ferrofluid in jbl 2420 drivers?

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Hi -

I have some JBL 2420's laying around that I plan to use for a compact two-way BR project w/a 8" Faital W8N8-200 woofer. To keep doppler distortion down as much as possible, I was planning to xover around 800hz, which is fairly low but still well within the intended frequency range of the 2420. However, since the 2420 seems to have a low resonance frequency, perhaps as low as under 300hz, and I don't really want the diaphragm responding that low, I was thinking that if I added ferrofluid to the gap, this would reduce out of band excursions & work stressing of the diaphragm surround while having a negligible effect above 800hz and might allow me to run a bit lower frequency wise without distortion increase.

Has anybody done this with these older 1" JBL drivers, or for the 2440/2441?

Also, I couldn't help noticing that the large majority of newer design 1" compression drivers have much higher minimum recommended xover frequencies - 1.5Khz and 2Khz are common, as compared to the 2420 which specs 800 hz and even down to 500 hz with a 2/3 power derating (the Altec 800 series is similar). Presumably that's the price paid to squeeze that last watt of power handling capability out of a small format compression driver - maybe ok for pro use, but not optimal for designs such as this.
 
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Hello

Do you have a way to measure them?? Do you know which type of ferro to add?? Why would ferro reduce LF out of band excursion??

Why not just use steeper crossover slopes. Remember JBL did cross them over as low as 500hz in some of the early systems. Do you have the original aluminum diaphrams or Ti replacements?? If you have the Ti I would not worry all to much about work stressing them.

Rob:)
 
Hi -

Ferrofluid is widely touted as providing significant mechanical damping of voice coil assemblies (and improved heat transfer), especially at their fundamental resonant frequencies, and reducing resonant impedance peaking. For my application, I wouldn't mind trading a few db of diaphragm excursion at 300hz for no cost or effort except applying ferrofluid, so it seems like it might be a relative win-win. I have a particular HF xover topology I like to use which is basically a third order elliptical tweaked to approach a first order phase and amplitude response near xover. (For instance, with a -3db point of 800hz, a 200hz notch center is practical for a worst case stopband rejection of over 30db) With the right ferrofluid application, I'm thinking I could approach overall out of band amplitude rolloff characteristics of a 4th order LR filter (w/o ferrofluid) with this xover type while still reasonably approximating a first order filter transfer function to a half octave or more away from the xover frequency.

As far as the original aluminum diaphragms or Ti replacements, that's still a tbd because I actually have 6 of these drivers (I was originally going to use them for HT surround channels but went a different way). I am going to open them up and see what each has and what condition it is in. Hopefully, I will be able to get one pair out of them with aluminum diaphragms in good condition. I also prefer aluminum over titanium for top quality reproduction. So maybe send those two out to be remagged and put them back together and sell off the other 4.

Btw, I also have a bunch of JBL 2344 100 degree x 100 degree horns (the originals with the aluminum bolt on throats) and freshly reconed 2206J's that I'll probably want to sell off, eventually. I still think that combo would make a pretty cool medium size 2 way, or perhaps with 2204's instead of 2206's, but just decided to do something else for HT surround channels.

As far as the proper ferrofluid to add, that's a good question. I actually have contacted JBL pro about all that. Hopefully, they'll respond with some positive advice.
 
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Hello

So you have a crossover already picked. They never used ferro in those drivers, hell they have been around before there was ferro! I would be surprised if you get an answer from JBL. I can tell what they use in the 435Be and 435Al. It's made by Ferrotec and the part number is APGS38n. They only use 100ul for each driver. Not a lot just enough to wet the coil all the way round. It's has some viscosity like 30 weight oil. Hope this helps if you can't get an answer from them.

I think the 2344 and 2204/2206 would be a killer in surround set-up. I used the 2344 for years still like them warts and all. I think the 2344 sounds better crossed over at 1.5K than lower. I had mine is a 3 way matched up with 10" 2123's as mids.

Rob:)
 
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