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Micro Grand Utopia
Merry Chirstmas everyone. I've been working on this project for a couple months now, but haven't posted anything yet because I was not sure if it would really work. I now have one prototype cabinet constructed and have taken a set of preliminary "proof of concept" measurements. The graphs look pretty good, so I decided to go ahead and start a build thread.
Each channel consists of five independent cabinets arranged in an adjustable curvilinear array. From top to bottom, the layout is "MMTMM" + sub (see drawings below).
Each midrange and tweeter driver is given its own small cabinet and each small cabinet is, in turn, connected to the other small cabinets using slotted masonite brackets. This makes it possible to adjust the relative position of each driver in the "X" and/or "Z" dimensions based on measurements and listening tests.
I initially plan to use very short 2 and 3 inch long slotted masonite brackets that will allow for very small driver micro adjustments. These small adjustments will be somewhat similar in concept to those used by Focal's Grande Utopia or Wilson's WAMM Master Chronosonic loudspeakers. If larger adjustments are needed, I can simply swap out brackets of different lengths.
INITIAL DRIVER SELECTION:
Tweeter: Bozhen CQ76B ribbon (1 per channel).
Midrange: Tectonic TEBM46C20N-4B BMR's (4 of them per channel).
Woofers: GRS 12SW-4HE (1 per channel).
Crossover targets: T/mid: 1-3kHz passive LR2.
Mid/woof low pass: Plate amp set to 150 to 200Hz.
Mid/woofer high pass: 150 to 200Hz passive LR2 with parallel notch to dampen the impedance peak.
Comments
I put together a list of my design goals & methods:
1) Portability goal: Keep the size and weight of the speaker low to save my back when loading and unloading at DIY events.
2) "Swapability" goal: Make it simple to swap out drivers in the middle of the build without having to scrap the entire project and start over. With this multi-cabinet scheme, it should be fairly easy to swap out tweeters or midrange drivers by simply rebuilding or replacing just two small cabs or baffle boards.
3) Directivity goal: Use several small diameter midrange drivers with directivity similar to the tweeter. Attempt to solve the "tweeter-beaming-above-10kHz" problem by designing for a flat response at roughly 20 degrees off axis.
4) Time alignment goal: Evaluate the effect of proper time alignment by arranging the cabs in a curved linear array. Independently move the tweeter and/or midrange cabinets, along the "X" and "Z" axes, to achieve the best possible integration at the listening position. Experiment with "feathering" the vertical "toe-in" and/or driver spacing distances to achieve the best possible imaging. Stop experimenting when I come up with something that measures and sounds good.
5) Measureability goal: Design the enclosure shape to make it easy to capture both vertical and horizontal spin measurements at a height of about 50 inches above the floor. Accomplish this by extending the depth of the 5x5" tweeter cabinet out to 18.5" and then use this extension to flip and clamp the array in either a horizontal or vertical position.
6) 150Hz mid/woofer xover goal: Develop a very low 2nd order high pass midrange xover using a passive parallel notch filter that provides the full 6dB of BSC. Tweak this notch so that it also suppresses the large midrange impedance peak and keeps the overall impedance well above 4 ohms.
7) Frugality goal. Use inexpensive 3/4" thick laminated "value shelving" for all panels. Use relatively inexpensive drivers that measure and sound good. Make inexpensive masonite brackets to screw all the cabs together.
I found my GRS 12 to be pretty noisy when played that high.
Where are the woofers going.
Maybe I will have to swap over to my pair of TB W6-1139SIF subs. The TEM46's won't go lower than about 150Hz.
Directly below each one. The two sub cabs will be 25" high, which will place the tweeters about 41" above the floor.
Here are a few construction pics from the past few months:
Making all the boxes for two channels:
The tweeter baffle is only 5x5" so I needed to cut clearance dados for the Bozhen CQ76B plastic enclosures:
Routing the tweeter flange:
Routing the TE TEBM46 flanges:
Flange routing done for all 5 boxes:
Boxes screwed together using slotted masonite brackets:
I mounted all drivers and did some measurements yesterday. Will post a few more pics and graphs as I go along.
WoW!
I've been kicking around a similar idea as a kind of "final hurrah" build, one intended to be my last entry into serious design work. Convincing wife to let me spend four or five grand is a different story.
Yeah, slowly gearing up for "one last build" for myself. Everything I've learned. using the best drivers on my shelves.
Just an idea, what if you used 10-32 screws and a bracket in the rear of each box/section for a precise angle adjustment. One full turn would give a 1/32" adjustment.