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Mandolin Curved Cabinet Tower build (Bagby Design)

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  • Mandolin Curved Cabinet Tower build (Bagby Design)

    Edit 6/2/2017 : These speakers got chopped up, and are being converted to Mandolin MTM! Still using Jeff Bagby's crossover design for the MTM.
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    I previously made a thread for these. It somehow got deleted. After 2 months talking to mods, I determined I should just make a new thread. Apologies to anyone who posted in the previous thread. Mainly I wanted to post this for reference for anyone who likes curved cabs or wanted to know more about this design.
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    First off, I would like to thank Jeff Bagby for answering all my stupid questions, PETT member TimK for helping me tremendously with CNC work, and Meniscus Audio for customizing my order, answering my questions, etc.

    After much consideration, I decided to build the Mandolin by Jeff Bagby from Meniscus Audio. This was based on Jeff's comments on the design and reputation, Zaph's measurements of the drivers used, and my requirements. I kept looking for a “small” and “narrow” speaker although eventually I settled on a design that was 8.5” wide. The size limitations are due to WAF.

    The Mandolin uses the SB Acoustics SB29RDC-4 1 1/8” Ring Radiator tweeter and the SB Acoustics SB17RNXC35-8-UC 6" midwoofer. I was excited with the build quality of the SB accoustics midwoofer- which is excellent. Rear cone area is mostly unobstructed, the motor is overbuilt, and the motor venting is robust- seems like quite a bargain.




    My twist on the standard Mandolin enclosure design is adapting them into a "small tower" format, slightly changing the crossover layout, and building a (hopefully) very rigid and visually appealing cabinet. Speaking of which, here's a prototype I hacked together in Google Sketchup. I suck at using sketchup, FWIW.


    The internal structure parts were made with assistance from local PETT forum member Timk, who used his expertise and equipment to make flawless CNC-cut Braces and a precision rabbeted panel. The side panels will be 6 layers of 1/8" material, 5 layers of 1/8” Baltic Birch plywood, 2 layers of ¼” MDF, and 1 layer of HDF, bound with a gallon of Titebond 2. I'm going to use strap clamps to form and fit the sides to the curve I need.

    The T/S parameters for the SB17RDXC-8 are interesting. The SB Acoustics provided parameters appear optimistic, and Zaph's measured T/S parameters are much different. I consulted Jeff Bagby and his parameters were closer to SB's, on a fairly fresh woofer. I have suspected that either Zaph does not thoroughly break-in drivers before testing, or his measurement system differs from what others commonly use. Therefore, using this and some woofer break-in data from Danny Richie, I formulated "broken-in" T/S parameters. We'll see how well I did later. After many of simulations in Unibox(using all 3 different T/S parameters), I arrived at the conclusion that there was little to be gained by increasing internal volume much from the standard enclosure size. Therefore, only about 1 L of additional displacement was added, carefully calculating the displacement of the port, crossover, and internal braces. Plus the blueprints for the origional were only about 14.1 Liters net (undersized, slightly).
    Last edited by djkest; 06-02-2017, 08:15 AM.
    -Dan
    Mandolin Curved Cabinet Floorstanding; Dayton Reference 18" sealed Subwoofer; Sealed 12" Dayton Reference Subwoofer ; Overnight Sensation builds

  • #2
    Re: Mandolin Curved Cabinet Tower build (Bagby Design)

    I like the looks! I'm trying to get a curved tower build finished up myself. I'm looking forward to watching your build progress.

    I have one tip on your glue choice. You may want to see if you can get Titebond Extend somewhere close by. Titebond II only has an open working time of 5 minutes. When you're spreading glue on large panels, that 5 minutes goes by VERY quickly! Titebond Extend will give you 15 minutes of open working time and is the recommended choice for bent laminations.

    Comment


    • #3
      Re: Mandolin Curved Cabinet Tower build (Bagby Design)

      At work I have SolidWorks, and I used it to model my internal braces. By using different amounts and placements of stress, I was able to optimize the size and placement of the 2 holes for the internal brace. Sorry, I can't post the SolidWorks data on my proprietary machine.

      I made these prototypes using a home-made large-radius router jig and a jasper jig.


      I soon realized that it would be difficult for me to make a bunch of these that were dimensionally identical. While I can make the shape, it's hard to replicate it exactly.


      The overall size is going to be 8.5" wide at the front, coming down to 4.5" at the back. 35" tall (before the stand/feet) and 12" deep (with the front baffle).

      Here's the information I passed on for the CNC machine:


      After TimK made the CNC parts for me, I went to work cutting the one rabbeted panel into smaller pieces, and routing out circles. Then I assembled everything for the initial frame.

      side view




      top view


      front view

      In the previous thread, a lot of people were ... upset that I didn't put a large woofer in the bottom (or subwoofer) and turn this into a 3-way. It wasn't my intention, and actually the woofer chamber takes up more than 1/2 of the total volume. These are really bookshelf speakers with built-in stands.
      Last edited by djkest; 03-19-2013, 02:44 PM.
      -Dan
      Mandolin Curved Cabinet Floorstanding; Dayton Reference 18" sealed Subwoofer; Sealed 12" Dayton Reference Subwoofer ; Overnight Sensation builds

      Comment


      • #4
        Re: Mandolin Curved Cabinet Tower build (Bagby Design)

        I decided that I needed more bracing, because I was going to be putting a lot of stress and strain on this frame while bending the sides on.

        I made some additional bracing using MDF scraps and then used my router to round the sides a little.





        I was hoping to add some weight to the bottom by adding some solid chunks of MDF. The idea is that even though these lower chambers are largely useless, they do contribute to the mass and stiffness of the enclosure.

        -Dan
        Mandolin Curved Cabinet Floorstanding; Dayton Reference 18" sealed Subwoofer; Sealed 12" Dayton Reference Subwoofer ; Overnight Sensation builds

        Comment


        • #5
          Re: Mandolin Curved Cabinet Tower build (Bagby Design)

          Dan what about stealing a page from Wolf and filling the bottom chamber with kitty litter or something? This would also add to the stability of the enclosure from being accidentally tipped over.

          Edit: sorry I should have realized you were still posting...:o
          HAGD,
          Marc

          Even though I try to tell everyone upfront, understand that I am still a Newb. I wish the status of Seasoned Veteran/Senior Member, etc. was earned with time not posts...

          TMWW thread

          Maurbacs DCR Tower

          Comment


          • #6
            Re: Mandolin Curved Cabinet Tower build (Bagby Design)

            But, it didn't really work out as well as I'd hoped. Here's my first attempt at a first layer using 1/8" Baltic Birch plywood (3 ply).



            Notice the crack... :/



            And then trying to trim some of the excess off, I took off too much.

            These were dark times. It was really a struggle. And it only got worse.

            Here's the 2nd layer:






            Okay so the 2nd layer went a lot better. But that didn't last for long.

            3rd layer:


            Because of many factors, this third layer didn't work at all. The glue dried prematurely, the wood didn't want to bend, it just didn't happen.

            I ended up ripping and scraping the whole thing off. It took a really long time to scrape all the wood off, and then sand it smooth before I could continue. I estimate 4-6 hours. It was a dark time for me, lol.

            Luckily I figured out that my back piece had been cut at much too severe an angle, and that was causing some of my problems. I carefully made these wedge shaped pieces on my tablesaw. It took a lot of trial and error to get them just right. Without a tablesaw, I would have been screwed.



            -Dan
            Mandolin Curved Cabinet Floorstanding; Dayton Reference 18" sealed Subwoofer; Sealed 12" Dayton Reference Subwoofer ; Overnight Sensation builds

            Comment


            • #7
              Re: Mandolin Curved Cabinet Tower build (Bagby Design)

              Originally posted by WernerM View Post
              Dan what about stealing a page from Wolf and filling the bottom chamber with kitty litter or something? This would also add to the stability of the enclosure from being accidentally tipped over.
              I should have done that, and still may. I could drill a 1" hole through the back panel and fill it with sand or kitty litter, and then plug it up. There isn't much free air in the bottom chamber, but the next one up has quite a bit more.

              I am contemplating a 1/8" layer of hardboard on the back to cover all the seams. Stay tuned.

              Continuing on:
              The gap that the wedges fixed on speaker 2 are pretty apparent on speaker 1. This is going to be have to be fixed with bondo. Lots of bondo.




              ----------------------

              A router with a flush trim bit gives us a peek into the layers that were applied. I think this is the "good" one.




              Oops, I left some stuffing and crossover stuff in there from testing.
              -Dan
              Mandolin Curved Cabinet Floorstanding; Dayton Reference 18" sealed Subwoofer; Sealed 12" Dayton Reference Subwoofer ; Overnight Sensation builds

              Comment


              • #8
                Re: Mandolin Curved Cabinet Tower build (Bagby Design)

                So it came to making the front baffles. Crazy me, I decided they needed to be solid wood. So I went to Home Depot and got a big chunk of Aspen. It was painstaking work getting the holes routed accurately.
                Apparently I couldn't wait to listen, I just clamped everything on. Don't tell my mother.









                -----------------

                Oh yeah, what about crossovers?
                -Dan
                Mandolin Curved Cabinet Floorstanding; Dayton Reference 18" sealed Subwoofer; Sealed 12" Dayton Reference Subwoofer ; Overnight Sensation builds

                Comment


                • #9
                  Re: Mandolin Curved Cabinet Tower build (Bagby Design)

                  Crossover:
                  I made my crossover boards out of 1/4" MDF, with through-holes drilled for zip ties. I sealed the boards with sanding sealer, then spray painted them. It's a waste of time, but I enjoyed it anyway.

                  Doing a bunch of reading around the web has provided me with a large variety of opinions on the subject of “crossover component quality”. I decided to take a “middle-ground” approach regarding component selection. I decided that better quality tweeter capacitors may matter enough for me to spend some extra money on the crossover. With that in mind, I upgraded them to the low end Clarity Caps from Madisound. These weren’t particularly expensive and are high quality pieces. I also upgraded the resistors from the generic sandcast resistors to Lynx resistors. The woofer capacitors are Solen which are high quality poly caps, and purportedly work well for this purpose. The cost difference was fairly minimal and it gives me some warm fuzzies. The inductors are of course hand-wound "five nines" Meniscus Audio inductors. The tweeter section uses 18 gauge air-core I believe, and the woofer is an iron-core, which helps reduce DCR. Does it make it sound better? I'll never know, but I'd like to think so. Thanks to Wolf for exchanging PMs with me about Capacitors and helping me pick components out. I feel that we achieved a balance of cost and quality, without going overboard.

                  Crossover Rev 1:

                  (note, this photo has been edited to hide the component values)

                  I also redid the crossover boards to be a little smaller painted a 2-tone “sunburst” on the crossover board. Yes, it’s overkill but it didn’t take me much time. I became concerned that a few of the capacitors may be “squished” by the overzealous tightening of zip ties, so I went out and bought a new multimeter that could measure capacitance. What I found was that these “dented” capacitors were extremely close to the target values. I was no longer concerned. Perhaps the dielectric has prevented damage.

                  Last edited by djkest; 02-03-2015, 12:35 PM. Reason: edited for clarity
                  -Dan
                  Mandolin Curved Cabinet Floorstanding; Dayton Reference 18" sealed Subwoofer; Sealed 12" Dayton Reference Subwoofer ; Overnight Sensation builds

                  Comment


                  • #10
                    Re: Mandolin Curved Cabinet Tower build (Bagby Design)

                    So yeah, so I made that crappy Aspen Baffle, right? I soon realized that Aspen is a pretty bad material for a speaker baffle. It's hardness and stiffness aren't very high.

                    So, I did some research on materials properties of various types of solid and composite wood products. This lead me to Bamboo, which would have been awesome, but it was costly, difficult to find, and a general pain in the ****. So I settled on American White Oak, which is similar to Red Oak. It's very hard, very strong, and looks awesome. I think I paid about $40 for the wood for these 2 baffles. I think I bought 4 feet, not entirely sure.

                    I provided this drawing to TimK:



                    And he made the baffles. Then I took them to my dad's house 120 miles away to use his drill press.





                    You can see how perfect his flush-mounting is.





                    And lastly, the dorky temporary listening setup I was using. (I threw a blanket over my TV to reduce HF reflections).

                    Last edited by djkest; 02-03-2015, 12:36 PM.
                    -Dan
                    Mandolin Curved Cabinet Floorstanding; Dayton Reference 18" sealed Subwoofer; Sealed 12" Dayton Reference Subwoofer ; Overnight Sensation builds

                    Comment


                    • #11
                      Re: Mandolin Curved Cabinet Tower build (Bagby Design)

                      Originally posted by djkest View Post
                      Luckily I figured out that my back piece had been cut at much too severe an angle, and that was causing some of my problems.
                      I had that EXACT same problem, but mine didn't appear to be as severe as yours. I know the inside joint is sealed and secure, so I'm just going to fill the voids on the back and sand it flush.

                      Comment


                      • #12
                        Re: Mandolin Curved Cabinet Tower build (Bagby Design)

                        Great job guys, so......Dan any more thoughts as to the overall "finished" sound in "the dorky temporary listening setup", which looks like a normal 2 channel critical listening area to me.

                        Robert
                        -Robert

                        Comment


                        • #13
                          Re: Mandolin Curved Cabinet Tower build (Bagby Design)

                          Ah, the subjectives are always so subjective. I think they have great clarity, nice space and seperation, good imaging. They have kind of a sweet sound, very nice. Norah Jones's voice is portrayed so wonderfully here. I love Jones and Dianna Krall, and both of them sound wounderful on these speakers. I've listened to everything you can imagine on these, and have not been dissapointed. Hip-hop, Electronic Music, Rock, Metal, Jazz, Classical. Joshua Bell sounds great, his violin dances through my speakers. I know this is taboo, but often times the music sounds "live" or at least gives me the illusion of live sound.

                          I was impressed that listening to Disturbed with their electric guitars sounded good, I wonder if paper cones are just the right thing to reproduce heavily distorted guitars. Some accurate speakers can be kind of "harsh" or "clinical" with some music, especially poorly recorded music. I did not detect any harshness here.

                          Bass performance is hard for me to comment on. These are tuned to 38 Hz which is pretty low. I think I am going to redo the tuning to 42 Hz, it won't much matter once I have them crossed over to a sub anyway. Depending on taste, musical selection, and room size, you may find them adequate. I like mine with a sub.
                          -Dan
                          Mandolin Curved Cabinet Floorstanding; Dayton Reference 18" sealed Subwoofer; Sealed 12" Dayton Reference Subwoofer ; Overnight Sensation builds

                          Comment


                          • #14
                            Re: Mandolin Curved Cabinet Tower build (Bagby Design)

                            Very nice. I applaud the persistence :applause:. It's heartbreaking when you have to spend a ton of time ripping appart something that you just spent a ton of time putting together :( Been there a few times.

                            Any curved builds i work on going forward will probably have clamping forms (made with the negatives of the internal braces) to apply pressure to the curve. I had problems with strap clamps as well.

                            What's the plan for a finish on these guys?

                            Comment


                            • #15
                              Re: Mandolin Curved Cabinet Tower build (Bagby Design)

                              Originally posted by lowpolyjoe View Post
                              Very nice. I applaud the persistence :applause:. It's heartbreaking when you have to spend a ton of time ripping appart something that you just spent a ton of time putting together :( Been there a few times.

                              Any curved builds i work on going forward will probably have clamping forms (made with the negatives of the internal braces) to apply pressure to the curve. I had problems with strap clamps as well.

                              What's the plan for a finish on these guys?
                              Thanks for posting. Yeah, it was really tough at a few points. It looks really easy when something like PassingInterest does a build and everything comes together beautifully. I'm just a hack with some tools and a big imagination. Luckily TimK is a pro and he helped me out a lot through PMs and with his CNC work.

                              The clamping forms would be a really smart idea. And with that I'd like to present my "lessons leaned" with regard to curved cabinet building in the method that I used.

                              1) I would not use Baltic Birch again. It's really strong, but it doesn't like to bend and it's picky which way it wants to bend too. And I wouldn't use bending ply for speakers based on my limited experience with it. 1/8" MDF works awesome for this, although it isn't hard to break or mangle it on the first layer. It bends well, is very uniform, and has some nice density.
                              2) If you can, I do recommend you do the final layer as 1/8" HDF. It's great stuff to worth with, bends well, and has a very smooth / less porous surface. It should make finishing a lot easier.
                              3) It's twice as difficult and takes 3 times as long to do this as you think.
                              4) You need more straps.
                              5) As someone previously mentioned, extended dry time glue would be helpful, especially since I did both sides at once. Take your time, it's harder to fix mistakes than to do things right the first time.

                              Oh, and Ben, your curved cabs look awesome, way cleaner than mine are. I may claim to have decent hearing, but I'll never claim to be a master craftsman.
                              -Dan
                              Mandolin Curved Cabinet Floorstanding; Dayton Reference 18" sealed Subwoofer; Sealed 12" Dayton Reference Subwoofer ; Overnight Sensation builds

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