Another FHXL Build

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So, I've been living with my FHXLs for about 2-3 months now. I enjoyed them when they were first completed, though my first reaction was that they were a bit bright on top for my tastes. Compared to the previous speakers in their place (Atlantic Technology System 350 THX), they cast a much bigger sound stage, but were still bright to me.

I finally found some time to carry them down into the basement and conduct some measurements today. As I suspected, there was a pronounced wide-band hump in the 5kHz to 15kHz region. It stood out on some recordings, but not on others. The primary place where it was obvious was with spoken voices - television shows broadcast in 2.0 sound format were typically the worst. After adding a simple notch filter, the difference is astounding! The upper-mids and treble are reigned it a bit from where they were, but is still plenty bright for my tastes. The part that I didn't expect was that bringing the treble in line with the lower midband range allows more of the bass to come through!

I know I said this before, but WOW - I am just FLOORED with the bass that the FHXL design puts out! Especially after adding my filter tweak! I was enjoying them so much, I got myself in trouble with my the rest of the family for excessive volume after spinning a few albums... At first, I actually had to get up and touch my sub to make sure it was really off! I tend to be a bit of a bass head, so this is really quite something!

I'm just beyond giddy with delight! These speakers sound AMAZING - I'm so glad I spent a few hours measuring and tweaking. I ended up paralleling a 1.0uF cap + 0.27mH coil + 28R and putting this in series with the positive speaker terminal. It brings that 5-15kHz hump down by about 8dB.
 
And some data from the FHXL and Alpair 10.3 driver. It seems nearly pointless to discuss speaker response without adding a few graphs...

In the attached graph, the purple line is the speaker straight from the amp with no correction at all. After a few hours of playing around, I hit on the green response curve after putting a 0.27mH coil, a 1.0uF cap, and a 28R resistor in parallel and attaching this bundle in series with the positive speaker terminal. As usual, it's probably best to ignore everything under 200-300Hz.

Of course, the irony here is that the results of my fiddling around are probably very similar to the response plot from the 10P driver all by itself... :rolleyes: Though, this result is cheaper than buying another set of drivers and selling these. Plus, it comes with the added ability to adjust the response just how you like it. Do you like more treble? No problem - reducing 28R to 20R adds back 2dB in this range. Reducing it further restores more treble. For the ultimate in flexibility, add a variable resistor and dial in exactly the amount of treble that makes you happy!
 

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Not content to leave things alone, I also spent a few minutes measuring the difference that the stuffing below the woofer makes. I turned the speaker around and place my SPL meter on the floor just behind the rear horn vent to measure the low end response.

Below are two graphs, one comparing 100% stuffing (80g) with 50% stuffing and another one comparing 100% stuffing with no stuffing. In both cases, the 100% stuffing level is represented by the gold trace. Reducing the stuffing results in a few dB more bass at the low end, but visibly adds to the peaks at 1500Hz and 5-8kHz. In the end, I left the stuffing at 100% as my FHXLs sit in the corners of an alcove in the room.
 

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After tweaking my notch filter for these speakers, I'm loving them even more than before! So much so that I've started on a center speaker with the materials left over from my original build. Turns out that two 5x5 sheets of Baltic Birch is enough for an LCR array. I also had two larger pieces of Bubinga left over from my FHXL build that I joined side-by-side for the front panel.

Since I didn't do a good job documenting this with my original build, I made a few more images of the tear-out that seems to happen with figured hardwood when it passes through the planer. The shallow ones sand out without too much effort, but the deeper ones need bit of attention. I saved some of the dust from when I sanded the faceplate, dabbed a bit of glue into the tear-out, and pressed some sawdust into the hole. I repeated this two or three times, sanding in between, until the hole was filled. After a few rounds, it blends in quite nicely - you really have to search for the hole.

Now I need to order another driver and some additional felt...
 

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Here are some updates on my center channel build. Construction technique is the same as the left and right speakers: 18mm baltic birch throughout, 20mm bubinga hardwood laminate to the front baffle, and driver fully recessed. The center brace is about 30-35% holes and is completely lined with 1/2 felt. I had a hard time trying to figure how to mount the brace so it was flush against the back of the driver as well as the rear of the cabinet. I came up with an idea for using two wooden shims. I mounted the driver, glued the brace, put glue on the shims and shifted the shims around until it was all nice and snug, then let it dry. I cut the shims after everything was dry and added some felt to the face of the brace as well.

It's all sealed up now and waiting for the first coat of finish to dry. This is always the part that tries my patience...
 

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In my frequency response plots posted above, please ignore the dB scale on the left side of the image. This is essentially irrelevant as I did not make any effort to achieve any specific SPL level. The point is simply to show the relative frequency response across the audio spectrum and what happens when you add a notch filter or alter the stuffing below the driver.
 
Five coats of tung oil finish later, here is my finished center. Turns out that one 4x8 sheet of veneer is enough to cover all three speakers in the front array, with just a bit left over for another small project.

I didn't spend the time to break this driver in like I did with my left and right channels, so I'll reserve any comments on sound until after another few weeks go by. For now, I've left the more dense felt in place until I can locate something that is softer. I also want to make some measurements before I go playing around with things...

Patience, patience... ;)

Edit: Hmmm... looks like I have some reflection across the top of the speaker from something else in the room. The color is more even than it appear in the images.
 

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I like that speaker input plate, Dave. Thanks for the link!

Zia: Thanks - it is a nice match. The original bubinga wood that I used for the left & right channels was 48-49" long, so the baffle for the center is the bottom that was cut from the left & right baffles, but-joined side by side as book-match, and then laminated to 3/4" bb ply. They sure look nice as three matching speakers, but the center stands out visually a bit more now as the previous speaker was black grill cloth across the entire front. My wife says, "Boy, that one sure stands out more than the last one..."

These three as the LCR array match one another in tonality much better than my previous Atlantic Tech LCR array. The previous ones were THX certified D'Appolito designs and the center always sounded just a bit funny to me (a bit too much lower mid range - so it sounds slightly bloated). The Alpair drivers are clearly more "open" (for lack of a better word) and make the sound appear to be originating somewhere in the middle of the room rather than directly from the boxes. Exchanging the center speaker especially is quite a stark difference, but it's also not broken in yet...
 
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