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My A7 Project - an almost mini RS100-4 and ND20FA-6

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  • My A7 Project - an almost mini RS100-4 and ND20FA-6

    Here is my latest project. I hope some of you will find it interesting.



    Two summers ago, I won a pair of RS100-4 midwoofers at the Atlanta DIY get together. They were in bad need of a home so I came up with this little speaker. I was intending to use them for side surrounds, but in the end they ended up as my son's computer surrounds.

    These sound way better than their small size would indicate. However, they are not the most efficient speakers you will ever hear, so feed them well. 50 watts/ch should be plenty and less will do if you don't need to play them real loud. They also reach down to 70 hz easily, so they can be used alone or crossed to your favorite sub.

    I discussed these briefly in Ryan's thread about his little speaker project, but I thought a few more details might be helpful for those who are looking for a speaker like this for a small room or maybe a computer.

    These are a basic two way with a crossover around 3500hz. I say around 3500hz because the crossover is a bit atypical. The woofer rolls off above 2000hz and the tweeter kicks in electrically at 4300hz. I was trying to keep the parts count as low as possible, but still not compromise on the accuarcy. You can see this in these in room measurements:



    The chart is a bit busy. First there are measurements of the woofer (with and without the notch filter) and tweeter separately. Then the composite with the tweeter in and out of phase electrically. The lowest point on the darker blue response curve with the tweeter in electrical phase with the woofer is at about 3500hz which corresponds to the null. (3500hz also roughly corresponds to A7 on the piano, and thus the name.) The null is not extremely sharp, which indicates that they are not 100% phase matched, but since the tweeter out of phase response is so flat, I didn't try to get a better reverse null. It still took eight components in the crossover to get this close and I didn't want to make it any more complicated since this was designed to be as simple as I could get it.

    Here is a drawing of the crossover and the box plans. The 10" X 8.25" X 5.5" box is 1/2" MDF with a 3/4" front baffle. I didn't think I needed thicker MDF on such a small box and indeed it's pretty dead in the knuckle wrap test. But the cross brace is an absolute necessity. Sorry, but I don't know any of the cad programs to draw it nice and pretty, so I'm afraid this is all you will get :



    The crossover shows two variations. (I have other variations that I'll discuss later.) I ended up with the 4.3uF and 3.0 ohm tweeter highpass version. In the response chart above, I ran the two variations a few db higher to show them separately and so you could see the difference. The 4.3/3.0 version corresponds to the lower, light blue trace. The 5.1/2.2 version (purple) didn't sound bad at all, in fact some of you might prefer it, but when I used the speaker as a center channel, the over equalized high end on our local TV stations just made it too bright. But for regular music use, both versions sounded very detailed and quite musical without ever sounding harsh.

    I'll go into more details later, but if this looks like something you might like, I think the RS100-4 is on sale for one more day at PE.

    -Bob
    Attached Files
    -Bob

    The PEDS 2.1 mini system
    My A7 Project - another small desktop speaker
    The B3 Hybrid Dipole - thread incomplete and outdated

  • #2
    Bob,
    Those look real sharp. The RS drivers are great, so I'm sure it sounds good.

    I now see why you were so shocked by the crossover in my project. Did you use software to design it?
    - Ryan

    CJD Ochocinco! ND140/BC25SC06 MTM & TM
    CJD Khanspires - A Dayton RS28/RS150/RS225 WMTMW
    CJD Khancenter - A Dayton RS28/RS150/RS180 WTMW Center

    Comment


    • #3
      I didn't have frd files to use design software, so I used a combination of on line calculators and a good bit of trial and error. I'm not so sure that design software would have helped a whole lot since the tweeter doesn't have a very smooth rolloff on the bottom end and the woofer rolled off more than I expected, so it took some tweaking to get them to work well together.

      -Bob
      -Bob

      The PEDS 2.1 mini system
      My A7 Project - another small desktop speaker
      The B3 Hybrid Dipole - thread incomplete and outdated

      Comment


      • #4
        Tonight, I have just a few notes on the box size and bass alignment. First, I measured the TS parameters for the RS100-4 using a WT3. I expected a closer match to the published parameters, but since both of my samples after break in were very close, I used the measured parameters to determine the box size:



        The Fs and Qts were much higher than spec, but since they were consistant, I used them.

        I plugged these into WinISD and got this box and tuning:



        Since the Q on the woofers was rather high, it was difficult to use a ported box without some minor compromise. In this case, I had to accept a small 1db bump between 100 and 200 hz. But this slightly reduced the amount of baffle step correction that was needed, so it was easy enough to compensate. You can still see about 1 db higher before rolloff before bass rolloff, but it is not noticable. It would have been higher, but I stuffed the box pretty full of poly fiberfill to reduce the base bump to this low level. In WinISD I changed Qa to 5 to model the stuffing.

        To those who say that WinISD is not very accurate, I will strongly disagree. It is accurate below baffle step. You just have to add baffle step correction after you build the box to what WinISD tells you and you will be fine.

        In this case, the 5.5" baffle width creates a need for a rolloff starting between 700 and 800 hz. With a 4 ohm driver, this works out to about a .75mH coil. Indeed, I tried everything from .3 to 1.0 and .7 was just right. It kept the output at 1000 hz about 1 db below the 100 hz SPL.

        Here is a close mic'ed response graph of the woofer with no compensation, and with .54mH and .72mH (the measured value of my .7mH coils). In each case I had the notch filter for the high frequency ringing engaged.



        First, notice that the output is very flat, unlike the in room graph in the first post. This is the real output of the driver and the rough look in the first post graph is strictly room interaction. Put this smooth output with the flat output of the tweeter, and this speaker is much flatter than the in room response graph would indicate.

        Second, notice that .7mH provides a full 6db of baffle step compensation. If you use these close to a wall or on a desk, .5mH might be better, but even used directly next to my TV screen, I didn't need to change it. But in any case, .5 was just not enough in listening tests.

        More later.


        -Bob
        Attached Files
        -Bob

        The PEDS 2.1 mini system
        My A7 Project - another small desktop speaker
        The B3 Hybrid Dipole - thread incomplete and outdated

        Comment


        • #5
          I know this is an old thread, but work got in the way and I never finished it. With the release of Paul Carmody's Speedsters and Mark65's Dadiels over on the PE board, it seems like "Tiny Two Way Week", so here we go again.

          First, I have a few last comments on the box and crossver. Here is an impedance sweep of the final box with a WT3 using the 4.3 uF cap and 3.0 ohm resistor in the tweeter circuit. You can see that while it is a 4 ohm load, it actually only goes slightly below 4 ohms in the very deep bass. Above 30 hz, it stays at or above 4 ohms, so this should be safe for small amplifiers rated to handle 6 ohm minimum impedances:



          The crossover is designed to be as flat as I could get it when placed 2.5 ft off the floor and 2 ft from the back wall, like with a stand mount or placed on the end of a low entertainment center. If you want to use these closer to a wall or as desktop monitors for a computer, I would suggest a that you reduce the BSC by reducing woofer inductor to 0.4 or 0.5 mH and using a 5.1 uF cap and 2.2 ohm resistor in the tweeter circuit.

          If you want to experiment with tweeter levels, these values will maintain the same 4.3 khz electrical crossover point on the tweeter:

          C1 R1 Approx. change from default value in db

          6.2 1.5 +3
          5.1 2.2 +1.5
          4.3 3.0 default flat
          3.6 4.0 -1.5
          3.0 5.1 -3.0


          For the crossover, I used 20 ga inductors, 10 watt resistors (except for the 1.0 ohm 5 watt for the notch filter on the woofer) and poly or mylar 250 volt caps. Since I was trying to keep this simple, I did not think that I needed 16 ga inductors or higher quality caps. I used Erse, PE and old parts box parts, but you can get all the parts at PE or Erse. The 5 watt resistor is listed under electrical components at PE - not speaker components - and you have to click on show ALL at Erse to see them.

          The crossover was designed using actual measurements of each driver in box. It took a lot of tweaking once I got it close. I used ARTA and a PE mic calibrated by Cross Spectrum Labs. Here are the A7s next to my old TV.




          -Bob
          Attached Files
          -Bob

          The PEDS 2.1 mini system
          My A7 Project - another small desktop speaker
          The B3 Hybrid Dipole - thread incomplete and outdated

          Comment


          • #6
            Now, on to box construction. I used 1/2" mdf for the sides, back and crossbrace and 3/4" mdf for the front baffle. I used a 1/2" roundover on the front baffle and a 3/8" roundover on all the other edges. The port is made from standard 1 1/4" PPVC pipe from HD. This has an actual inside diameter of 1 3/8" and an outside diameter of 1 11/16" so I have no idea why they call it 1 1/4" pipe.

            Here are a few pictures of the box under construction. Note the required camfer on the back of the woofer cutout:





            The box is lined on four sides with 1/2" Sonic Barrier and a piece is also wrapped around the cross brace. You could just as easily use carpet padding if you want. I didn't use any on the front or back panels since the
            brace and stuffing should be sufficient to prevent any internal standing waves. But I did find that I needed a lot of stuffing. Without it, it had that all too familiar one-note ported box sound. Sort of like saying "wub" in your deepest voice. I've heard that in every ported box I've ever built and found I always needed stuffing to minimize it.



            The port is located close to the bottom floor of the box and just barely clears the Sonic Barrier damping material. I did keep the port area open and free of stuffing so that the port funtions as it should.




            -Bob
            Attached Files
            -Bob

            The PEDS 2.1 mini system
            My A7 Project - another small desktop speaker
            The B3 Hybrid Dipole - thread incomplete and outdated

            Comment


            • #7
              The crossover assembly was a bit tricky since it had to be small enough to fit in a rather small box. I found that the best way was to attach it to the inside of the back panel. I used a 3 1/2" X 5" piece of pegboard and mounted it on four brass screws with bolts and washers. I used brass to prevent any interaction with the inductors, although the screws are probably not close enough to change the inductance even if they were standard steel screws.







              You can try your own parts layout, but this was the most compact I could come up with. It worked out well since it has the two inductors at right angles to each other to prevent interaction. Wiring is standard 18 gauge
              speaker wire with connectors on the speaker ends of the wires. You will need .205 and .110 inch sizes.


              -Bob
              Attached Files
              -Bob

              The PEDS 2.1 mini system
              My A7 Project - another small desktop speaker
              The B3 Hybrid Dipole - thread incomplete and outdated

              Comment


              • #8
                For the finish, I just used Rust-Oleum Hammered Black spray paint over a coat of primer. It's presentable, but many of you have done far better. I found the spray was OK, but it was difficult to get a consistant finish. It
                took several coats and it only worked well on horizontal surfaces. When I tried it on vertical surfaces, I had problems with drips and didn't get the hammered finish look very well. You had best lay the box with the side you want to paint facing up and then spray. Repeat on each face separately. It took nearly a whole can per speakers.





                That's it. I hope this project gave you some ideas. The drivers and crossover parts are right at $100 and wood, paint, screws, terminals, stuffing and wire add another $30 or so if you don't have them already. Let me know if you build a pair and what you think.


                -Bob
                Attached Files
                -Bob

                The PEDS 2.1 mini system
                My A7 Project - another small desktop speaker
                The B3 Hybrid Dipole - thread incomplete and outdated

                Comment


                • #9
                  Bob,
                  That looks great. I'll try and get you added to the "Guide" thread. I could probably do it even quicker if you filled out the table.

                  People are going to ask for a complete BOM.
                  - Ryan

                  CJD Ochocinco! ND140/BC25SC06 MTM & TM
                  CJD Khanspires - A Dayton RS28/RS150/RS225 WMTMW
                  CJD Khancenter - A Dayton RS28/RS150/RS180 WTMW Center

                  Comment


                  • #10
                    Thanks, Ryan. I'll work on a BOM tomorrow. It's a little too late here in the Eastern time zone.

                    -Bob
                    -Bob

                    The PEDS 2.1 mini system
                    My A7 Project - another small desktop speaker
                    The B3 Hybrid Dipole - thread incomplete and outdated

                    Comment


                    • #11
                      I'll give you until Sunday night.
                      - Ryan

                      CJD Ochocinco! ND140/BC25SC06 MTM & TM
                      CJD Khanspires - A Dayton RS28/RS150/RS225 WMTMW
                      CJD Khancenter - A Dayton RS28/RS150/RS180 WTMW Center

                      Comment


                      • #12
                        Looks great. I'm surprised by how low it goes!

                        Nice job
                        Bianchi C2C Peerless XLS Sub

                        Comment


                        • #13
                          Per Ryan's suggestion, here is the Bill of Materials for a pair:



                          The RS100 uses #6 screws, so you might want to get some of those as well.


                          -Bob
                          Attached Files
                          -Bob

                          The PEDS 2.1 mini system
                          My A7 Project - another small desktop speaker
                          The B3 Hybrid Dipole - thread incomplete and outdated

                          Comment


                          • #14
                            Hey Bob! I'm up in your general area of town quite a bit now (I'm a full-time student at Life University) so we should get together. Do you remember what port length you used? I just skimmed the thread but did not see it listed. If I built these (or got someone to build them for me since I don't have space or tools anymore) I'd like to mount the drivers on the 10" x 8.25" side and wall mount these for surrounds.

                            Wanna build some cabinets for me? ;-)

                            Comment


                            • #15
                              Hi Brian,
                              The port is 5.5" X 1.375". However, these weren't designed specifically for wall mounting. I was just making an educated guess about using a smaller inductor and the increased tweeter level circuit. Plus, mounting the drivers on the wide side of the box would change the XO some as well. But its a simple XO that we should be able to modify relatively easily.

                              Unfortunately, speaker building time is at a premium right now, so I couldn't build them for you. If you can get the boxes built, I could help you with crossover adjustments. Send me a PM when you get started.

                              -Bob
                              -Bob

                              The PEDS 2.1 mini system
                              My A7 Project - another small desktop speaker
                              The B3 Hybrid Dipole - thread incomplete and outdated

                              Comment

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