your idea will ever yield adequate bass output. When you passively mate a woofer (no EQ, no boost, and no active crossover with a second amp) to a full range driver you are looking for two properties, high Qts and an efficiency that is 6 to 10 dB greater then the full range driver value. Both of these characteristics are used to counteract the rolling off low end that is driven by the baffle size and shape. While your method may increase the Qts of the FE-207E it will not lead to a rise in low frequency efficiency so in the end you will not gain very much in the way of additional bass output. I recommend you look for an appropriate woofer(s) and design a passive crossover, that combination works really well.
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Old 31st March 2009, 02:54 PM   #3
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Hi Martin, thank you for your reply. I cannot doubt that you are correct.

The baffles are 46" wide (24" baffle proper, with two asymmetrical wings). I'm not looking for anything below 100hz, which I should probably get anyway. If I could separate the two magnets, it would be an interesting thing to be able to adjust the magnet strength.

(A poor man's passive field-coil, sort of). However, maybe I should quietly pursue this foolishness in private! Thank you again for your reply. I will bear it in mind before any hacksawing.
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Old 31st March 2009, 03:06 PM   #4
MJK is offline MJK  United States
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Quote:
(A poor man's passive field-coil, sort of).
Before taking drastic actions like cutting on the magnet, why not wind a field coil around the magnet OD and then power it in reverse polarity. You could then adjust the BL term up and down by adjusting current and current direction. I guess there would be some risk of demagnitizing the permanent magnet but if that did not occur then the experiment would be totally reversible. Once you cut away magnet material it would be kind of hard to put it back.
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Old 31st March 2009, 03:39 PM   #5
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No matter what you do to the FE207 it will still be excursion limited.

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Old 31st March 2009, 03:41 PM   #6
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Hi Martin, that is a fantastic idea! Continuously adjustable and (presumably mostly or entirely) reversible. I will see if I can attempt this, cautiously. Thank you for the idea!
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Old 31st March 2009, 03:42 PM   #7
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Quote:
Originally posted by planet10 No matter what you do to the FE207 it will still be excursion limited.
Hi Dave, true. Aren't you curious though?!
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Old 31st March 2009, 03:51 PM   #8
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Quote:
Originally posted by rjbond3rd
Aren't you curious though?!
Not in this case

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Old 1st April 2009, 01:07 AM   #9
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If you want a field coil, have you considered throwing the magnet away, & replacing it w/ a field coil?
Building some Lowther conversions now, you'll hear Phil's...
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Old 1st April 2009, 03:27 AM   #10
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Quote:
Originally posted by planet10 Not in this case
Dave, I like your civilized yet candid style.

Quote:
Originally posted by serenechaos If you want a field coil, have you considered throwing the magnet away, & replacing it w/ a field coil?
Building some Lowther conversions now, you'll hear Phil's...
Hi Robert, that sounds really exciting, and I hope to learn enough to be *gulp* dangerous. The idea of adjustability is really appealing.
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