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stevie

12" Cab Diary Continued

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Hey guys, I can't stress enough about sourcing good plywood. I went to my local timber yard and just asked for a sheet of 3/4 plywood. It looked great and even had a really nice grain in the top layer. Later when I inspected along the cuts, I noticed a lot of empty little voids between the plys of timber. Even with good sharp blades in my cutting tools the ply splintered and and some quite large chips came out.

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[quote name='The Shrek' timestamp='1489696507' post='3259092']
Hey guys, I can't stress enough about sourcing good plywood. I went to my local timber yard and just asked for a sheet of 3/4 plywood. It looked great and even had a really nice grain in the top layer. Later when I inspected along the cuts, I noticed a lot of empty little voids between the plys of timber. Even with good sharp blades in my cutting tools the ply splintered and and some quite large chips came out.
[/quote]We have chosen poplar for our builds and they prototype cab was made from Spruce. The little voids you see are knots and they fall out once they have been sawn. All you can do is fill them with sawdust and PVA glue. If you use wood filler, TuffCab will not stick to it. There is no such thing as knot free ply, at least not at a realistic price, although a B/BB grade will give a pretty good outer veneer.

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I began the build this week and so far I am pleased. The poplar supplied was 7 PLY not 9 as they show on their website. It seems the Timber Merchants and importers are at the mercy of the producers as to what is supplied. I had heard that the producers and exporters of Poplar ply could be erratic and that may be why it is hard to find. Anyway the outer faces are lovely and it almost seems a shame to cover them with Tuff Cab, Vinyl or Carpet. However it has to be done.

The cuts were good and all the panels except the struts were within 1mm of the specified size. The struts were cut too long, but reviewing my own cutting drawing, I can see how that could happen. Lesson 1: separately detail the size of each panel.

Once I got the panels home, I started a dry fit of the cabinet. I held the panels square and flush using some corner clamps from Toolstation. [url="http://www.toolstation.com/shop/p85872?table=no"]http://www.toolstati...p85872?table=no[/url]. I drew a line 7mm from the edge and used No 8 by 1.5" screens to hold them in position. Once the cabinet is glued and the glue has dried, I will replace all the screen with dowels. During the design phase, we had a number of discussion about how to construct the cabinets. Phil Starr is in favour of battening each corner along the whole length to give a greater gluing area and to avoid the need for buying lots of clamps. There is no reason why you cannot do that for this build although you would have to make your own decisions as to how to design the struts. It does make construction much easier although the end result will be a heavier cabinet. Incidentally I weighed the screws to see how much extra weight they add. They weigh 2 grams each so if you decided to uses screw and leave them in place, the extra weight is negligible. Where screws are a problem is when you try to round over the edges.



You can see the clamps in the picture here. The baffle is just pushed in.

The picture below shows the line 7mm in from the edge with the three screw heads on that line. Not quite in focus is my countersink bit. I counter sunk all the holes to avoid the edges splitting. I would avoid relying on the modern self-countersinking screen here as the extra pressure need by them could create the split we are trying to avoid. On the longer sides I used 4 screws. It is important to keep the screws away from the extreme edge of the board as there are two edges there and the material is weak until glued. Clearly I should have vacuumed the top before taking the picture but the sawdust adds realism.




It is important when you do a dry fit to be sure that the panels go back in the same way when adhesive is applied. The picture below shows how I am doing that. I was writing upside down when I did this but my handwriting really is that bad



The next step is to remove the excess wood from the struts. In order to get two cabinets from one sheet of play, we only had two lots of two struts cut, knowing that the other two could be made form plywood offcuts. To Be Continued.

Edited by Chienmortbb

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Well I now have the two complex struts cut out/ More about that later. You will see from the pictures that they are not geometrically perfect. My original idea was to use round holes of the same area as the rectangles that were drawn. I them decided I should build it to the drawing too see if that was a good way to go. Stevie designed the struts based on individual softwood battens that were screwed and glued to the side of the cabinet. It seems sensible to replicate that in a single panel of ply, with corresponding cut-outs. After spending some hours trying to get the struts to look like the drawing, I believe that road holes or individual pieces of wood are better.



On strut L, I did use my jigsaw to round off the ends. Unfortunately I did not have the correct blade for tight turns so it is not perfect but it does save a lot of time. The tear out you can see was my fault. In order to cut some of the longer runs faster I used a coarse blade on scroll mode. It was mistake.

I cut Stevie's strut (great name for a song) K at the same time but that was a little more geometrical. We have had a discussion as to whether the rear of strut L should be a figure of 8 rather than a single long hole. We agreed it would and foolishly, I forgot to mark it on the drawing. I will make Stevie's a figure of 8 then we can compare the strength. [size=4]I have some work to do now but will attach [/size][size=4]the[/size][size=4] struts to the back later. [/size]

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I quite like the idea of using round holes for this. I imagine it would be a lot quicker.

Edited by stevie

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[quote name='The Shrek' timestamp='1489696507' post='3259092']
Hey guys, I can't stress enough about sourcing good plywood.
[/quote]

Totally agree with this. If you are going to spend your valuable time building a cab like this, don't dry to save pennies on the wood. You can make this cab from just 1/2 sheet - so it's not that much more to get the good stuff. Cheap plywood will just not do the job properly. If you're going to use normal ply, get Finnish or Russian birch plywood. If you're going to use poplar ply, get the Italian stuff. Unless you're a wood expert, avoid anything from China.

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[quote name='stevie' timestamp='1490025283' post='3261536']
Totally agree with this. If you are going to spend your valuable time building a cab like this, don't dry to save pennies on the wood. You can make this cab from just 1/2 sheet - so it's not that much more to get the good stuff. Cheap plywood will just not do the job properly. If you're going to use normal ply, get Finnish or Russian birch plywood. If you're going to use poplar ply, get the Italian stuff. Unless you're a wood expert, avoid anything from China.
[/quote]I totally agree and for strength and total durability Finnish Birch would be the first choice. Unfortunately it is also the heaviest being 1.75-2.0 times heavier than Poplar but about 30% stronger on average. Or to put it another way a 15mm Polar cab is stronger that a 12mm Birch one.

I weighed a complete kit of the poplar boards and the weight came out at about 7Kg, a 15mm birch cab would weigh between 12-15Kg. add the drivers and crossover (approx 6Kg for mine) plus a further 0.5Kg for the connectors and handle. My 65 year old back says Poplar.

As I have said before, it is inevitable that there will be knots in the ply, and some of these will fall out when an edge is cut. It is one of the drawbacks of the material but is not a big deal unless you go down to 12mm or even 9mm ply.

Edited by Chienmortbb

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This is why I spent the time sorting out the bracing. With a birch cab, a circular brace above the driver, one behind the driver and one on the top panel would be more than enough. If weight isn't an issue, that would be my first choice for performance.

7kg sounds good though. :)

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The cabinet is slowly coming together. The two main struts are attached to the back and the sides are about to be glued too:



As we go into Wednesday, the two sides have been glued and I will post a picture tomorrow. All other struts will be created from off-cuts of plywood similar in size to the bracing pieces used on the prototype cabinet from which the drawings were created.

I have created a drawing to allow an accurate cut-out for the PH-170 P-Audio horn, despite its complex shape, Sadly when I try to put it on here the dimensions are altered. I will keep working on that or PM me for a copy of the template drawing in PDF form.

Edited by Chienmortbb

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Sorry for the delay in posting, I had the start of a virus when I was st Stevie's on Tuesday and it has laid me low ever since. I have made some progress and will post photos tonight.

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I have been trying to do the odd bit on the cab despite wanting too curl up in the corner. I finished putting the braces/struts into the cabinet and put the other side on.



Then I cut the baffle hole in both my baffle and Stevie's. Mine went OK but lost concentration with the router and ended the rout of Stevies with black sawdust on the baffle. I had routed through the live wire but being a class two device, there was no danger of a shock. I have rewired it now but the cable is 35cm shorter :unsure:.

Next trick, dry fit the baffle. That is the next thing to go on so it needs to be a good tight fit and it is.



Sadly when I spaced out the braces, I calculated that the full brace above the baffle would sit just above the woofer hole...... except that I calculated to the centre of the brace and the is 7.5mm lower than the centre line and just [u][i]below[/i][/u] the woofer cut-out. You can see that in the image below where you can also see some of the sanding I need to do.


I am not too concerned as I think the angle to the speaker and the offset from the foam sealing will ensure that the brace is not in the way. The slight lost in strength is also offset by the better support for the woofer. I just have to keep repeating those two sentances t until I believe it. :tatice_03:

The good think about building one cabinet before the other is that these little issues can be corrected on the second cabinet. Stevie's should turn out perfectly :D

After I have tidied up the inside by sanding the rough edges of the braces, I will mark-out and cut-out the hole for the horn. I have a novel way to mark that out but it might not work! Then fit the wadding*, the baffle, the port and the connector hole(s) on the back. The top and bottom will be fitted last of all. I think we are still on track for the SW Bash Bash but I am going to bed now and hope to wake up feeling better tomorrow.

*The wadding is due Tuesday but I may have to glue the baffle before it arrives. Hopefully more progress tomorrow.

Edited by Chienmortbb

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These are the components that John will be fitting to his cab. A P Audio PH-170:



and a Celestion CDX1-1455 compression driver:



My prototype version looks something like this:



It is worth mentioning that 1" exit compression drivers come in different voice coil sizes. The most expensive versions have 1.75" coils. This one has a 1.4" coil, which offers a good compromise between performance and price as long as you don't try to use it too low. But it works well down to 2kHz. I have always found that compression drivers with 1" coils sound really tizzy and are only useful as supertweeters. Some bass cabs manufacturers use them, however, because to most people a compression driver is a compression driver.

Because we are unable to take advantage of the natural rolloffs of the drivers this time, the crossover is more complex. Also, this is a constant directivity horn, which by its very nature requires a more complicated crossover circuit. The design is finished, and I'll post the details of the crossover, the crossover layout, frequency resonse and impedance measurements as soon as I can.

Despite the fact that the horn and driver only cost around £50 together, this is probably the most advanced HF unit on any bass cabinet on the market.

Edited by stevie
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I am struggling to get this finished before the South West Bass bash on Sunday. I ordered some bits form Blue Aran, Handle, corners, screws TuffCab and that came today. i am still waiting for the wadding. Yesterday I finished the baffle and installed it together with a new brace to support the baffle between the Horn and Port. This is a weak area and I used a triangular ogg curt that attaches to the baffle and the Top. I painted the baffle matt black before installation. Tuff Cab does not adhere well to other paint so I was trying to avoid contaminating subsurfaces that might need Tuffcab.




The next picture shows the new brace. Im did wipe the excess glue after I took the picture.



I cannot put the top and bottom on until I have the wadding so next, the grille.

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The grille has been tricky. i decided that the weight of a good metal grille would be too much. I already had some old really tough plastic grille material ,and decided to use that. Here is a picture of the Grille almost complete.



The plastic is very string and tough and quite hard to work with. It is stapled on to the edges of the frame. It pulled the sides of the frame inwards so I added a brace as you can see from the clamps. The red triangles are thin pieces of plywood that both strengthen the joints and give me something to screw through to hold the grille in place.

If I had to do this again I would not do it this way and I may redesign the grille after the bass bass next Sunday.

Having tried a dry fit of the grille I have to say I am much happier with it. The two things that worry me now are whether I can get the fixing screws on and whether I will be able to get it off if I need too. To say it is a tight fit is an understatement.

Edited by Chienmortbb

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Supporting the section between the port and the horn is a good idea. A simple strut on the baffle would suffice, but a triangular piece is even better.

I like the grille too. It's a lot tougher than your usual fabric but a lot lighter than perforated steel. Let's hope we can iron out the assembly snags.

Another thing..... I've found that you can apply Tuff Cab to other finishes all right. I think that the distributors are perhaps being a bit over-cautious with their advice. Of course, there's always a first time. :)

Edited by stevie

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Tuff cab seems OK with water based finishes. However I guess any other finish will act as a barrier between it and the woodand might affect the way it bonds, meaning it may chip easier later. I've had no problems so far though.

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We're a bit under time pressure here, what with the SW bass bash looming. I've wound the coils for John's cab and he will be picking them up tomorrow. All very skin-of-your-teeth.

So here is the LF section of the crossover with the board layout. Please shout if you see any silly mistakes. There will be a PDF pinned to message # 1 in due course.

Edited by stevie

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Here's the HF section. Can anyone recommend anything better than Microsoft Paint for this kind of stuff?



[IMG]http://i64.tinypic.com/n2g16q.jpg[/IMG]

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I'll give it a go. I've just tried three other highly recommended drawing programs, but they're not as intuitive to use as Paint. I just need a copy-and-past feature so that I don't have to draw everything from scratch, and the ability to save text as text, so that I can edit rather than erasing and re-writing.

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[quote name='stevie' timestamp='1490895827' post='3268931']
We're a bit under time pressure here, what with the SW bass bash looming. I've wound the coils for John's cab and he will be picking them up tomorrow. All very skin-of-your-teeth.

So here is the LF section of the crossover with the board layout. Please shout if you see any silly mistakes. There will be a PDF pinned to message # 1 in due course.


[/quote]

You've got A connected to C, which bypasses the entire LF crossover.

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[quote name='stevie' timestamp='1490908332' post='3269041']
Here's the HF section. Can anyone recommend anything better than Microsoft Paint for this kind of stuff?




[/quote]

A is connected to B, which will bypass the entire HF crossover.

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Oops! I knew there would be something. Thanks for spotting this, tauzero. I'll fix it now.

Edited by stevie

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