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Wolf
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Posts: 173
Joined: Mon Feb 20, 2012 7:36 pm
Location: Indiana
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"Fenghuang" Airborne FR151B822F/RT4101 build...

Wed Sep 17, 2014 4:26 am

This started by me winning some FR151B822F 'wood' cone wide-rangers at the InDIYana 2013 event, which I host annually. The FR of the 151 is 'acceptable' all the way up by itself, but you usually lose off-axis treble response when you use any wide-range driver larger than 2.5" diameter. This got the ball rolling as to how I wanted to augment the top-end. Airborne also makes an inexpensive planar-ribbon driver, the RT-4101, and I had not seen it used in a design. This planar is really long and skinny, and would look absolutely stupid above a midbass when mounted vertically. So- I opted to mount it alongside the 151 to keep the center to center distance shorter, gain top-octave response, and hopefully avoid any weird lobing issues. (By chance, a commercial offering called 'Iota', by Neat Acoustics, has a similar arrangement.)

So- I set out to model the box using measured T/S...

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NOTE: Notice the power input level, the output at about 95dB, and the Xmax basically riding the line. The Mms of this midbass is only 13g, and according to a Klippel measurement I've seen, the AC of the coil is set a touch forward of optimal position. IT IS EASY TO PUSH THIS DRIVER INTO DISTORTING BASS RANGES. Point blank- if you use it in a large room, use a subwoofer and a cap of 150uF or smaller to avoid it farting around. If you use it nearfield or against a wall in a smaller room with limited output, it will likely be just fine. For the demo at Meniscus a couple weeks ago, I used the 150uF caps, and my Triumph sub design. (http://s18.photobucket.com/user/wolf_te ... t=3&page=1) I think these would be great as a set of bridge-monitors.

I used a sealed box, stuffed with Ultratouch denim damping to hopefully protect it from overexcursion. It also happens to be a PE knockdown cabinet at its heart.
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The 4101 is centered at the 2" mark from the closest side-edge, and the midbass within a 1/4" from its other side, both centered vertically on the baffle face. The 4101 is an open-back of sorts, and I was able to flush mount it within the thickness of the baffle to keep it isolated. No damping in the planar cavity, as there is not room. This element will also not take quick disconnects, so solder is a must. I ended up pre-tinning the wires for attachment with solder, and laying them lengthwise alongside the terminals and applying heat for bonding. There really is not an easier way.

Xover arrangement...

I tried both for this set of drivers in terms of method, both series and parallel topologies. In fact, I have a parallel version wired up but have not tried it. Anyway- It was easier to get a good result with fewer parts using the series xover method, so I went with that one. Note- I don't normally use an SXO with a ribbon/planar driver, as it can be harder on tweeters than the parallel modus. Being that I have a xover point of 7.4kHz, and that I used a 0.05mH/16AWG coil (wound it myself, along with the 1.35mH) in the main leg of the network, followed by a 2.0uF cap for a second order electrical; it left me at ease in doing this under safe operations. The CTC is about 3", and that approximates 4.5kHz for xover point, so it is a half-octave above that estimate. Oh well...
The 4101 has a stiffly peaked response centered about 12kHz, and rolls off hard to either side. The 0.05mH coil shunts off level to help flatten this out, and the cap steepens the order. Listening to it in sweeps, you basically have no output below about 5-6kHz range. There is a bit of excessive ripple in the top-end of the 151, and that helps balance out the top-octave rather nicely in tandem. Even though this has a full xover, and is not just a super-tweeter implementation, it is mainly to help the off-axis response the 151 just does not have.
The 151 has a resonance in the upper midrange that makes it sound a bit forward, so I added a notch there for tonal balance; and listened a LONG TIME for preference on the resistor value. 10 ohms to 12 ohms is the range. The 10 seemed a bit shouty, the 12 seemed a bit dark. Depending on your application, this could be beneficial to pick what's best for you. I chose the 11 ohm Lynk resistor Meniscus offers to be the best compromise for me. I also did something I don't normally do for the midbass- I overcomped the baffle-step a tad. I set that at about 8dB, as sometimes these wide-range drivers can sound a touch lean or lack warmth. Coupled with the highish sealed Qtc, it sounds good. The wood does a good job with the warmer character.

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Reverse-null for those who want to see it...
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Positions on baffle:
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(before I swapped in the Lynk and soldered it up)
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(After the Lynk swap)
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The bump at ~1kHz made me want to forge ahead with the large radius half-rounds I had planned, filled of course with my proprietary 'unused feline clay sand'. I used biscuits and Gorilla-glue to bond them, and a couple #9 x 4" or so screws for clamping and hold while curing. These screws remove any twist that might be present in the semi-circular additions. Nothing a little clamping pressure can't fix in this case.
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I skinned the top and bottom with hardboard, and brush-coated the entire cab with Elmer's glue-all white glue, nondiluted. (There is packing tape over the planars to keep them safe. It can't be seen in the photo. It was easier to leave them in with the soldered contacts than remove them at that point in time.)
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I did not have the exact size corrugated tubes to match the depth of the cabinet, or I would have. The roundover I placed above the drivers on the top-front edge is to fix a mistake I have done too many times- flush trimming into/against a driver rebate. I had to fill the second one, but it masked the first. Yes- I DID IT TWICE! You can see the fill on one of them.
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A bit of Rustoleum Midnight Blue Satin, and VHT Burnt Copper, and you get this below:
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I used some coat-hooks to set the proper angle when placed on average speaker stands, as these have tight vertical off-axis response. The imaging is VERY good because the 151 covers most of the range of the design, including pretty much all dialog and vocals. If the speaker can fire straight forward at you from a higher level off the floor, then please omit feet. I wanted a bit of an antique feel with these, and I feel I succeeded.

Thanks for looking!
Wolf
 
DEQXter
Posts: 6
Joined: Tue Apr 30, 2013 8:02 pm

Re: "Fenghuang" Airborne FR151B822F/RT4101 build...

Wed Oct 15, 2014 1:11 am

Wolf,
I'm intrigued--for the last year or so, i have been day dreaming along similar lines as a way of getting rid of diffraction--or honestly just reducing it so the FR trace doesn't look like a lousy sine function extending across the spectrum. As aesthetic as a 1" roundover is, I suspect it falls far short of the bill for actual remediation.Two inches--maybe some.

One of my many hobbies is flying large and dangerous amateur rockets and so I have tubes of many dimensions that could be simply cut in half and made tangent to the baffle and meet up somewhere on the side. maybe stuff the seam with felt when it comes back full semicircle. But from friends in the model airplane hobby who cut any shape from foam, they thought I might be able to use core foam to make more of a french curve shape with the round part adjacent to the baffle and then a gentle taper back to the speakers side panel.All hot knife stuff using minimal templates and a steady hand I'm told.

Of course all of what i described could look pretty cheesy, and I would be compelled to vacuum bag the foam or phenolic tubes to carbon fiber, or fiberglass. In any event I am very intrigued. Hope youll keep us posted with results and msmts.

Regards,
DEQXter
 
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Wolf
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Posts: 173
Joined: Mon Feb 20, 2012 7:36 pm
Location: Indiana
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Re: "Fenghuang" Airborne FR151B822F/RT4101 build...

Wed Oct 15, 2014 2:34 am

The box is an off-the-shelf 0.23ft^3 (as advertised) knockdown cabinet from another vendor.

What else is it you want to see?
Wolf

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