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Helping a friend build a new bass cabinet.


isaac42

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Design philosophy:

 

Ideally, the 2240H should be in a 10 ft^3 cabinet, but that's impractically large. Also, he wanted it to match the looks of his 4x10 cabinet. So I went with a cabinet 30" high x 24" wide x 20" deep, about 6 ft^3. Target tuning is 30.9 Hz, to minimize cone motion at the fundamental of the low B string. The resulting frequency response isn't particularly flat, but is has a nice, smooth roll off on the low end which should suit his tone and keep it from being boomy even in small rooms.

 

Here's the cabinet as delivered:

 

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Yesterday, we rounded off the corners so that the corner pieces he bought will fit, installed the t-nuts to mount the driver, made the hole for the input jack and installed the ducts in the ports.

 

I did a bit of playing around before we got busy. I put the JBL into the box, then connected a signal generator to it. I roughly measured the cabinet tuning at about 47 Hz. Even tuned that high and with little more than a volt to drive it, the low bass coming out was nice. Audible down to 20 Hz, though I suppose that might have been mostly harmonic distortion. Then I turned the frequency down to 5 Hz and we watched the driver move in and out. Kinda cool.

 

Now we wait for the handles and casters to arrive.

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One other thing that might be interesting. The holes for the ports were cut slightly too large. I already had tubes for the ducts. What to do? We carefully wrapped the cardboard tubes with masking tape until they fit snugly in the holes. Then I put white glue on the wood in the holes and inserted the tubes. I made sure they were flush with the front baffle, then added a bit more glue to fill in any gaps. After curing for a day, the tubes are rock solid.

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Handles and casters are still on back order. Well, at least, they have yet to arrive. So no progress.

 

One other issue we'll have to address: gaps. The guy who built it used butt joints. I have no problem with butt joints as such, but in this case, there are gaps. Specifically, there are gaps between the top and both the front and back panels. I have a plan to deal with them. On the front, we need to put in a framework for the grill. That can double as a seal for the gaps there. For the rear, I intend to install a ¾"x¾" piece of molding to cover the gap.

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  • 1 month later...
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For the rear' date=' I intend to install a ¾"x¾" piece of molding to cover the gap.[/quote']

 

 

Are the gaps that severe?

 

If the gaps are small, gap filling with slow-curing epoxy will do the job.

 

 

 

 

>>> JBL 2240

A speaker of that size must mean reggae. :rastaphil:

 

 

 

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Good thought, and one which wouldn't have occurred to me (hammers, tools, nails, and all).

 

No, not reggae. 80s rock and metal. I neglected to mention that this cab is going to be a subwoofer rather than a full range cabinet. The main sound will be coming from a 4x10.

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Good thought' date=' and one which wouldn't have occurred to me (hammers, tools, nails, and all).[/quote']

 

 

Just mix up some "30 minute" epoxy on a painter's pallet (make a painters pallet from some scrap A/C plywood with a little block handle on the backside). Take tour time and trowel in and pack the epoxy into every gap with a plastic throw-away sheetrock taper's knife completely filling the crack or gap. Use a powerful flashlight to detect gaps.

 

Also, fill in any divots, cracks, gouges, dents, and imperfections on the flat external surfaces with this epoxy where you will be applying Tolex covering. Yes, Bondo will work, too, but epoxy is even stronger.

 

 

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I neglected to mention that this cab is going to be a subwoofer rather than a full range cabinet.

 

 

A while back -- maybe about 8 years ago, for about 9 months -- the market kinda got flooded with factory-refurbished Peavey Black Widow PA speakers. You could get these 15-inch and 18-inch 1000 / 1200 watt drivers with big ol' huge honkin' magnets on the back for relatively low $.

 

I knew of a few people that built a front-firing speaker cabinet around one or two of these speakers, with a Crown or QSC PA amp driving them from the preamp feed from their bass amp or bass preamp.

 

 

 

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A while back -- maybe about 8 years ago, for about 9 months -- the market kinda got flooded with factory-refurbished Peavey Black Widow PA speakers. You could get these 15-inch and 18-inch 1000 / 1200 watt drivers with big ol' huge honkin' magnets on the back for relatively low $.

 

I knew of a few people that built a front-firing speaker cabinet around one or two of these speakers, with a Crown or QSC PA amp driving them from the preamp feed from their bass amp or bass preamp.

 

That's the idea. Cosmetically, we're trying to make it look like his 4x10, as if they were intended as a set.

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That's the idea. Cosmetically' date=' we're trying to make it look like his 4x10, as if they were intended as a set.[/quote']

 

 

With that sort of weight, you need to have casters with fairly large wheels so that cracks and gravel bits can't impede rolling the cabinet to and from stage and van. I'd be tempted to fabricate a large-ish "shopping buggy" style rear handle to mount on the back of the cabinet to aid in transport and handling, like what Ampeg and a few other do with their 6x10 and 8x10 cabinets.

 

Something else that has always concerned me slightly with these sort of high-powered speaker cabinets, is the front panel that the loudspeaker mounts onto / into. I'd probably go a bit overboard and glue two 3/4-inch plywood sheets together for the front panel to maximise stiffness. Of course, you can also use 1.0-inch or 1.5-inch "furniture grade" voidless ash or birch plywood, if you have access to a lumber store that carries such supplies. An alternative to a thick front panel would be to screw'n'glue in a pair of vertical 2x4 braces on both sides if the speaker.

 

For the sides and back, 3/4-inch is sufficient, but the front panel gets far more mechanical abuse from the loudspeaker than any other part of the cabinet. If the front panel waffles around in a sympathetic manner with the loudspeaker, sound quality will suffer.

 

 

 

 

 

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  • 1 year later...
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I can hardly believe that cabinet has been sitting in my garage for over a year now. In the meantime, my friend has had a heart attack and quadruple bypass surgery. He's feeling better now, though, and came over yesterday. We did a bit more rounding, then routed out depressions for the handles and drilled holes for the casters. Before he came over, I went ahead and put in a piece to block off the gap. Last thing was to glue on the top. So some progress was made.

 

Next thing is to put on the "rat fur" covering, then install the casters, handles and input jack. I hope it won't be another year!

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  • 2 years later...
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It's been a while.

I did some work on the cabinet, if only to get it out of my garage! We decided to pass on the rat fur, and painted it. Black all around, except for white on the baffle board. I installed the casters and handles. Yesterday, I vaccumed out the spiderwebs and stapled in some fiberglass insulation. Then, I installed the driver. Looks like this:

image.png

It's functional as it stands. Still needs a grill to protect the driver. I need to fire it up today, to hear how it sounds.

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Played through it yesterday. It has a lot more midrange than you might expect. Sounds very clean. Not boomy at all. One might even think it doesn't have enough low end, but that's because it isn't putting anything in that wasn't there before. The goal was for the bass to be deep rather than boomy, and it seems to be a success. Want more bass? Turn up the bass control, and it's all there. I should run a few tests on it. Maybe this weekend.

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  • 6 months later...
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Tom came over yesterday, and we finished the cabinet. He was very pleased with the sound: deep without being boomy. Exactly what he wanted, and exactly what I was shooting for in the design.

And now, I have a little bit of room in my garage!

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