Adding magnets to small full range drivers?

[BHTX]

I have a pair of FF85K's on the way. If/when I get around to actually using them in a system, it'll be a simple 2-way (FAST?), most likely mated with 6" to 8" woofers. Anyway, I'm curious about adding magnets to the back of them to lower Qts and raise sensitivity. Obviously, a Dayton or S&L woofer tester would come in handy here, which is something I've had on my list for a long time anyway.

I found an old thread from '04 discussing this topic:
adding magnets

It's my understanding that since the additional magnet wouldn't be part of the actual magnetic circuit, beneficial gains obviously wouldn't be nearly as high as one might first expect. However, with a 3" full-range in a 2-way, I find the chance of any increase in sensitivity of great interest, particularly when the procedure is so easy to do. Sensitivity is a big limitation in any system involving a conventional 3" driver. On top of raising sensitivity, lowering Qes should also indirectly lead to slightly lower distortion and a slight increase in power handling of the system, especially if the electrical crossover point can be raised slightly.

I was just looking around at magnets on ebay. There's a couple of decent possibilities there. On the other hand, the cheapest way to do this might actually be to scrap old/cheap drivers. This way, any size, shape, and strength is possible and readily available.

I have a pair of 5.25" (I think?) woofers with shielded magnets from a pair of PSB Alpha Mini's my (deceased) father bought me when I was like 14. The Alpha Mini's were destroyed several years ago, so I scrapped the parts and tossed the shredded cabinets. Unfortunately, the magnets on both woofers had also come loose. Upon closer inspection, small screws holding the baskets to the magnets are broken off. This might prove to be a problem if I attempted to use those slugs to add to the back of the FF85K's, not to mention ridiculously huge , as I might not be able to remove the broken screws from the magnets after shredding the woofers to get to them.

Anyway, I found this topic interesting and thought I'd bring it back up to see what anyone else out there was doing. Something I might like to toy around with. Curious about what results can be obtained from doing this.

Here's a couple of Japanese pages with several photos of someone adding magnets to a Fostex FE126En and FE126E:
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[planet10]

Do let us know how you make out. FF8wk is a very good driver.

dave

[BHTX]

It's the FF85K, not the FF85WK.

Was your MTM with the FF85K and EL-166's passive? If so, what'd you end up with for a crossover? At first glance, might be able to get close to 400 Hz LR2 with a cap on the FF85K, but impedance will likely be a problem. Speaking of which, I remember your "mid TL" for Tysen. Interesting approach, but it's just too large for implementing into a small 2-way like I was imagining. As for the woofer, that's still kinda up in the air at this point, plenty of possibilities. I'll likely end up with either 1st order or LR2 on the woofer, depending on which woofer is used. I can see either approach working.

[planet10]

FF85K is very good. FF85wk takes it up a notch.

The MTM had a passive series XO. 3mH + 30uF. Completely phase/time coherent due to 1st order series + essentially coincident drivers.

The aperiodic midTL used in both Tysen & the MTM go a long ways to flattening impedance.

dave

[planet10]

Originally Posted by BHTX

I remember your "mid TL" for Tysen. Interesting approach, but it's just too large for implementing into a small 2-way like I was imagining.

There is the tinyTysen. A thread for Tysen and variations on FAST



dave

[BHTX]

Forgot about that one! Nice. I should take a another look at that thread.

Any idea why the glue process became different towards the end of production on these? My set looks nothing like the spec sheet.. they're both the type with excessive black goop on the dust cap / cone. Got any of those ugly K308 grills?