Last updated on 10/25/98
When my wife and I bought a house back in 1995, I made her a couple of promises about the living room. One promise was to get rid of the ugly black 19" EIA equipment rack in which I'd installed our stereo equipment, and replace it with some type of cabinet that fit into the room's decor. Another was to replace my then-current speaker system (a pair of Fried B/2 satellites, with a Fried T subwoofer cabinet the size of a coffee table) with something a bit less visually intrusive.
I fulfilled the first promise by having a local cabinetmaker design and install a set of shelving and cabinets... an "entertainment center" they call it... in the industry-standard white Melamine. This holds the TV, the VCR, the stereo electronics, and has storage space for tapes. The whole stereo can be concealed behind closed doors when not in use.
The speaker system was a bigger job. I wanted something which sounded at least as good as the Fried system I'd be replacing, and which would be as close to "invisible" as possible. I looked at some commercially-available systems, but those with sufficiently good sound quality didn't come in sizes or finishes which would fit well into the decor and space in our living room. Nor was I eager to spend $5000 or more, to buy a system I knew I'd be happy with for years.
I'd built several speaker systems from kits over the years (the Fried systems, a set of Madisound Sledgelings for my office, and a set of Speakerlab systems back in my college days). I checked the current kit-offerings from several speaker companies, and saw quite a few interesting ideas, but nothing which really seemed to fit what I wanted. I decided to see whether I could design something from scratch.
I decided to use a trapezoidal cabinet, with the following approximate dimensions: front baffle 9" (rotated inwards 15 degrees, tilted back 4 degrees). Rear wall 15". Inner side 20" (perpendicular to the rear wall, meeting the front baffle at a 105-degree angle). Outer side 24" (perpendicular to the front baffle, thus meeting the rear wall at a 75-degree angle). Height 40".
The trapezoidal shape, and tilted baffle used in this design meant that most of the pieces of MDF used in construction would not be simple rectangles - they'd have cuts at odd angles, in one or both axes and on one or more edges. I spent quite a bit of time plugging numbers and formulae into MathCAD, calculating the actual dimensions of each piece and double-checking the resulting fit and cabinet volumes. If you want to see the calculations, follow this link.
I decided to sanity-check the design before I started cutting wood. I built a 40% cardboard scale model... by measuring 1 cm of cardboard for each inch of MDF, the calculations could be checked pretty easily. The cardboard model went together successfully, with no big ugly gaps, so I had some degree of confidence that the physical universe was willing to correspond to my theoretical calculations (or vice versa!).
I don't own, or have convenient access to a table saw or radial-arm saw. I do have a circular saw, a portable jigsaw, a router, and a reasonable collection of hand tools. Cutting these odd-shaped pieces of MDF using only these tools proved to be an interesting challenge. I constructed a good straightedge out of some pine 1-by-2, measured every cut at least twice before cutting, and became very familiar with the cutting-angle adjustment on the circular saw. By care, and with more than a small amount of good luck, I only messed up a couple of cuts, and was able to recycle the mis-cut pieces of MDF at a later stage in the construction.
You may hear people tell you that to cut MDF properly, you should invest in carbide-tipped saw blades, router bits, etc. Believe them. High-speed steel does go dull quite quickly when cutting MDF... I wore out two circular-saw blades and a router bit learning this lesson. You'll save money, do a better job, and (most importantly) work much more safely with carbide blades and bits.
The cabinets were assembled using a combination of screws, glue, and 2"-by-2" pine cleats in the corners. I started out using carpenter's glue (the yellow aliphatic type). Due to a combination of the odd cutting angles required, the lack of a table saw, and my own limited skill at precision cutting, I found that some of the interior pieces weren't cut precisely enough to allow for gap-free joints. For this reason, I stopped using aliphatic glue, and switched to a low-viscosity slow-cure marine epoxy (with a silica thickener added as necessary) for the rest of the construction. This particular epoxy blend (TAP Plastics) fills gaps quite nicely, is very strong, and isn't a strong skin sensitizer.
The subcompartment for the MTM drivers takes up the upper front quarter of the total cabinet. Its back is defined by a piece of MDF about a foot wide and 24" high glued to the sidewalls, 6" behind the front baffle. Its bottom is defined by a trapezoidal piece of MDF which extends all the way from the front of the cabinet to the back; I cut a large hole in the rear portion of this bottom piece, so it acts both as a sealed bottom for the subcompartment and as a sort of H-brace for the rest of the cabinet.
The remaining L-shaped area of the cabinet is dedicated to the NHT 1259 subwoofer. I used a router and a radius jig to cut the necessary opening in the side panel. The woofer opening is slightly recessed - not enough to allow the driver to sit entirely flush with the surface (this would have thinned down the edges of the mounting hole too much).
The front baffle was probably the most complex single piece of work required. I had decided to make it double-thick, in order to provide a rigid and well-damped support surface for the MTM drivers. The rear piece was cut to the size of the cabinet front. The front piece was cut 1.5" undersize in each dimension (plus a fraction of an inch more undercut horizontally to take into account the narrowing of the baffle). I cut 3/4"-radius pine "quarter-round" to match the dimensions of the front piece, trimmed the ends to a 45 degree angle using a miter box, and glued them around the edge of the front piece using epoxy and strap-clamps... thus creating a nice chip-resistant rounded edge to the front baffle.
I then marked and routed holes for the drivers in the front and rear baffle plates. The front plate was routed to allow the drivers to be mounted "flush" with the surface. The rear plate's holes were routed somewhat oversize. I drilled holes in the front plate for the driver mounting screws - the holes are large enough to hold "Wel-Nut" bushings, press-fitted from behind and then glued into place. I drilled similar holes in the main cabinet for the NHT 1259 mounting screws.
I then glued the two baffle plates together, using epoxy (in the areas around the driver mounting holes and in the corners) and an acrylic-latex caulking compound (in the "blank" lower portion of the baffle). My hope was to create a two-layer baffle which would be both rigid (around the drivers) and have some useful amount of internal damping... it seems to have worked, as the baffle lets out only a dull "thuck" when tapped. I glued the baffle to the cabinet with epoxy, and drove screws through the baffle into the 2-by-2 cleats holding the top and sides together.
I sealed the insides of all of the cabinet seams using a siliconized acrylic latex caulking compound, to ensure airtightness.
At this point, the cabinet assembly was largely complete (by now, it was sometime in May). In July, I installed a set of drivers in one cabinet (running temporary wires out of the subcompartment) and tested the midwoofers using a CD player and a small amplifier. Although the treble was absent and the deep bass was missing, I was impressed by the naturalness of the midrange - Judy Collins sounded really nice.
I lugged the cabinet into the back of my minivan, and drove it up to Concord, CA to the offices of A&S / Just Speakers. I left the cabinet there for a week while on vacation, and picked it up when I returned. During that time, Brian Smith of A&S measured the frequency response of the system and designed a crossover. Fortunately, this proved to be quite simple, as my system is similar in many respects to A&S's own "Signature 717" design - Brian was able to make a small modification in the Signature 717 crossover design and get excellent results. He didn't even charge me for the work. I bought the necessary crossover components from Brian (his prices were competitive with other suppliers). If you're interested in a crossover schematic for this sort of system, follow this link.
I "glopped" the interior of the cabinet with two coats of Shur-Stik Permanent Patch 100 - a patching compound which dries to form a tough, rubbery, and mechanically "lossy" form. I ended up using one gallon per cabinet, forming a layer about 1/8" thick across most of the accessible panels and braces. My tests had shown that a layer of this stuff did an excellent job of damping resonant vibrations in a sample piece of plywood. In practice, to seems to have damped the MDF quite nicely... the cabinet is quite "dead" when tapped.
I was concerned about the risk of the MDF chipping or crumbling badly if struck (it's not the strongest of materials). The risk of this happening seemed to be worst at the bottom of the cabinet - it'd be easy to drop or bang this area when moving the cabinet around. I decided to reinforce the bottom to keep this from happening, so I glued on a sheet of 1/4" plywood and then trimmed the edges flush with the rest of the cabinet using a straight router bit. I ran a 1/4"-radius router bit along all of the corners (top, bottom, and the backs of the sides), rounding them slightly to reduce the risk of chipping.
I had quite a bit of filling to do: screw-holes, dings, dents, and hairline cracks at some of the seams. I filled most of these with water-base wood-filler paste or with drywall spackling compound. There was one big section of filling I needed to do - along the sides of the front baffle. Because the baffle meets the inner side at a 105-degree angle, the 3/4" pine quarter-round I'd glued along the edge of the front baffle plate didn't quite line up with the side of the cabinet... there was an underhang of about 1/32" inch. As this was a fairly large area, I didn't want to use a filler which might shrink or weaken. Instead, I mixed up another batch of slow-cure marine epoxy, and used "microspheres" as a thickener and matrix. This product is a white powder consisting of tiny air-filled plastic spheres... it turns epoxy or other resins into a stiff paste which hardens to a reasonably light, sandable, non-shrinking solid. I filled the baffle-side gaps with this mixture, creating a nice smooth transition between the cabinet side and the quarter-round. I used another batch of this mixture to smooth the junction at the bottom of the baffle, between the bottom quarter-round and the sheet of 1/4" plywood.
After the various filling compounds hardened, I rented a belt sander for a few hours and got busy. It took about an hour per cabinet to shape and smooth things to the point at which I was happy with them. I returned the belt sander, and then did touch-up filling and sanding by hand.
Finally, it was time to paint, using an airless paint sprayer (noisy, but fast and efficient). I sprayed on one coat of an oil-based primer/sealer, followed by three coats of Kelly-Moore "Dura-Poxy" (a water-based semigloss acrylic latex which hardens to an extremely tough surface). This took several weeks, as I couldn't spray more than one coat per day and only had limited time free on the weekends. I sanded lightly between coats, to remove dust caught in the paint and provide a good "tooth" for the next coat to adhere to.
In this rear view, you can see the L-shaped subwoofer chamber, the wall stiffeners, the 2-by-2 cleats and braces, and the grey Shur-Stik coating.
The same cabinet, viewed from a slightly different angle, shows the "outer" side of the cabinet, the mounting hole for the NHT 1259 subwoofer, and (through the hole) the side-to-side brace which stiffens the subwoofer compartment. If the inside of the cabinet looks a bit odd, it's because I had to use The GIMP to enhance the image - the original photo has such high contrast that nothing was visible in the dark upper area.
The same cabinet, viewed from the front, after one of the "finish coats" has been applied. Note the high-tech spray cabinet I constructed out of plastic dropcloths, string, and duct tape. The little white rectangles positioned around the driver mounting holes are bits of foam tape stuck over the Wel-Nuts to keep them from being clogged by paint.
The external crossover box serves several purposes. It puts the crossover out where I can get to the components, thus permitting me to "tweak" the circuitry if necessary. It keeps the crossover inductors well away from the driver voice coils and magnets, thus avoiding interactions. And, its walls stiffen the rear panel of the cabinet.
In this picture, you can see the left cabinet just before the rear panel is glued on... you can see the feedthroughs and the black silicone caulking around their bases, the Shur-Stik glopping on the panel surface, and the areas I marked out to avoid glopping in order to allow the epoxy a clean surface to grab onto. Note the unglopped square near the upper standoff - this "mates" with the end of the 2"-by-2" front-to-back brace you can see sticking out of the wool stuffing. The silver nuts at the very top of each rear panel are the mounting for a set of metal handles I installed... these give me a secure handhold if I want to move the cabinets around.
I ran the midrange and tweeter wires through a set of holes I'd drilled into the subcompartment, caulked the holes, and then stuffed the subcompartment and mounted the midwoofers and tweeters. Finally, I stuffed the bottom half of the subwoofer compartment and mounted the subwoofers. I ended up using a total of about 5 pounds of long-hair wool, and some acylic pillow-stuffing as well.
The drivers were all mounted using a tacky poster-mounting compound to form a sort of home-brew gasket, and screwed into the Wel-Nuts. The gasketing material isn't entirely airtight, so I ran a bead of black RTV silicone around the outer edge of each driver, filling the gap between the driver and the routed-out recess.
As I had not yet assembled my Marchand XM9 crossover, I hooked up the MTM array alone. My first impression was that the systems were a bit bright, and bass-shy. After a few weeks of breakin, the bass opened up a bit, but was still low in impact. This didn't surprise me, as I'd known the 18W8545s would roll off at a relatively high frequency when used in a sealed enclosure.
Over the weeks that followed, I assembled the Marchand crossover, dug out my spare amplifier, and switched over to a biamplified arrangment. Much better! The bass is now clean, solid, and honest - no boominess at all. It sounds extremely natural - string bass, drums, etc. sound neither anemic nor exaggerated. Driver-to-driver integration is extremely good.
Some of the excessive brightness remained after breakin. This didn't surprise me too much - Brian had warned me that I might want to increase the value of the tweeter "padding" resistor a bit. Before I did that, though, I built a set of grill covers for the cabinets (using a stiff sheet of plastic foam for the frames, and an acrylic fabric for the covers). The grill cloth tamed the brightness in the treble... in fact, it dulled the treble too much. I installed a set of shorting jumpers across the tweeter padding resistors in the crossover, and the sound achieved a very pleasant balance.
I committed fully to the new speaker system this weekend. I sold my venerable Fried B/2 / T setup to another net.audiophile. Fortunately, he had strong arms and a large vehicle...
I built a new set of grill covers, replacing the old ones. The original grill-cover design suffered from a couple of problems. The acrylic fabric I used had a fairly close weave - it was not acoustically transparent. The frames, constructed of 1/4" plastic foam, were not very rigid - I couldn't stretch the fabric taut without distorting the frames, and as a result the fabric was becoming rather "droopy" and actually touched the lower woofer surrounds. And, finally, the covers were fastened to the baffle using wads of poster-tak, which wasn't holding all that well - they kept coming off.
The new grill-covers are of a more conventional design. I made frames out of 1/2"-radius pine quarter-round, mitered at a 45-degree angle at the ends and glued with epoxy. Inside each corner, I glued a triangular piece of 1/2" pine, to serve as a strengthener for the corners and as a place to put a ball-and-socket fastener. The balls went into holes in these braces (press-fit, reinforced with epoxy), the sockets went into corresponding holes drilled in the front baffle (press-fit, no epoxy needed), and the frames were given a quick spray-coat of white paint. I cut pieces of white grill-cloth fabric (ordered from Parts Express, along with the fasteners), ironed, tensioned, and stapled. Voila! Much less trouble than I thought they'd be.
The new grill covers are much more sonically transparent than the old ones - I've removed the shorting clips around the tweeter padding resistors. They're visually semi-transparent, too - one can see the drivers behind them to some extent.
My hope is to find a crossover setting which largely avoids the "floor reflection" cancellation effect, which tends to cause an audible dip in the midbass. The original 100 Hz crossover is vulnerable to this suckout, since the midbass frequencies are being routed to the MTM array several feet above the floor. By pushing the crossover point to a higher frequency, the midbass would be routed to the side-mounted NHT 1259 woofers - these are close enough to the floor that the floor-reflection cancellation would fall above the crossover frequency and would have little or no audible effect. As an additional benefit, this change may also reduce distortion in the 18W8545s, as they'll be handling a smaller range of frequencies, with less power and less cone excursion.
The first new module I tried out was 275 Hz (it was the only one for which I had suitable resistors in my supply box). I wasn't too happy with the result. The midbass frequencies were fuller, but sounded somewhat wooly and loose - they didn't integrate as well with the upper bass and midrange. I'm not too surprised at this - asking the side-mounted 12" 1259 drivers to handle frequencies this high probably isn't a terribly good idea.
I stopped by the Friendly Neighborhood Electronics Surplus Store and bought resistors for the 140 Hz and 200 Hz modules. I've installed the 200 Hz modules now, and am a good deal happier with the results. The midbass sounds fuller and more realistic than with the 100 Hz modules, but doesn't suffer from the looseness and poor integration I heard with the 275 Hz modules.
I'll listen to the system this way for a week or two, and then try out the 140 Hz modules. I might make up some more modules for intermediate frequencies... but I'll have to wait until the store renews their stock of 8-pin DIP component carriers, as I'd bought out their supply.
I did some tuning to the passive (MTM) crossover yesterday. When Brian Smith adapted his Signature 7 crossover for my cabinet, he deliberately set the value of the low-pass inductor to be a bit on the small side, in an attempt to give the 18W8545 carbon-fiber drivers something of the additional "liveness" of the 18W8546 Kevlar drivers he was used to working with. He warned me that this might possibly result in a bit too forward a midrange.
After listening to the system on a wide range of program material for an extended period, I decided to try his alternative suggestion: increasing the value of the lowpass inductor from .87 mH to 1 mH. This causes the midwoofers to begin rolling off a bit sooner, reducing the system's output in the midrange region.
I also did the final set of cosmetic work on the cabinets - I fabricated a set of grill covers for the side-mounted NHT 1259s and mounted them into place. The construction was quite similar to the method I'd used for the front covers - 3/4" pine quarter-round, sawn at 45 degrees in a miter box, epoxied to form a square frame (18" on a side), equipped with ball-and-socket mounts, spray-painted white, covered with white grill cloth. At this point, the system no longer has any "bare" drivers - all four drivers per side are dimly visible through the grill cloth but are not visually intrusive.
Here's a photo of the systems in our living room, during the 1997/1998 Christmas holiday season. This shows both cabinets in place on either side of our fireplace, with all of the new grill covers in place.
You can see that my wife's fond of having twinkling Christmas lights, and plenty of Christmas decorations! She'd probably have been happier if the speakers were physically smaller... but she's content with them, as they're a lot less obvious than the old ones.
This is a closer view at the left-hand cabinet, in its location by the fireplace.
My total investment in this project was somewhere on the rough order of $2000... including tools like the router, circular saw, and paint sprayer that I'll certainly be continuing to use for other purposes, but not including the assumed value of my time (many, many hours!). It was a valuable learning experience and a lot of fun. I think I ended up with a better set of speakers for the dollar - and a much better match for what my wife and I wanted - than I could have gotten on the commercial market.
This was an extremely aggressive and challenging speaker design for me, as it was my first "from scratch" design. A lot of things could have gone wrong. Fortunately, very few did.
I'm very grateful to Brian Smith of Just Speakers for his willingness to share the Signature 717 crossover design with me - he's clearly done a lot of work to optimize this design, and it certainly works better than any sort of "cookbook" design I could have done on my own.
If you're interested in my plans and circuit schematics, you can go here for further information.
Feel free to send me email if you have comments or questions concerning this project.
Go here to reach the Jade Warrior home page.