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The original idea - big speakers in front, monitor speakers in back.

“Baby” speakers.

A word about near-field monitor speakers – you want to hear them, but you don’t want them to be all that loud.   Loud sounds at close range destroy your hearing.  Loud monitors close to your microphone cause feedback, even with the best “unidirectional” mikes.  And while you want them to be heavy enough that loud sounds coming through them don’t melt their voice coils, they should be used at such a moderate level that smaller and cheaper drivers are perfectly fine.  An advantage of smaller drivers is that they tend to emphasize the midrange, where the intelligibility of vocals and speech is determined anyway.  You don’t need thunderous low range, and you don’t need sparkling highs either, so don’t design supertweeters into your near-field monitors; you’ll damage your hearing and end up having to carry a bottle of Advil in your gig bag.  The same principles apply to regular floor monitors, where decent, high-powered cone drivers in 8”, 10” or 12” sizes will do everything you need a true vocal monitor to do.  If anyone in your group can sing a note below the lowest range or above the highest range of such drivers, check their DNA quick because they may not be actual humans.  Yet the ready-made market is full of enormous wedge-shaped floor monitors with heavy 15” woofers, and supertweeters with range beyond audibility.   This makes no sense to me, but I also see five-foot “soccer moms” hauling 50-pound kids to school in SUV’s the size of office buildings, so maybe that’s the American way.  Oh, well, on with the story.

Replacement speakers for “factory” nearfield monitors are available in the $30-50 range and up, usually rated for 100 watts or so.  Since I had room for two speakers for each “channel” (i.e. two for vocals, two for drums) I bought four less-expensive drivers from MCM (www.mcmelectronics.com) via the web.  They have aluminum cones and rubber surrounds (NEVER buy a foam-surround speaker for use outside the home, and never use anything but an accordion-surround speaker for heavy performance cabs, but a rubber surround is OK for low-power, sealed-box applications such as this one).  They have 16 oz. magnets and one-inch voice coils, so they are reasonably heavy-duty for a speaker of this size, and they were about $10 apiece.  They have very compliant suspension, appropriate for a smallish box.  Plus I think the aluminum cones look cool, even though no one’s going to see them but me (and maybe you if you keep reading).

The Heavy Hardware.

 For the front-facing speaker, I got an 18”  “Dayton Pro” model from Parts Express with a 4-inch voice coil and a 109-ounce magnet which is functionally identical, except for diameter, to some 15’s I have used for PA service - so I know it can take some abuse.  I paired that with a pair of CTS 1188A piezo drivers mounted on threaded horns.  This piezo driver is rated to reach as low as 800hz, which is good in this instance since the big 18” starts running out of frequency response not far above that figure.  I found a fan-cooled Peavey power amp in good condition on eBay.  This combination will produce sound-pressure levels adequate to level a small town, at least if the town has completely inadequate building codes.

Wiring plans

Okay, I clearly want the front speaker much, much louder than the rear ones.  I will accomplish this in three ways. 

First, while the front and drum-monitor speakers will be wired in parallel, I will wire the two 8-ohm monitor speakers in series with one another.  The amp will “see” two loads in parallel, one 8-ohm load (the 18”) and one 16-ohm load (the two 5’s in series), for a total impedance of 5.33 ohms (s1xs2)/(s1+s2).  Electrons are a lot like me and tend to take the path of least resistance.  Twice as many of them will choose to visit the 18” speaker, and assuming equal efficiency (SPL) among all the drivers, the 18” speaker would be 3db louder than the pair of 5’s.  In fact, the 18”, rated at 97db/1w/m, is substantially more efficient than the 5’s (87db/1w/m), and happily that will further double its loudness relative to them.  I will wire the other two 5’s I will use for vocal monitors in series for the same reason.  This sort of selective parallel- and series-wiring exercise can give weird and unpredictable results when all the drivers are facing forward since their effective impedances are varied, affecting their loudness and hence the balance relative to other drivers in a multi-speaker array.   But for this application, the series-wired drivers are the only things facing to the rear, so “balance” doesn’t matter; I am “unbalancing” them intentionally. 

But the Dayton is rated to handle several hundred watts RMS; the little 5’s, tough as they are, are only rated at 50 apiece, or 100 for each pair.  I don’t want to forego the full capabilities of the front speaker, just to keep the rear speakers operating within their design capabilities.  I would thus like to further reduce the relative impedance ratio between the rear and front speakers as a starting point, and one way to do that is to introduce more resistance between the amp and the pair of 5’s.  I will start with a 10-ohm, 20-watt wirewound resistor in series with the 5’s, bringing the total load the amp sees from them to approximately 26 ohms.  With one branch of the parallel hookup (the 18) presenting an 8-ohm load and the other presenting a 26-ohm load, I’m pretty close to a 3:1 ratio, and the total load on the amp is now 6.1 ohms, well within its design parameters.

That doesn’t mean that if I get zealous and drive the front speaker with 300 watts, I want my monitors to spew out 100 watts’ worth of sound.  I may want even more than a 13 db difference between the two, since these little 5’s will be aimed pretty much at my head, at close range.  Yet if I have the front speaker turned down for a small venue, I still may want the monitors clearly audible.  So my final step will be a 100-watt L-pad, or variable resistor, wired after the fixed resistor has cut down the current to the monitors (L-pads, even big ones, can be fragile).  The L-pad will give me a big ol’ knob on the back of the cabinet to adjust the relative volume of the monitors.  So if you will excuse my crude excuse for a wiring diagram, I envision the 18 and two of the 5’s being hooked up like this:

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Figure A-36(e)(7)(iii).  Arrangement of parallel and series-wiring to control relative volume.

I didn’t know how to draw the little semicircle thingies that indicates a wire crosses another without being actually connected, so I drew the “plus” output from the amp in red, and the “minus” in black.  These wires don’t actually touch each other except where indicated by a little dot.  A graphic artist, I’m not.

Now I will introduce one more option, by installing an input jack wired to the 5’s as an alternate input source, and a DPDT switch that will alternately (1) let them share the amplifier output going to the front speakers, as described above; or (2) switch them completely out of that circuit and hook them only to an external jack – which could receive the same signal from the other channel of a stereo power amp, or could receive a different signal altogether – whatever I want.  “DPDT” simply means “double-pole, double-throw,” so there are three pairs of connectors.   The middle set goes to the speakers; one of the switched sets goes to the jack, and the other goes to the speaker wire coming from the front speakers (this is only a single switch, despite the way it’s shown in the diagram).  Now I have maximum flexibility in determining what I want these two speakers to do.  The switch has a “center-off” feature so I can switch the monitors off altogether and use this cabinet as a conventional front-firing speaker in any other application.

The other two 5’s will get similar treatment, but instead of being switched or hooked in with any of the front speakers, will simply be hooked to a jack on the cabinet exterior to be “daisy-chained” with one of our larger, floor-wedge monitors, where the same relative-loudness principles will apply (our wedge monitors are 8-ohm, 12” speakers; we use Peavey Black Widow 1203 drivers designed for guitar use because of their accentuated midrange).  Viewed from the back of my new cabinet, the two left monitor speakers will carry PA/vocals; the two right monitor speakers will let me hear my own drums; and each pair will have an L-pad beneath it for volume control (and feedback suppression if necessary, since my microphone will not be far away).  If all this works as planned, I will have my entire speaker setup contained in one box taking up no more stage-floor space than a conventional acoustic kick drum (although it will certainly be taller and heavier).  I have determined that the cabinet cannot be more than 41” high overall or I won’t be able to see over it, and the height of the actual box can’t be more than 36”, because the casters I have will add 4-5 inches to the cabinet height.  I would never build a box this big and heavy without casters.  Or else its own running gear, engine, and steering mechanism.

Oops, another idea.

I’ve designed a box I can just see over while seated, with monitor speakers conveniently built in and flexible switching; life is good.  But note that first photo again – I have to amplify this beast somehow, and with that big cab sitting in front of me, I’ll have to find another place for an amp, which used to sit conveniently on top of the smaller cabinet.  If I placed it on top of the new cabinet, I would have a lovely view of, well, a bunch of wires coming out the back of an ugly old amp.  So in this design, the amp will have to sit somewhere else.

At the same time, a lot of the volume in the box is simply air, which is good for that big 18” but adds to the cubic volume of the stuff I have to carry around.  I generally plug my drum module directly into a power amp and speakers.  Can I integrate an appropriately-sized power amp into this cabinet?  More sketching, and I start noodling with this design:

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Amp-in-cabinet design promotes convenience at the expense of size, weight.

There are two disadvantages to this idea.  One is that the big box, heavy driver, and power amp in combination would be very, very heavy – probably 30+ pounds heavier than the unpowered version I originally conceived.  The other is that as I lose volume in the main box, the 18” loses low-frequency response.  I will do some rough computer-modeling to see if that’s critical.  But there would be huge advantages common to any powered-cab design.  Let me ramble on about those for awhile.  Like you have a choice, right?

Powered Cabs.

You are starting to see a lot of “self-powered” or “powered cab” speakers for sale from major PA manufacturers, although usually at very high prices.  A “powered cab” can be a useful item in your group’s arsenal of equipment, for a number of reasons:

        It allows a “modular” approach – for the little gig, you can just take your little PA.  For a larger gig, just add a powered cab or two with few changes to your usual settings.

        It permits a close match between amp and speaker power ratings – with the power amp and speaker integrated into one box, you shouldn’t end up driving mongo speakers with an underpowered amp, or vice-versa, and you will quickly learn what amp settings your speakers will allow without overdriving.

        It simplifies connections, in a way.  When connecting additional speakers to an existing power amp, you have to give consideration to speaker impedance ratings (4-ohm load, 8-ohm load, and so on) so that your amp isn’t fried from too low a load, or rendered inefficient from a load that is higher than necessary.  With a powered cab, you’ve already matched the amp and the speaker(s) it drives.

        If your powered cabs are full-range, heavy-duty units, you can use them for pretty much anything that has a “line-level” output – PA, guitar, bass, your Walkman when you’re goofing in the garage.

The disadvantages?  Well, you could lose some flexibility in arranging amps and speakers, and a powered cab is always going to be heavier than an unpowered one.  Plus you have to think in terms of line-level and AC power connections, which introduces different stage-layout concerns than simple speaker cables.   Basically, though, a powered cab is pretty much like the “combo amp” used by most guitarists – you walk in with a box, set it down and plug it in.   In my experience, guitarists need strong hands and quick fingers, but the "combo amp" makes brains pretty much optional.

If you’re designing a powered cab from scratch, you probably want to make its internal width (i.e. side-to-side, inside the cabinet) at least 19-1/2” or so.  This is to accommodate the width of “rackmount” power amps, which are relatively plentiful for those of you with fresh tax-refund checks in hand.  The cleanest look is a cab with interior dimensions exactly “rack width” so that the face of the amp fills its horizontal space, and any speaker or combination of speakers in 15”, 12” 10” or smaller sizes can be fit in that horizontal space easily.  You can even give yourself the flexibility to convert to a “combo amp” setup by adding a preamp and/or other rackmount signal-processing equipment if you use standard prethreaded rack rails and leave yourself a few extra rack spaces.  You can of course make the cab wider than the rackmount equipment, with little filler panels on the sides, if you need to accommodate 18” speakers or are retrofitting an existing cabinet.  So that’s what will have to happen in the project I’m describing today.

Most modern power amps have two channels.  For powered-cabinet use, and unless you are planning a stereo speaker setup for some arcane reason, make sure your amp has a “mono bridge” mode that allows connecting the two channels for a higher-power, monophonic output.  In my design I will have the flexibility to use the “mono bridge” feature to drive the entire cabinet with higher effective power, or (with the switch described above) to drive the front speakers with one channel and the rear speakers with the other.  With an external patch cord between the two sets of monitors, I can even drive all four of them with the same signal.  

Amp-mounting considerations.

Where the amplifier should be mounted in a powered cab depends on the physical attributes of the amplifier itself.  Ordinarily, it is most convenient to mount a power amp in the top of a powered cab, where any controls are convenient to an ordinary standing human, or a squatting NBA star.  There are lower-powered amps that are not terribly heavy and are cooled by finned heat sinks on the back which lend themselves to such mounting, just like a standard guitar amp.  Really powerful rackmount power amps, however, tend to weigh as much as several marble-finished bowling balls and require support not only on their rackmount face, but also at their rear corners unless they’re sitting on something flat and solid.  Mounted at the top of a powered cabinet, they can make the whole thing top-heavy and very inconvenient to carry with side handles that aren’t perfectly balanced on a front-to-rear axis.  I would mount such an amp in the cab bottom, where its weight is better positioned and where the bottom can provide extra support – but I would still secure that heavy amp at its rear, because there is no guarantee your cabinets will always be transported bottom-side-down.  Ideally all your equipment should be able to be laid on its back, face, sides or top without scratching anything delicate, and without having gravity or bumps in the road make anything come loose.

The cab should be deep enough that the amp knobs, if any, don’t project beyond the front face of the cabinet, and so that the rear with all the connectors, plugs, etc. in place, doesn’t extend beyond the rear face either.  With a really big power amp, this can mean a cabinet depth that is a little unusual for performance-speaker cabinets – sometimes up to 20” or more.  As amp manufacturers increasingly try to squeeze their designs into one or two vertical rack spaces, the resultant amps seem to get deeper and deeper.  A shallower amp (even at the expense of more vertical rack space) is good for a powered cab, because it doesn’t dictate unusual depth for the entire cabinet, which of course has to be carried from place to place for the rest of its life.

Most performance speaker boxes  have the speakers mounted in a sealed volume of air, usually with ports to relieve the backpressure and reinforce low bass in some helpful way that you’ll have to read about somewhere else (again, guitar cabs are the exception, many of which have open backs, like hospital gowns).  In a homebrew powered cab, you want to give the speakers their own sealed chamber, apart from the amp.   Power amps don’t lend themselves to airtight mounting, since both the faces and the backs need to be accessible, and power amps generate tremendous heat that you don’t want to trap inside a sealed box.  So you will just need a partition inside the cabinet, roughly the same dimensions as its top or bottom, to divide the amp space from the speaker space, and it will need to be secured as well as if it were the actual top or bottom of the cabinet. 

You could direct-wire the speakers to the binding-post connections on the amp.  I prefer, even at the expense of complexity, to install a jack on the partition board and wire the speakers to that.  Then I use a small jumper wire from the amp connections to this jack.  In an emergency situation, I can hook another amp to this speaker very easily.  I can always hook this amp to another speaker.  This overcomes the objection that a powered cab is not “flexible” for different configurations.   At the same time, the jumper wire can be left in place practically forever since it doesn’t stick out of the cab. 

Modeling performance

OK, so I’d like to make this a powered cab for my convenience.  Then I can just plop down my drums, wired to the module, roll in this behemoth, plug the module to its input and plug the power amp to an AC source, and I’m in business.  Now the question is, if I rob space from the cabinet to accommodate a power amp, will my cabinet still have decent bass?  Luckily I don’t have to build the cabinet to find out; I can do a little computer-modeling and get a good idea of the result.

There are a vast number of commercial, shareware and freeware box-modeling programs out there, as I have mentioned in prior articles.  So far as I know, they all work well and give similar results.  For this project I will do my modeling with a free utility called “PEBox,” downloadable from the Parts Express website.  It’s a DOS-based program and its interface is not very attractive, but it has two advantages for me.  First, my Dayton woofer is a “house brand” from Parts Express, and PEBox’s database already “knows” all of the parameters of the speakers they sell, so I don’t have to enter these manually.  More importantly, PEBox has a little pop-up utility that allows you to see the predicted results of smaller-than-optimum enclosure volumes on low-end response.

I pop up PEBox and identify my driver as #295-085.  Like most such programs, PEBox gives me the optimum volume for a vented enclosure, which is 9.17 cu. ft., and suggests dimensions of 41” x 25” x 15.5”.   Remember, these are interior dimensions, so if I use ” plywood all around, the exterior of this “optimum” box when finished would be 42.5” x 26.5” x 17.”  This is a little wider and a lot taller than I want; I was aiming for 24” exterior width (22.5” inside), 18” depth (16.5” inside), and 36” high.  So if I build the box the size I want and need, it will have a nominal interior volume of 22.5” x 16.5” x 34.5”, or 7.4 cubic feet (the cubic-inches-to-cubic-feet conversion factor is 1,728).

Nor is this an “empty” box.   If it is 22.5” wide and the little monitor sub-chamber I build will be 6” x 10”, that subtracts another 1,350 cubic inches from the interior volume.  If I choose the powered option and invade the “box space” for 3 rack spaces at 1.75” each, and an extra “floor” or partition, that eats up 6.5 vertical inches, or 2,413 cubic inches.  1” x 2” interior bracing (it’s actually milled to ” x 1-1/2”) eats up 1.125 cubic inches for every linear inch, so I’ll probably have 200 cubic inches of that.   I can minimize all this somewhat by cutting down on the size of some of these “deductions,” but for the most part they’re necessary to the design.  So from my nominal interior volume, I will lose approximately 4,000 cubic inches, leaving between 5 and 5.5 cubic feet of usable air in my cabinet.  What is that big Dayton woofer predicted to do in a cabinet with that sort of volume?

The quick-and-dirty way to find out is to press “F8” in PEBox, which will plot the predicted low-frequency rolloff of the selected driver in cabinets of varying volumes.  In my case, it tells me that the F3 (the point where low-frequency response rolls off by an audible 3 db), which was 32hz for the optimum 9-foot cabinet, will be raised to about 40 hz.  If I really wanted those last 8 hz, I could make the cabinet considerably deeper, but then it would probably stick off the front of the stage or something.  If I would forego the “powered” option and reclaim the 1.45 cubic feet it consumes, the F3 would only drop from 40 to about 37 hz.  If I were designing a cabinet for some other instrument this might be worth it, but the fundamentals of a bass drum may be most important in the range just above 40hz, see http://www.sospubs.co.uk/sos/jan02/articles/synthsecrets0102.asp, so maybe I can live with an accelerated fall-off below that.  It will still beat the F3 of the smaller 15” cabinet, and I won’t have to haul a separate amp around.  And the 18” was previously mounted in an “X-Audio” cabinet of only 3.5 sq. ft., and sounded surprisingly good even in that relatively tiny volume.  So, a powered cab it is!

Arranging things.

In a previous PA-speaker project I recommended arranging your intended drivers on a piece of newspaper the size of your planned baffle board.  With speakers on both the front and rear of the cabinet, this project is a little more three-dimensional, but my rough calculations indicate I should be OK.  I plan about a 1-1/4” front setback of the baffle (i.e. a 2” setback of the mouting framing, less the ” thickness of the baffle) and my horn/driver combination (actually I will use two, for power-handling) is about 12” deep – that’s the important measurement, because I want the horn at the top of the cabinet for projection (and the 18” is only about 9” deep anyway; usually your horn drivers are the critical depth measurement).  So in the remainder of the cabinet depth, I have room left for the 6” deep sub-chamber I envisioned, sitting behind the horns.  Tight, but adequate.

Looking at the face of the baffle, I have an 18” diameter speaker to fit in a 22.5” X 34.5” baffle.  I will center it along the shorter, 22.5” axis, leaving 2.25” space on either side.  I decide to position it that same 2.25” from the bottom edge of the baffle, both for looks and to keep the bouncing waves from the back of that huge cone at least a little ways away from the nearest internal wall.  That leaves me 12.25” of remaining vertical space, 22.5” wide, for horn(s) and port(s).  But I can’t really use the uppermost 1.5” of that without notching or cutting down the bracing behind it, so as a practical matter I have a space 10.75” by 22.5” to play with. 

As mentioned above, I would like to use two of the piezo drivers to enhance power-handling and to more nearly match the capabilities of that big, powerful 18”.  I have seen a little threaded “Y” connector to attach two drivers to a single horn in “siamese” fashion, but I’ve searched parts houses and websites and can’t find such a thing for sale (if I found one it might add too much depth for this project anyhow).   So, to get two high-range drivers I’ll have to use two horns.  I can get large, 15” x 5” horns to fit my drivers from a number of suppliers, but if I stacked a pair of those in the remaining cabinet space there would be no room for ports.  So I will go with smaller horns 11” wide or less, placed side-by-side, to fill the width of the cabinet but leave me some port space on the face of the baffle.  There are two downsides to this: the output of horns placed this way tends to change radiation and horizontal-dispersion patterns somewhat; and smaller horns are less resonant help in the lower midrange.  But I decide it’s worth it to double my power-handling capability in this application, and get a pair of Selenium HC23-25 horn lenses, each approximately 10” x 6”.

After that, I have 4.75” in vertical space left to fit my port(s).  I decide to tune my port-cabinet combination a bit below the 40hz approximate F3 cut-off; this will raise distortion below that point and be a little tough on my woofer, but I’m not building it to reproduce the contrabass in the Boston Symphony anyway; I just want it to make loud, thumping noises.  PEBox tells me (F2 function key) that a 5.5 cubic foot cabinet for my driver should have a 40.13hz “f3,” a box-port tuning frequency of 34.89 hz, a response peak/dip of 3.20db, and should have a minimum round-port area of 6.06 in. so air won’t “whistle” through the ports.  The PEBox port calculator (F3 function key) advises a single 6-inch port tube 7” long, or two 6” ports 18” long (which won’t fit front-to-back in my cabinet).  With 4.75” of vertical space left for my 6” port, I have a problem.

Sometimes you can fit a large port in the additional space left by the curvature of the speaker, especially if the speaker is somewhat offset, but this of course places the port very close to the speaker and the cabinet wall.  The accepted wisdom is that most ports should be spaced their own diameter away from either the speaker or the cabinet sidewall to best capture the rear output of the speaker and have a chance of emitting that in phase with the front output.  One alternative, and exception to this principle, is the “slot port,” a rectangular port that hugs one of the cabinet sides.  There is a handy online guide for calculating slot ports at http://www.carstereo.com/help2/Articles.cfm?id=31.  I tell it I have a 5 cu. ft. cabinet I want to tune to 27hz, and just for grins I tell it to calculate a slot port 21” wide and 1.5” tall.  It tells me that my dimensions are too long and skinny for optimum performance, but I have a space problem here and ignore the warning.  Then it tells me the walls of the slot port should be 8.16 inches deep, which will easily fit in the depth of my cabinet.  Why did I select 21” wide, you ask?  Well, that’s the width of the interior of my cabinet, less two ” thicknesses of bracing material on the inside corners.  Why 1.75” tall?  Because (1) I only have 4.75” of vertical space to play with, and (2) a piece of ” x 1.5” bracing material attached next to the corner brace will form a 1.5” high frame, to which I can simply attach a 22.5” x 8.16” piece of plywood and have the suggested 21” x 1.5” x 8.16” slot port!  Despite what was indicated in my initial sketches above, I will place the port on the bottom so people can’t look right in there and see all the rough-finished guts of the cabinet, or stick things in the port.  A 41” high cabinet is almost precisely at eye-level with my friend’s 4-year-old, who frequently comes to band practice and looks for things to destroy.  So from bottom to top, the order will be: amp, port, woofer, horns.

Building the box.

I will use the same construction techniques, or lack of them, outlined in my earlier article on this site.  I know my box dimensions already; I’m just building a box with another, smaller box inside, an extra floor, and some other details. 

 One thing I always do, which I forgot to mention in the previous article, is project my dimensions onto a sketch of a standard 4’ x 8’ plywood sheet, to estimate how much lumber I will need.   Sadly, I can’t build this box from a single sheet of plywood – but my local lumberyard sells precut 2’ x 2’ sheets as well, so I can make do with one big sheet and one little precut one for the main panels:

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Planning your panel cuts.  Your sketch doesn't need to be pretty, just accurate.

 

 

 

 

 

Missing in this diagram are the pieces for the inner sides of the monitor subchamber and the 22.5” x 8.3” side of the port.  If I don’t have scrap plywood in the garage of the right dimension, I will probably just buy 2-foot precut lengths of 1” x6”, 1” x 8”, and 1” x 10” pine boards for these.  For the rear of the monitor chamber, I will be screwing from the cabinet exterior into the edge and ends of the 10” plank, and plywood doesn’t take screws driven into its edge very well (or very strongly).  The rear baffle will be screwed into the edge of the sub-chamber floor, same principle.  The top of the port will have an edge that extends to the front face of the cabinet, and pine is easier to final-finish than plywood. 

I start by building the main box with bracing in the front down to the top of the port only, and then with the two extra planks the width of the interior, attach the “sub-chamber” inside.  Unlike the main chamber, this will not be ported; the little 5’s with their floppy rubber suspensions like a fully-sealed cabinet, and don’t put out much bass anyhow.  My sub-chamber is 10” high, giving me room for bracing and speaker mounting, and the bracing to support the baffle board is recessed to leave room for a grille using smaller, ” X ” bracing around the face of the sub-chamber to save space while the main chamber gets larger, 3/4” X 1-1/2” material for strength.  The vertical wall of the sub-chamber will help brace and dampen the top of the overall cabinet.   Here’s the cab about midway through assembly – sorry, the port and amp chamber are painted flat black so they’re hard to see:

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Rear and front of raw cabinet.   Putty and sanding are in preparation for spray-on finish, described below.   Left photo shows monitor sub-chamber and some of the crap in my garage; right photo shows additional bracing on interior of sealed cabinet back.

An alternative covering.

As you can see from the photo, while this is still a fairly simple rectangular box, it will be difficult to cover with carpet in the traditional wrap-all-four-sides way, because of the horizontal elements of the speaker-amp partition and the port.  To cover all the exposed areas of this cabinet with carpet, I would have to do a lot of cutting and patching, and would have numerous seams.  The situation is similar if you build a traditional “trapezoidal” cabinet; the non-parallel sides force you to have carpet seams in places that may take abuse, and create a lot of carpet wastage. 

After a little research, I decided to try a spray-on finish.  While ordinary black paint provides little protection for the underlying wood, there are heavy textured finishes designed for rough exterior use that can be as tough as traditional speaker carpet, using formulations developed for non-skid walking surfaces and pickup-truck bedliners.  These are generally polyurethane or epoxy-based finishes that provide a rough, textured coating that is impervious to the elements, and to most forms of abuse.  There is an entire cottage industry that has sprung up around these finishes, which will prep and spray-apply them to the interior of a pickup-truck cargo bed (usually for a few hundred dollars US), the interior of a cargo van, or what-have-you.  These places may be more prevalent here in Texas, where there are an awful lot of pickup trucks running around, but there should be one in or near just about any major city.  Plug “spray-on bedliner” into your web search engine or check the Yellow Pages ads under “truck accessories” or something similar.  There is also a roll-on, do-it-yourself kit sold under the brand name “Herculiner” for about $100 US, which is a suspension of small particles of rubber in a two-part mix of polyurethane, but it comes sized to do a small truck bed and once it’s mixed it has to be entirely used, so I will save that experiment for a time when I have more or larger cabinets.

A textured polyurethane finish is thicker than paint, but does not have nearly the error-hiding capabilities of carpet, so it requires a fairly smooth cabinet finish.  What that means is that I have to putty over screw holes and seams, and sand the exterior of the cabinet fairly smooth, just as I would for any paint finish.  This requires an electric sander, several pieces of sandpaper, and a tolerance for sanding dust wafting all over the place.  But hey, you’re going to keep this handcrafted cabinet for a long time, right?

With the exterior of the cabinet essentially finished and all holes puttied and sanded smooth, I took the box to a local “Line-X” franchisee (see www.line-x.com) who agreed to apply their finish to the box exterior for $70 – more than carpet would have cost, certainly, but the results in this case were spectacular and even with all the filling and sanding, I saved a lot of labor.  They took the cabinet into a huge booth where a guy in a protective space-suit finished it beautifully in about five minutes.  They explained that the two parts of the formulation are actually mixed at the last minute, at the sprayhead, and that no real noxious solvents are used (an apparent disadvantage with the do-it-yourself formulations, which use highly volatile and flammable solvents).  The coating will stick to unfinished wood as well as it sticks to painted metal.  The result was a wrinkle-textured finish that was dry to touch almost immediately upon application, and in ten minutes I was headed home with my beautifully-finished cabinet.  I know photos can’t convey texture all that well, but hopefully you can get some idea of the result:

    LINEXF_1.JPG (84972 bytes)LINEXB_1.JPG (89159 bytes)

Raw cabinet after spray-on finish.

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The finish, close-up.  Looks like "Tolex" but a lot tougher.

This stuff is tough, but you can drill and saw through it, as I did to attach casters, handles, corners, etc.  It is also thick enough that there is no point precutting any removable baffle(s) until the finish is applied and you can tell how much allowance you need to make in baffle dimensions.  I did not have the finish applied to the baffles, intending to paint or carpet-cover them since they will be largely hidden behind the grille covering anyhow.  I did, however, flush-mount a nonremovable back on the main chamber so that it has the same finish as the sides of the cabinet.  I will have interior access through the baffle (and when it is installed, though that huge 18” hole in it), so a removable back would only have added needless complexity.  Come to think of it, having a removable baffle was probably a needless complexity as well.

Final assembly.

 By building the entire monitor baffle, with the jacks and l-pads, in one piece and recessing it relative to the box edge, I simplify the wiring for those as uncoordinated as me and help protect those L-pad knobs from being sheared off in a tragic it’s-late-I’m-tired-throw-it-into-the-van accident. 

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The rear monitor baffle subassembly before wiring.

The amp chamber gets side trim to a 19” front width; I couldn’t find precut rack rails 3 spaces high so I decided just to bolt an amp right into holes in well-braced hardwood.  The rear of the amp is fastened to L-brackets bolted to the cabinet floor.

From this point it’s pretty routine, as I have described before.  I foam-line the main cabinet and sub-chamber (the interior of a slot port needs to be unobstructed, so no foam for it), install and wire the speakers as designed, and screw the front and back baffles in from the exterior with black bolts that hopefully won’t show much.

Oddities I encountered along the way.

L-pads are generally available with short shafts, to allow “panel mounting,” or long shafts, for mounting through thick wood.  I got the latter, with a faceplate and knob attached, because that was easier than mounting the equivalent of another “jack cup”.  But it was a lot longer than the thickness of my monitor baffle board, so I had to put another thickness of wood on the back side so that I could snug the L-pad against the interior without having it stick waaaaay out the back.  With a thicker baffle I probably wouldn’t have experienced this.

I always prefer to front-mount speakers to a thick baffle.  However, the little 5’s had so little depth that in a ” or 5/8” thick baffle, the speaker terminals where the wires attach would have been pocketed down in the hole cut for the speaker.  Since this is a low-power application with small baffle dimensions, lots of metal things bolted to it, and the additional bracing of the divider between the speaker grille and jacks/L-pads, I went with a 3/8” thick plywood baffle for these little guys.  The front baffle, where the stress of high power will be felt, is ” plywood.  As explained in my prior article(s), T-nuts and bolts are used throughout to attach handles, casters, baffles, etc.

The amp I found, a Peavey 4C (400 watts, bridged), is three rack spaces high, hence the three-space void at the bottom of the cabinet for it.  If in the future I install a two-rack-space amp, I can simply install a “blanking panel” in the extra space, or even a power strip to give me an outlet for my drum module and/or other things.  I incorporated a small “lip” of bracing material at the bottom rear of the amp chamber so I could throw cords, etc. in there, including wadding up the amp’s own power cord, and have some hope they will stay, just like the guitar guys do with combo amps.  With three rack spaces of height, there is still plenty of room to reach in there from the rear and make connections.  And since the PV-4C amp has two parallel sets of input connections, I can run another line-level cable out of it to an empty PA channel and put just a touch of e-drums into the PA mix to help keep everyone in sync with me.

I used a round metal grille for the woofer – not my favorite compared to “full-screen” metal mesh as recommended in my other articles, but I didn’t have the depth from the front lip to use that – especially since the Selenium horn lenses stick out the front of the cabinet almost an inch (won’t make that mistake again) and would require that a full-screen mesh grille be stuck waaaaay out there.

So here is my powered cabinet, finished and ready to roll.  Thankfully I found I did not need any minor adjustments to the fixed monitor resistors, wiring scheme, or anything else. At over 100 pounds, it’s a two-person job to lift this thing, but with 4” caster wheels the necessity for lifting is minimized.  The bedliner finish is wet-rag cleanable.  And the whole thing has a very, um, purposeful look.  The other people in the band immediately christened it “Ampzilla”:

ampzilla2.JPG (73541 bytes)The finished cabinet - heavy and imposing, but it works as advertised.

However, when they heard it reproduce a kick drum the giggling subsided pretty quickly.  Total cost was a little under $650 US, including $200 for the amp, used from eBay, $160 for the 18” driver, $90 for horns and drivers, $40 for the monitor speakers, $70 for the finish and about $100 for the wood, accessories, etc.  The results are pretty much exactly what I’d hoped for.   It will reproduce a deep bass drum and tinkly cymbals, and everything in between, at sound levels which approach the point of pain.   The monitors are well positioned and give me a perfect (and adjustable) sample of both my drums and the PA output.  Best of all, I connect three cords (AC power, line-level input from drum module, PA monitor cord) and I’m up and running, which gives me more time to fiddle with the PA (or my pathetic comb-over) before a gig or practice.  If I want to use it to play bass, keyboard, guitar, or even use it as a supplemental PA cabinet, it will handle anything I throw at it.  And I know that there is not another cab just like this, anywhere in the world!

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Ampzilla in action at an outdoor concert, February 2003.  Worked great!.


You can send comments, questions, or bits of freeze-dried lark's tongue to lmundy@attbi.com.  In the Dallas-Ft. Worth area, you can check www.yournamehereband.com for local appearances by "Ampzilla" or links to a video of an exploding whale and other deplorable stuff.


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.

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