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A Stereo Guitar Speaker Cabinet

04/03 - 2003 - Larry Mundy for Shavano Music Online  http://www.colomar.com/Shavano/stereocab.html


Note: Larry Mundy has written a number of speaker-design and speaker-construction articles, most of which are posted here on the Shavano Music site.  He can be reached at the email address at the end of this article, or through his band's website at www.yournamehereband.com.

Why a Stereo Cabinet?

Everybody knows guitar and other instrument cabinets are mono, right? Some of those giant four-twelve cabinets come with a stereo-mono switch that no one ever uses, because most guitar heads are mono, and no one wants to use only half the speakers in his cabinet. But there are some times when having a stereo cabinet is useful. With the aid of only one six-pack of Coors and a tattered legal pad, we’ve come up with several potential uses for the project described in this article:

1) When your head (or combo amp) is stereo for a good reason. We have a rhythm guitarist who uses a combo amp with stereo chorus. I know there are lots of mono amps with chorus effects, but there really is a different (and better) quality to the chorus effect if the delayed/pitch-altered sounds are coming out of different speakers, although the speakers don’t have to be very far apart to capture this effect. This amp has outputs for two (stereo) extension cabinets – it would be so much easier to have a stereo speaker setup in a single cabinet and just use two speaker cables.

2) When you use two heads (or a stereo amp) for some other reason. Our lead guitarist has a midi pickup on his guitar, and the guitar signal goes to the guitar amp, while the midi signal goes to an Axon midi processor and another amp, which can make his Strat sound like a saxophone, a horn section, or even a chicken being dismembered with a chainsaw. With a stereo foot-controlled volume pedal, he can fade the input from only guitar, to only midi, or some blend of both. In the past, two amps has meant two speaker cabinets.

3) When you want to distribute the sound of other instruments across the stage. Bear with me here. Our preferred stage setup has two "lead" instrument players with a predominant midrange (keyboard and lead guitar) positioned some distance apart. Each complains that on a broad stage, they can’t hear the other because of the "directivity" of midrange and treble sounds (bass sounds don’t have this problem; they go in all directions at once). So we have been planning to run an extension speaker from the keyboards to the area where the guitarist is standing, and vice-versa. With a pair of heavy-duty stereo cabinets, each producing one guitar signal and one keyboard signal, the sound from both instruments would be "positioned" across a broader stage and better audible to everyone on stage. Also, each of these players would have a better idea of his instrument’s volume relative to the other "lead" instrument, hopefully leading to better dynamic control when we are performing without a "sound man."

4) When you need to conserve space, or take the least possible equipment to a gig. One stereo cabinet might be smaller than two mono cabinets, and is only one thing to move into position. For a small club, if two instruments with two amp heads plug into a single, powerful cabinet, you’ve just saved a bit of stage "footprint" for that tiny club where the stage was designed for Barbie dolls.

Our group has had all of these needs at one time or another, so this article will describe a construction project for a relatively straightforward stereo cabinet, primarily for guitar use.  Since I want to save as much stage space and "lugging weight" as possible, this will be a medium-sized cabinet with two, 12-inch speakers. When the cabinet is used with separate amps, each will get only one speaker. But if I use sufficiently heavy, powerful speakers, this should not be a problem. 12-inch drivers rated to handle hundreds of watts are readily available.

What's In The Box

For this project I will use a pair of Eminence Delta Pro 12’s. This is a heavy speaker which combines serious power-handling capability with impressive efficiency of approximately 98 db across its range. Here are Eric Kirkland’s comments from a review in the January 2003 issue of Guitar World:

"Historically, many great players have chosen neutral-sounding speakers with ultrahigh power handling so that the only distortion in their sound is that which originates at the amp.  The Delta Pro 12A is designed in that tradition.  It boasts an 80-ounce magnet and a 2.5 inch voice coil, and is capable of handling 300 watts of power.  With a lightning-fast response and minimal tonal coloration, the Delta Pro 12A performs equally well over a wide frequency range and delivers exactly what's coming from your amp--from test-condition clean tones to searing metal crunch.  And for those who think that speakers of this ilk are cold and lifeless, I suggest you listen to any SRV or Zakk Wylde album."

One of these speakers will perform acceptably for my purposes (down to 50-60hz) in a simple ported cabinet of less than two cubic feet. Two of them, then, will work fine in a ported cabinet of 3.5 cubic feet or so. They do start to fall off a bit in the midrange, so I want to add tweeters for a little high-end reinforcement. I recognize that many guitarists, particularly those who play "heavy metal," don’t want the additional high-end sizzle of a tweeter. No problem; one cheap double-pole switch and I could take the tweeters in or out of the circuit. To save space, cost, and the complexity of a crossover, I will use CTS/Motorola 1165A tweeters, which are part of the "Powerline" series I have praised in other articles but are only 4" square. They are rated at 100-200 watts and "protected" by the Powerline circuit to 400 watts. At 92db, they are relatively inefficient compared to more conventional tweeters, so their output should already be lower than that of the Delta-12’s and I shouldn’t have to "throttle them back" with resistors or an L-pad.

This driver combination has a current "street price" of a little over $200.   I could have saved about $50 by using the stamped-frame version of the Delta woofers, which have very similar performance and also come in a 16-ohm version in case there's a need to have the cabinet present no less than an 8 ohm load to the amp when used in mono mode (i.e. both drivers wired in parallel).  All current Delta models are now rated for 400 watts of input power, although mine are the older, 300-watt-rated models.  I went with the "Pro" series because a cast aluminum frame promises a little better performance than stamped steel under extreme stress due to its non-magnetic nature and slightly better heat-dissipation capabilities, but that choice was probably overkill for this application.  Obviously, there are lots of pro-audio speakers which would work for my purposes, but in my experience the Delta series represents a "sweet spot" in the Eminence lineup, with an excellent price/performance ratio and very high efficiency, and a response curve well-tailored for guitar.  I would use heavier-duty drivers for a bass cabinet, though. 


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The Delta Pro-12’s are 8 ohm drivers. When used in parallel for mono sound, they present a 4-ohm impedance to the amplifier, well within the design limits of most pro amps.1165B.jpg (8910 bytes)

The KSN1165 provides high power-handling capability in an inexpensive, 4" square package, and since it is a piezoelectric tweeter, it doesn't require a crossover.  At about $12 each, this is an inexpensive way to add high-end "sparkle" to your sound.  If the result is a little too "sparkly," experiment with fixed resistors as described in my other articles.


My sketches indicated that two 12" speakers, side to side or on top of each other, would require one cabinet dimension to be at least 28" or so – the sum of the cabinet-wall thickness, two 12" drivers, space on either side of the drivers for the underlying cabinet bracing, and an inch or so between the drivers so that the baffle board isn’t paper-doll "thin" from having two large holes very close together. I gave myself some slack with a 29-inch dimension. This is about as wide as even the most gigantic "Marshall-style" amp heads. The other (vertical) cabinet-face dimension had to be at least 19" – a 12" driver, a 4" tweeter, and an inch between and on either side of both of them – again I gave myself a little slack, going to a 20" dimension. This will make the cabinet 20" high when in a typical, "Fender-Twin" configuration, like so:

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However, it will also allow the cabinet to be 20" wide when set on its side – which is happily, the same dimension as most of our rackmount equipment cases, which could then sit on top and look like the whole thing was designed to be used that way. I have seen many cases with little rubber "feet" both on the bottom and on one side, to give this sort of flexibility of placement, and that strikes me as an excellent idea for a cabinet which does not have casters installed (with casters, sitting a cab on its side results in casters sticking out the side – which not only looks dorky, but the "unloaded" casters can rattle while the cab is in use).  We're old farts, though, and put casters on anything heavier than a ham sandwich.

Note in the diagram above that the tweeters are placed at the sides of the cabinet to maximize any perceived stereo effect when the cabinet is used that way. The cabinet ports are positioned "inboard" since they produce primarily nondirectional low frequencies.

This cabinet will not be used for extreme bass frequencies, and will not have an excessive front-to-back dimension – perhaps 14", which after setbacks for a flush back and  front recess, plus the depth of the back and baffle themselves, will net me an interior depth of 11.5" or so. And I plan to use copious internal bracing. So, to save weight I will use the thinnest material I would ever recommend for a performance cabinet – half-inch plywood. Within my 29" by 20" "face" dimensions, then, this dictates that the baffle be nominally 28" by 19". Actually it will be a little less after the carpet wraps around the front lip, so I will subtract " (i.e. 1/4" per side) when cutting it, to 27.5" x 18.5", to save trimming later. Right now, I’m just seeing how those dimensions will "map" onto a sheet of plywood, and I got sort of a happy surprise: the top, bottom, sides and baffle can all be derived from a half-sheet (4’ x 4’) of plywood, saving me the extra expense of buying a 4’x8’ sheet and storing the wastage.

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Since I only need to buy a half-sheet of plywood, I will splurge on high-quality birch, which will also help control resonance.  And, that remaining 20.5" wide plank (with the "X" through it in the diagram at right) is within 7" of covering the remaining rear face of the box. At the expense of some complexity (and a couple short pine planks), I decide to get a little fancy with the cabinet back, using the 20.5" plank for a back and filling the extra 7" with a recessed panel, made of a small piece of scrap, to hold the switches, jacks, etc. While traditional "jack cups" are somewhat recessed, perhaps a half-inch, traditional " phone plugs used for speaker connections always stick out the back where they are susceptible to various forms of damage. If I recess the entire jack-and-switch plate relative to the rest of the cabinet back, maybe just an inch or so, I can protect the plugs and switches a little better, at the expense of a little less cabinet volume.  I plan to use a plastic jack cup, or two separate metal ones, for the reasons explained at the end of this article.

Computer modeling

With interior dimensions of 28" x 19" x 11.5" I will have a resulting box of about 6,118 cubic inches, or 3.54 cubic feet, less the "inset" for the back jack panel and the interior bracing – I’ll spare you the math but the resulting cabinet has about 3.35 cubic feet of air in it. My computer-modeling program declares this completely workable, achieving optimal (flattest) response with two 3-7/8" diameter ports, each 4.08" long (I can buy this diameter from Parts Express in a longer length and cut them down with a hacksaw). If your computer program does not allow you to design a box with two identical drivers, just cut your theoretical box in half and design for one speaker, then double everything.   I will in fact isolate the two woofers from each other to a small extent, using a separating plank attached to the baffle board that extends toward the back of the cabinet about four inches, covered in absorptive foam, so that sound waves coming sideways from the back of one cone don’t crash directly into the back of the other cone and cause it to "speak" when it should keep quiet. This might be a concern if one speaker is playing a soft, delicate passage while the other one is bouncing around to thundering chords. Since the divider does not extend to the back of the cabinet, however, both speakers will have the advantage of the entire cabinet volume and when they are reproducing the same thing, this should assist their low-frequency reach somewhat.  My computer-modeling program tells me to expect useful bass response down to about 60hz:response.jpg (30655 bytes)

This will be a perfectly ordinary, rectangular, carpet-covered box, the easiest kind. I’ve already discussed most of the details of building such a box in another article, but I’ll include a few photos of this one, too, as we go along. First, since I will be using a metal-mesh grille suspended from the baffle-board face with 5/8" or 3/4" rubber feet, I recess the front bracing (3/4" x 1-1/2" trim, placed sideways) about two inches from the front lip of the cabinet. After the " thickness of the baffle board, that will still give me a 1-1/2 inch recess; the rubber standoffs and grille will use up about 3/4" to 1" of that and the grille will still be slightly recessed, which is good since the raw edges of a metal-mesh grille can slice your pinkies open with absolutely no remorse. Then I attach the top and sides to one another with wood glue and drywall screws, maintaining a 90-degree angle with a corner clamp while the glue sets.

endpanels.JPG (67450 bytes)End panels with bracing preattached.   Front lip setback is 2", rear is 3/4".  Bracing is wood-glued and screwed from exterior with 1-1/4" drywall screws.

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Top, bottom and remainer of front bracing attached, wood glue and drywall screws again.

 

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Drivers and ports laid out on baffle board.  Tight, but it will work.  Allow spacing on all sides for box bracing.

 

 

At the rear, I still have to figure out where to put the 7" of recessed panel. Since the cabinet doesn’t have a lot of depth, and I intend to use recessed handles which intrude into the interior, and the rear space between the speakers is larger than the space on either side, I decide to put the recessed panel in the middle. This means that in addition to the panel, I will have two backs, one on either side, created by sawing the 20.5" plank in half. This adds even more complexity (and the necessity for more bracing) but has the advantage of symmetry, and creates smaller back panels which are less likely to resonate. It’s long, unbraced or unsupported spans which cause the most resonance problems, since their greater length and mass makes them tend to resonate at lower frequencies. Or so I tell myself, since I’ve created a lot of extra work compared to a single, flat cabinet back.

Now I use the bracing at the rear of the cabinet as a work-stand and cut the holes for the speakers, ports, etc. in the baffle board. Remember, the "fit" of the baffle board and back(s) into the cabinet should be a little loose at this point since there will be carpet covering everything (and taking up space) later on.  I mark and drill the holes for the woofers, which will be bolted in with "-20 bolts and T-nuts. With over half the surface area of the baffle consisting of bolted-in cast-metal speaker frames, even the skinny " material will be plenty strong.  I will use silicone sealant and ordinary drywall screws to attach the tweeters, which are flimsy plastic things anyway, and I will make the ports a tight press-fit and then secure them with silicone sealant from the back of the baffle. Before I cover the baffle, I attach the divider board between the speakers with glue and drywall screws.  baffle divider.JPG (64935 bytes)

It would be difficult to carpet the box with the dividing panels between the jacks and the backs in place – so I will build (and carpet-cover) them separately, then screw them into a framework already designed to receive them after the main box is covered. This sub-assembly is made of two carpet-covered pieces of bracing material, to which the "control panel" screws from the rear (i.e. from the inside of the cabinet). Then the whole thing is screwed into the bracing that will support the back adjacent to it, forming a solid central divider.

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Above: divider panel attached to back of baffle.  Left: test-fit of rear central panel; it will be removed again for carpeting.  Below: test-fit of all panels.  The baffle should be somewhat loose - perhaps 3/8" - as the wrap-around carpeting on both the box and the baffle will make for a tight fit.

testfit.JPG (58695 bytes)

 

I run an electric sander with coarse-grit paper along all the edges and corners, just enough that the carpet doesn't have to make any really sharp bends over splintery wood-cuts.  Then for a bit of extra cosmetic insurance in case the carpet is torn in the future, I paint the cabinet exterior with flat-black paint anywhere I think a carpet tear might happen, which is, um, pretty much everywhere. I cut the holes for the recessed handles, positioning them just a touch (maybe ") closer to the cabinet front where the bulk of the speaker weight is, for somewhat better balance.  Then I sweep up all the debris and sawdust and wet-mop the floor, to keep dirt and sawdust out of the carpet in the next step.

When the floor dries it’s time for carpet, applied just as in the PA-speaker article, with 3M "77" spray adhesive and a new blade in the old razor knife.  The cabinet’s "circumference" is 98 inches (29 + 29 + 20 + 20), so 3 yards of carpet cut about 20" wide will do the main box, with some of the leftovers usable for the "control panel" section. It will take another square yard or so each for the baffle and backs, but most such carpet comes wide enough (48" or 54", probably) that you have enough left from your 3 yards to finish them too.

carpeted.jpg (80862 bytes)Baffle is test-fit again after carpeting, and in this case it needed further trimming to be able to fit after it was carpeted.  Shown with handles installed.

Attach the handles and corners and you have a far easier-to-handle box, plus it’s starting to look good. Eggcrate foam is glued and stapled to all interior surfaces of the box. I next attach the carpeted baffle to the box with just a few black screws and black finish washers positioned where the grille standoffs won’t go. I install the speakers and ports, then use long-enough screws for the grille standoffs that they penetrate the baffle and extend into the bracing, tying the whole thing together very solidly. This thing is starting to look very businesslike:  

cabfront.JPG (66360 bytes)cabback.JPG (74675 bytes)

Now in back, the central panel with jacks and switches is installed and wired to the drivers, which is a piece of cake with those two big holes still in back.  Now's the time to test your wiring by feeding signals to the cabinet.  If everything works as advertised, the backs are carpet-covered, foam attached to their inside surface, and the result is screwed to the bracing with drywall screws and finish washers. It’s done, looks mean and sounds great!

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At the same time, for maximum flexibility, it would be nice if a stereo cabinet could be switched into mono mode for use with a single instrument or for additional power-handling capability. This is easy to do with an inexpensive switch that chains both "halves" of a stereo cabinet in parallel, or with a parallel-output cable like the one described in an article on this site. The cable has the useful advantage that it’s relatively incapable of being misused, with one plug for the amp and two for the stereo speakers. The switch is more convenient because it can be built right into the cabinet, and is harder to lose or break. But if misused (i.e., switched to mono when two amps are attached) it can feed the output signal from one amp into the other, with potentially-harmful results. I will probably retrofit a switch into this project someday, but I will label it clearly and warn all potential users of the potential for disaster. The Marshall 1960 cabinet has the same disaster potential, see http://www.bright.net/~robertso/marshall/1960cab.html, and I have yet to see one of them chucked into a landfill in disgust.  Marshall (and Peavey, I think) have made stereo/mono cabinets where the switching is done via switched input jacks, but I don't trust a switched jack to last over the long haul.

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This diagram makes it look a lot more complicated than it is; basically a double-pole switch (get the kind with no "center-off" feature) connects (mono) and disconnects (stereo) the + and - terminals of the two jacks from each other.  Be sure to use a plastic terminal cup, or separate metal terminal cups, so the jacks aren't already grounded together by a two-jack metal terminal cup.  This is no problem for mono use, but with separate stereo amps plugged in, a metal ground connection at the terminal cup connects the ground side of the amps together, which might be a bad thing depending on how the amps are grounded.  I'm using a Parts Express #260-290 terminal cup, which is plastic, comes with two jacks preinstalled and is easy to drill for installation of a switch.

Publications by the same author: Design and Build Your Own Live - Sound Speakers - A book about How to build your own Pro-Audio Speaker systems - 117 pages of excellent information. ISBN 1-4120-2998-8.

In the Dallas-Ft. Worth area, you can see and hear any of the projects I've written about at any local performance of "Your Name Here;" check www.yournamehereband.com.  I'd love to shoot the breeze (during a break) about how our home-built stuff works and sounds.   Or those of you who live in a more civilized part of the world can e-mail me

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