waveguide with a little fostex

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Hi all,

Ive been playing around with 2 drivers on an open baffle. A little Fostex ff85k fullrange and a JBL 123a 12" woofer. I think they sound really great together. Having said that it seems I can never leave well enough alone...

I was thinking about mounting the ff85k in a shallow waveguide roughly the same diameter as the JBL (12") and finding a good crossover point that integrates them directivity wise. Figured this would help in time alignment and stands a chance of making things sound better....ha!

I should say this isnt a "FAST" system. The ff85k is only handling upper mids and higher.

Any experiences/thoughts out there on using a midtweet/fullranger this way?

MrKramer
 
Thats good to hear. How did it effect the sound? Did you notice any difference?

As for the x-over, I guess it depends on whether or not a waveguide thats 3" deep ( distance to align them) can actually control the dispersion. If so, Ill cross it right around where the JBL begins to beam. In the neighborhood of 900-1200hz.

Up till now, Ive been crossing it from 300-750hz. Sounds good no matter where it crosses but I do notice issues with excursion when I cross it lower and play louder.

MrKramer
 
123A ff85k

I designed an OB around this combo. Try using 10uF highpass and a heavy duty woofer grille with a 4x4 square of absorbent material (I used 1/4" F11 felt) centered on the grille to absorb highs from the woofer. The result is a near perfect 2k first order crossover. To get some directivity (less than you may think is required) around the edges of the ff85k I built up a few layers of felt for absorbtion. Tilt in the baffle was used for time alignment. There may be a writeup on enjoythemusic's DIY section soonish.
 
Yes. The felt is the only lowpass. One of many nice parts about it is that it makes the filter act as a 1st order electrical AND acoustical, so a textbook highpass on the fostex aligns nicely with the .6mH Le to create a near-perfect first order filter. The distortion products in high freqs are filtered by the felt as well as the top end of the response, which is an added benefit. It also seems to counteract lobing somewhat by interfering with opposite edge cancellations.

Sound is superb. They are the best speaker I've made, and amongst the very best I've heard. They're not without their limitations and are not very attractive, however. Being dipoles, they're definitely sweet-spot oriented.

The 123A, within its limited Xmax, makes some very serious deep sounds.
 
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Hello Badman,

Thanks for sharing your good work. A few questions if I may ask....

What is the size of your open baffle? And did you experiment on the 123a by varying the thickness(es) of the felt to achieve a lower crossover point. Perhaps, going down to 1kHz would be better for the 12" and get a little bit more of goodness from the FF85.
 
Hello Badman,

Thanks for sharing your good work. A few questions if I may ask....

What is the size of your open baffle? And did you experiment on the 123a by varying the thickness(es) of the felt to achieve a lower crossover point. Perhaps, going down to 1kHz would be better for the 12" and get a little bit more of goodness from the FF85.

The baffle is trapezoidal, attached is a dimensional drawing. I tried various XOs, but all those requiring electrical woofer compensation paled compared to the 2k "wide open" arrangement.
 

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