[marlbly]

Hello all, long time lurker, first time poster 

So I had these lying around, some 4" Fostex/Foster 100K03 apparently forerunners to the fe103s. Though they are rated 3W nom, 10W Max / 8Ω

I have four of them and I WAS planning on making something along the lines of bert doppenberg's bd-pipes with these or maybe the bipole miniOnken etc.

Just as a very rough experiment I decided to try the OB approach. Obviously not ideal baffles  but even so I was very surprised! Obviously set up like this bugger all bass, but the mids, the air, the separation & detail. Wow.



They were swapped out of a system with some Jamo 2 way BR bookshelf speakers and the difference was very surprising. I was expecting to go "oh god this sounds awful".... but even with those dodgy cardboard baffles I could hear for example in Shawn Colvin's - Facts about Jimmy, subtle background vocals that I hadn't noticed before. Swapping back the Jamo's I could pick them out again, but with difficulty and well ... eww smeary.

So I'm sold on putting a couple on open baffles. As it's going to be a for a whole new system I'm also looking around at the rest of the setup, been doing a lot of reading and fiddling with EDGE etc. So I'm considering a bi or tri amped setup. Something like the Topping TP20 to drive the Fostex drivers Full Range, active crossover or maybe minidsp haven't decided yet. Also have a couple of 10" woofers (REDBACK C3026 - 80W 8Ω/ fs 28hz/ 93dB)  I can use, but rather than on the same baffle I'd like to try something different maybe on a separate baffle (H frame maybe?) floor level with the fostexes on smaller baffles on stands. I'm tossing up mounting the driver on it's own brace, completely separated from the baffle so I can experiment with different baffle shapes and sizes.

So does this sound do-able? Does anyone have any advice or suggestions for a first time OB build, I can find very little info on the Fostex drivers themselves which doesn't help but they look very very similiar to the fe103 build.

[zmyrna]

Check out Linkwitz designs.
Ideally 4 ways; no problem with minidsp.
Add a 8" lower mid and a tweeter.
Woofers on an M baffle (a variation of H baffle which reduces vibration), two woofers on each side.
separate baffle is good:
you can do a stand,
or a bridge like Linkwitz's latest design,
or just make the upper section sit on top of the M frame with some shock absorbers in between (like I do).

[this_is_vv]

when i started i had an MDF board. cutteda eminence alpha 15 on the baord and on top side was a full range....alpha was from from 200 to down and full range wwere from 200 and onwards...

and that my friend was my start of journey...and i never went back to box sound....put a eminence alpha in an mdf board and put a full range first on it and you will see a good difference....


V

[JCS]

There are at least two schools of thought  as regards OB design.  I subscribe to MJKs wide baffle style.  I built a modified version of MJKs passive XO OB using Fostex FF85k and Eminence Alpha 15a.  Love the sound!  But as always, there are limitations and YMMV.

http://www.quarter-wave.com/OBs/OB_Theory.html

Cheers,  Jim

[zmyrna]

those vintage drivers won't make it down to 200Hz IMHO.

[JCS]

Most assuredly, the little drivers won't make it to 200 Hz with any great output on OB.

But OB design is a whole different kettle of fish from closed box design.  If you look at MJK's article, you will see that the XO points are 200 Hz for the Alpha 15a and 500 Hz for the little Fostex.  You can't extrapolate XO design from box enclosures to OB.  In OB you need to look at driver sensitivity & response, baffle response, and XO response.  The more sensitive Alpha 15a is brought down to the sensitivity of the Fostex  through a combination of baffle response and XO slope, which fills the 'hole' you might be expecting between 200 & 500 Hz.  Sounds crazy until you work through the details.

Cheers,  Jim

[marlbly]

Sorry everyone for the late reply! Been a crazy couple of days here lately.

Thank you for the info. I'll definitely have to have a proper read of MJK's paper.

At the moment I'm trying to use what I have on hand. I'll be in trouble with wifey if I spend any more at the moment.

Looking at the specs on the woofers I've got they're probably not suitable for OB having a Qts of 0.25. There isn't a whole lot of info on them available, but I might just put them in sealed boxes to make up the bottom end and use minidsp to run the fostex as per MJK and tweak the woofers to get a good frequency response ... at least as a starting point.

I was having a good look at the minidsp site the other day and noticed their "stacked" setup. I might even try out their system with the miniAmp for driving the Fostexes... that is assuming I can use the analog out of the minidsp simultaneously to drive the woofers through their own amp.

I'm curious about the different opinions on baffle width, I read in another thread somewhere someone mentioning not wider than 2.2 times the driver, but I've seen a lot of designs both wider and smaller. I'm only a newb, but I can guess at issues with having too wide and too narrow.. has anyone any ideas about where the "sweet spot" is for drivers like these?