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  • MTM with ribbon tweeter, my first complete design

    So here is my first crack at a design form the ground up. I wanted to use a ribbon tweeter since I heard some speakers with one quite some time ago, and really enjoyed them. That and because I like to be different/difficult. I chose the Fountek NEOCD3.5 based on the low cost and relatively low crossover point (per the spec sheet). I chose the Dayton RS150P-8A (originally mistakenly said RS180P-8A) based on the relatively high break up point judging from the spec sheet. I think I'm pushing it with the cross over point with these two at 3000 but I think it's doable. I'm not married to either driver if there is a better fit.

    My process just to verify that I didn't skip anything. I started with unibox and arrived at 46 liters and tuned at 50 Hz, then went into response modeler. I used data provided by dayton and some measurements I found online that someone did on the tweeter. I entered the baffle info, TS parameters, and the box parameters, but used half the volume, is that correct? I then went through the motions with that sheet combing box and baffle response and extracting min phase. I did this separately for each woofer and the tweeter. Anyway, I've attached some screen shots. With the crossover, I really wanted to keep it simple, but ended up with 4th order for both, with baffle compensation, l-pad and zobel on the tweeter. I could get the response curve pretty flat with second order on both, but then phase does not line up. I've done a whole lot of reading over this project but haven't figured out of this is a show stopper or not. If possible I'd like to simplify the crossover to reduce cost as well as just reducing the number of components in the signal path. Another concern is that the impedance dips quite low at the top end, but I can't seem to get it down while also keeping the top end from climbing too much. I'll be running this from a Kenwood home theater receiver which I think will handle 4 ohms at first and then intend to either build an amplifier or buy something vintage, maybe Marantz, either of which I believe would handle 4 ohms fine. With respect to cost, I've set a goal of $400, but could stretch it to $500 if it would get me something great. That's for drivers and crossovers and I think I'm right about there at $400 after estimating crossover components. I've already gotten lumber.

    If those with more experience could let me know if I'm headed in the right direction or lend any insight, I'd appreciate it. A little about my experience, I've done a ton of woodworking including building speaker cabinets and done tons of soldering, so I'm well equipped for the assembly, but just learning about designing from the ground up. Thanks in advance for the help.
    Crossover woofer response
    Last edited by fork; 04-11-2017, 08:19 PM.

  • #2
    On the tweeter padding, that solo 2 ohm parallel resistor is the problem. I would increase it, and then add some series resistance to get the needed padding amount. Then you will need to change other values in the tweeter x-over.

    Most of my x-overs have lots of parts, and that's because I feel they are needed to get the best results.

    Comment


    • #3
      Sorry, lots of parts and better xovers have absolutely nothing to do with each other. In fact most better sounding speakers have as few xover parts as possible.
      craigk

      " Voicing is often the term used for band aids to cover for initial design/planning errors " - Pallas

      Comment


      • #4
        Thanks, I increased the parallel resistance, then the series resistance wasn't really having much affect so I moved it from "after" to "before" and was able to tweak everything.

        Comment


        • #5
          I would continue tweaking. Its generally unnecessary to need a 4th order circuit on the woofers. 2nd, maybe 3rd but rarely 4th.
          https://www.facebook.com/Mosaic-Audi...7373763888294/

          Comment


          • #6
            Pushing a 7" woofer to 3000Hz and then handing off to a wide dispersion ribbon will result in very uneven off axis response. I'd not use anything larger than the RS150 for that task.
            R = h/(2*pi*m*c) and don't you forget it! || Periodic Table as redrawn by Marshall Freerks and Ignatius Schumacher || King Crimson Radio

            Byzantium Project & Build Thread || MiniByzy Build Thread || 3 x Peerless 850439 HDS 3-way || 8" 2-way - RS28A/B&C8BG51


            95% of Climate Models Agree: The Observations Must be Wrong
            Gravity is an overrated force on the cosmic scale. Physicists are missing the bigger picture. They fell into a black hole and were never seen advancing the understanding of the cosmos again.

            Comment


            • #7
              Originally posted by fork View Post
              So here is my first crack at a design form the ground up. I wanted to use a ribbon tweeter since I heard some speakers with one quite some time ago, and really enjoyed them. That and because I like to be different/difficult. I chose the Fountek NEOCD3.5 based on the low cost and relatively low crossover point (per the spec sheet). I chose the Dayton RS180P-8A based on the relatively high break up point judging from the spec sheet. I think I'm pushing it with the cross over point with these two at 3000 but I think it's doable. I'm not married to either driver if there is a better fit.

              My process just to verify that I didn't skip anything. I started with unibox and arrived at 46 liters and tuned at 50 Hz, then went into response modeler. I used data provided by dayton and some measurements I found online that someone did on the tweeter. I entered the baffle info, TS parameters, and the box parameters, but used half the volume, is that correct? I then went through the motions with that sheet combing box and baffle response and extracting min phase. I did this separately for each woofer and the tweeter. Anyway, I've attached some screen shots. With the crossover, I really wanted to keep it simple, but ended up with 4th order for both, with baffle compensation, l-pad and zobel on the tweeter. I could get the response curve pretty flat with second order on both, but then phase does not line up. I've done a whole lot of reading over this project but haven't figured out of this is a show stopper or not. If possible I'd like to simplify the crossover to reduce cost as well as just reducing the number of components in the signal path. Another concern is that the impedance dips quite low at the top end, but I can't seem to get it down while also keeping the top end from climbing too much. I'll be running this from a Kenwood home theater receiver which I think will handle 4 ohms at first and then intend to either build an amplifier or buy something vintage, maybe Marantz, either of which I believe would handle 4 ohms fine. With respect to cost, I've set a goal of $400, but could stretch it to $500 if it would get me something great. That's for drivers and crossovers and I think I'm right about there at $400 after estimating crossover components. I've already gotten lumber.

              If those with more experience could let me know if I'm headed in the right direction or lend any insight, I'd appreciate it. A little about my experience, I've done a ton of woodworking including building speaker cabinets and done tons of soldering, so I'm well equipped for the assembly, but just learning about designing from the ground up. Thanks in advance for the help.

              Is this speaker mostly for HT? How big is the room? Subwoofer?

              Comment


              • #8
                I changed to second order and tweaked some values. i got a pretty flat response, however the phase is way out of whack. My understanding is that the phase should be aligned or close to it where the drivers overlap. And thats not the case here, pretty near 45 actually.

                These speakers are for my living room which is about 15x20 with two large doorways in one wall, one large doorway in the wall opposite, all of them always open. Its a far from ideal room for critical listening. They will be used only for music, and I would like them to be able to play without a sub. I'd say 50 percent sitting in an ideal position (most important) , and 50 while wandering the house listening from all over, and the occasional party.

                Pete, I'll be honest, I did not consider off axis response at all in the design. I'm inferring from your comment that larger speakers have poorer off axis response at higher frequencies. But now that I go back to compare the RS180P-8A to the RS150P-8A. I see that the 180 does not exist and the 150 is what I actually used, haha. I'm going to edit the top post.

                Comment


                • #9
                  Image of the second order crossover.

                  Comment


                  • #10
                    Do you have an accurate Z offset? If not, then your crossover simulations will all be for naught. An accurate Z offset is required to ensure accurate phase relationship between the two drivers.

                    Comment


                    • #11
                      Hmm, I did not use any Z offsets. How would I go about figuring them out for the drivers, the data sheets and google search (for the tweeter) come up empty. Do I need to purchase the drivers and measure?

                      Comment


                      • #12
                        Originally posted by fork View Post
                        Hmm, I did not use any Z offsets. How would I go about figuring them out for the drivers, the data sheets and google search (for the tweeter) come up empty. Do I need to purchase the drivers and measure?
                        That's my conundrum as well, I tend to do a lot of paper designs without the drivers for a sort of geeky fun. I guesstimate the z offsets generally sticking the woofer about 3/4" back from the tweeter give or take based on where I feel the center dust cap will land, but they're not going to be quite right without actual measurements.

                        Honestly though (heavy IMO here), you'll get pretty close and with a good FR they'll sound quite good. They may not compete for a competition trophy without some actual measurements and design, but I gather that's not what you're going for.
                        Electronics engineer, woofer enthusiast, and musician.
                        Wogg Music

                        Comment


                        • #13
                          Originally posted by fork View Post
                          Hmm, I did not use any Z offsets. How would I go about figuring them out for the drivers, the data sheets and google search (for the tweeter) come up empty. Do I need to purchase the drivers and measure?
                          I'd ballpark a sim at 1/4" offset, and compare it to 1/2". Small changes to the x-over may need to be made to improve the response. Auditioning x-overs is fun. You will need extra parts. Actually you could use more parts in the x-over too! Obviously some here will disagree. Tilting the box a little will also change what's ideal, or correct for what is not.

                          Comment


                          • #14
                            Originally posted by fork View Post
                            Image of the second order crossover.
                            What is that huge discontinuity in the tweeter impedance?
                            R = h/(2*pi*m*c) and don't you forget it! || Periodic Table as redrawn by Marshall Freerks and Ignatius Schumacher || King Crimson Radio

                            Byzantium Project & Build Thread || MiniByzy Build Thread || 3 x Peerless 850439 HDS 3-way || 8" 2-way - RS28A/B&C8BG51


                            95% of Climate Models Agree: The Observations Must be Wrong
                            Gravity is an overrated force on the cosmic scale. Physicists are missing the bigger picture. They fell into a black hole and were never seen advancing the understanding of the cosmos again.

                            Comment


                            • #15
                              I will play with the offset and see what that gets me. I'm not sure about the tweeter impedance. I found the data here. http://www.audioexcite.com/?page_id=4178.

                              What are the implications to the sound if I don't get the phase right. Will there be strange nulls and cancellations, or are we just talking about getting the last 5% of potential out of the speakers.

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