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The Nexus - a budget-minded MTM tower

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  • The Nexus - a budget-minded MTM tower

    This is one of the designs I plan to bring to DDIY this year. I'm sure some of you will recognize them from when they were at DIY Iowa a couple years ago.

    Drivers: Peerless SDS-160F25PR01 on woofer duty and the GRS 1TM-4 tweeter. The woofers were selected because they modeled well in a reasonably sized cabinet and they were (and still are) attractively priced. The GRS tweeter was selected because it paired well with the woofer and fit the budget requirements.

    Cabinet: After measuring the woofers and modeling them in Unibox, a cabinet volume of 50L was arrived at. The woofers dictated the baffle width, as I used a method that Pete S. had posted on PETT awhile back where the baffle went from truncated edge to truncated edge on the woofers. This set the baffle width at 6.5", and I arrived at a baffle height of 46.5". The external dimensions of the cabinet then fell into place at 48" H x 8" W x 12" D. The cabinet walls behind the drivers are lined with eggcrate foam, but other damping materials could be substituted if desired. Fb/F3 comes out to 40Hz for both. The port is front mounted and centered 6.5" from the bottom of the cabinet. The woofers are centered on the baffle and are 5.5" and 15.75 from the top of the cabinet. The tweeter is centered on the baffle and is 10.5" from the top of the cabinet.

    Crossover: This was done by ear just for fun. That and I like tinkering with things so the 'trial and error method' is the most enjoyable part of the process for me.

    The woofers are wired in parallel with a first order electrical filter of .55mH.
    The tweeter is wired in reverse polarity with a second order electrical filter of 3.3uF and 1.0mH.

    Full write up with pictures, including bill of materials here: https://diy.midwestaudioclub.com/dis...-towers#latest (got sick of fighting with the forum software here)
    Failure is not an option. It comes bundled with Windows.

    We are passionate about great sound at whatever level we can afford, so don't let the audio atheists reduce the experience to a set of numbers and squiggly lines. - jbruner

  • #2
    Great job, nothing fancy, simple and solid. Love how you did the trial and error ear tuning. I don't have, or plan to get any software so I appreciate the approach.

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    • #3
      A Zobel across those woofers (6ohm+10uF - about $1) will pull the breakup at 4.5k down by another -5dB, giving a real 1st order rolloff.

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      • #4
        Looks a very interesting design, nice and simple crossover too: great for people like me who hate soldering! And the Peerless woofers look cool.

        How do you find the sound and quality¬*of the GRS tweeter? It seems to get a raspberry from most reviewers?

        Thanks

        Geoff

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        • #5
          Originally posted by Geoff Millar View Post
          How do you find the sound and quality¬*of the GRS tweeter? It seems to get a raspberry from most reviewers?
          Well, not quite "most"...7 of 13 reviews on PE are 5 stars. But the bad reviews are bad.

          it would be nice to hear an opinion, I've noticed that a lot of the lowest price item are like that....5 stars or 1 star.

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          • #6
            I believe It's the same tweeter used in the B452.¬* There's just not much to work with below 5K.¬* Distortion graph attached.
            John H

            Synergy Horn, SLS-85, BMR-3L, Mini-TL, BR-2, Titan OB, B452, Udique, Vultus, Latus1, Seriatim, Aperivox,Pencil Tower

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            • #7
              FWIW, the GRS is more of an Audax knock-off IIRC. There could be some shared lineage with the tweeter in the B452, but I have no experience with that particular unit.

              I have no complaints about the SQ, where it doesn't really do anything wrong. Classical, classic rock, rock, punk, metal, symphonic metal, vocal, etc. all sound as they should.

              That said, I'm reasonably certain the tweeter will go out before the woofers do if someone pushed them really, really, really hard.

              HTH
              Failure is not an option. It comes bundled with Windows.

              We are passionate about great sound at whatever level we can afford, so don't let the audio atheists reduce the experience to a set of numbers and squiggly lines. - jbruner

              Comment


              • #8
                Just for the heck of it, and to try to learn something, I modelled an MTM with this Peerless woofer and the VIFA BC25 in Xsim:¬*it came out looking quite good, at least in its frequency response.¬* I used the VIFA as I have a couple¬*lying around, and in Melbourne ¬*I don't have access to the GRS at a reasonable price.

                Thank you for the inspiration!

                Geoff

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                • #9
                  You're welcome Geoff!
                  Failure is not an option. It comes bundled with Windows.

                  We are passionate about great sound at whatever level we can afford, so don't let the audio atheists reduce the experience to a set of numbers and squiggly lines. - jbruner

                  Comment


                  • #10
                    Originally posted by S7910 View Post
                    You're welcome Geoff!
                    As a matter of interest, did you try Chris Roemer's' suggestion for a Zobel?¬*

                    I tried it in Xsim and it seemed to mess with the overall response, which can't be right.

                    Thanks

                    Geoff

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                    • #11
                      I hadn't tried it yet (been too busy with work), but have the parts on hand. From past experience I would expect some impact on the overall system response, however minor that impact may wind up being.

                      In this case I would expect the upper response of the woofers would be boosted some, making the mids sound a bit "forward", and possibly necessitating other changes to the crossover.
                      Failure is not an option. It comes bundled with Windows.

                      We are passionate about great sound at whatever level we can afford, so don't let the audio atheists reduce the experience to a set of numbers and squiggly lines. - jbruner

                      Comment


                      • #12
                        W/out it the woofer starts rolling off above 2kHz, then the cone breakup ramps back up +5 to +6dB from 3k to 4.5k. With it, there's not much change below 1.5-2k, but stays about level from 3 to 4.5k, then rolls off. It WILL change the FR around 4k, but I wouldn't want to hear that breakup like that anyway.

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                        • #13
                          Originally posted by Chris Roemer View Post
                          W/out it the woofer starts rolling off above 2kHz, then the cone breakup ramps back up +5 to +6dB from 3k to 4.5k. With it, there's not much change below 1.5-2k, but stays about level from 3 to 4.5k, then rolls off. It WILL change the FR around 4k, but I wouldn't want to hear that breakup like that anyway.
                          That's interesting, thank you.

                          With help from PETT members, I recently did an Xsim model for a TM with the Peerless 830657 and VIFA BC25 - the woofer has virtually the same specs and FR as the SDS-160F25PR01, but the latter has more Xmax.¬* Crossover point was 2,000 Hz.

                          I had tried a Zobel in that simulation, but couldn't get the FR to look right in that case, either.¬* However, the Zobel I tried had different specs than the one you suggested above (6 ohms and 10 microfarads).¬*

                          Would the same Zobel be used for a TM, please?¬* I've built the crossovers for the TM but could easily add the Zobel if desirable.

                          Edit: the Xsim used a small¬*0.22 capacitor¬*on the woofer inductor, which I 'borrowed' from many other designs which involve removing¬*peaks in the response.¬* This seemed to work better for Xsim than using a Zobel, but that might be because it's a sim and not the real world.

                          Fascinating thins, speakers.

                          Geoff
                          Last edited by Geoff Millar; 05-17-2018, 08:01 PM.

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                          • #14
                            For a TM (double the impedance) odds are that you'd need to go 12ohms + 5uF.

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                            • #15
                              Originally posted by Chris Roemer View Post
                              For a TM (double the impedance) odds are that you'd need to go 12ohms + 5uF.
                              Thanks Chris!¬* I didn't mean to hijack the thread, it's just interesting to compare designs with people who actually know what they're doing.

                              I'll try that in Xsim and see what happens.

                              Cheers

                              Geoff

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