One more spin of the M8 Box

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  • JonMarsh
    Mad Max Moderator
    • Aug 2000
    • 15259

    One more spin of the M8 Box

    Well, in between loads of laundry and fixing a friend's shower door, I finished building the long promised MkIV crossovers, and modifying the test box, so I can evaulate the design using the Vifa XT25 tweeter.

    We discussed the XT25 almost to death on this thread, including a lot about philosophy of design and measurement.

    Right now, I'm getting ready to hook up the monitor to the test speaker test computer, and see how it looks. What's quite pleasing, is that I've been listening to one channel on the test box, and I suspect I'm finally getting a little better at this computer modeling and cossover development stuff, because it *sounds* like I've been tweaking it for a few days or more- very nice balance, great mids, very easy to listen to. Crossover point is 1.25 kHz, yeah, we're pushing the envelope for an XT25, but then the acoustical slope is 48 dB/octave Linkwitz-Riley.



    More to come, soon. I've got my fingers crossed on this one. The actual proto boxes will use a slightly revised panel layout, based on simulations using BDS. At any rate, it will be the LAST version of this speaker I do- I think. At least for now. Too many other projects in the wings, like Arvo Pärt.


    Bye for now-

    Jon




    Earth First!
    _______________________________
    We'll screw up the other planets later....
    Last edited by ThomasW; 09 October 2004, 22:18 Saturday.
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  • JonMarsh
    Mad Max Moderator
    • Aug 2000
    • 15259

    #2
    Well, I got the test gear hooked up, though I didn't do a full fledged living room re-arrange, where I move the speaker and test setup out to the middle. I'm mostly concerned for this crossover measurment what's happening between 500 Hz and 4 kHz, to verify crossover behavior on the design axis and above and below, as well as to the sides.

    This is 15 degree off axis plot, which is pretty representative of what you'd hear with equivalent positioning; they're not out as far from the rear wall as they would be for listening, (so there's some rise below 400) and there's no baffle diffraction treatment- which may account for the roughness in parts of the tweeter range.

    Still, there's not a thing I'm going to touch before trying out the final proto cabinets. This is straight from the computer simulations, no tweaking components or adjusting levels. (this is how it's *supposed* to work, but usually doesn't) Lowering the crossover to 1.25 kHz and some other tweaks also improved the vertical window substantially; at 1 meter distance, you can't see any significant change in the crossover region behavior in an 18" vertical window. Hot dang!




    Better finish the Mk IV proto boxes, huh? 8)

    Regards,

    Jon




    Earth First!
    _______________________________
    We'll screw up the other planets later....
    the AudioWorx
    Natalie P
    M8ta
    Modula Neo DCC
    Modula MT XE
    Modula Xtreme
    Isiris
    Wavecor Ardent

    SMJ
    Minerva Monitor
    Calliope
    Ardent D

    In Development...
    Isiris Mk II updates- in final test stage!
    Obi-Wan
    Saint-Saëns Symphonique/AKA SMJ-40
    Modula PWB
    Calliope CC Supreme
    Natalie P Ultra
    Natalie P Supreme
    Janus BP1 Sub


    Resistance is not futile, it is Volts divided by Amperes...
    Just ask Mr. Ohm....

    Comment

    • Patrick Sun
      Super Senior Member
      • Aug 2000
      • 1380

      #3
      Jon, will you be publising the XO, box/porting details soon? Somewhat intrigued.




      PatCave; HT Pix;Gear;DIY Projects;DVDs; LDs
      PatCave; HT Pix;Gear;DIY Projects;DVDs; LDs

      Comment

      • JonMarsh
        Mad Max Moderator
        • Aug 2000
        • 15259

        #4
        Pat, if you're seriously interested when I'm finished, contact me by email and I'll provide info. This is being written up for a magazine article, and so won't be "published" on the web, per se.

        The box and porting details are very similar to the MkIII, as shown below, but driver locations are modified based on measurements and BDS simulations. Box tuning is 32 Hz with a 3" flared port. This version is designed to be used away from walls, with room placement similar to what George Cardas recommends. Otherwise, it will be too strong in the bottom end.




        Internal box bracing details have changed substantially since this CAD drawing.

        Completed MkIII looked like this:



        MkIII in room measurements with a long MLS period (relative to room size), necessary for LF response, looked like this:





        The MkIV will have the woofer a little higher, the tweeter a little lower and offset asymmetrically. This WILL be the last version of the M8 bookshelf, as I want to concentrate on the April's Fool Day Project, Arvo Pärt, a semi-full range dipole (mains, not surround, 70 Hz up). (well, aside from the MTM sealed version we've promised for another friend, and the smaller sealed version I've got to build for my daughter, patterned after the small version ThomasW did, which we designed with a somewhat different crossover for placement adjacent to a boundary. For such placement, I recommend going sealed, and take out the baffle step compensation. This increases the apparent efficieny. You can see the work on the "baby" version here. No rest for the wicked or those cursed with too much imagination for their own good! :B

        -Jon




        Earth First!
        _______________________________
        We'll screw up the other planets later....
        Last edited by ThomasW; 09 October 2004, 22:20 Saturday.
        the AudioWorx
        Natalie P
        M8ta
        Modula Neo DCC
        Modula MT XE
        Modula Xtreme
        Isiris
        Wavecor Ardent

        SMJ
        Minerva Monitor
        Calliope
        Ardent D

        In Development...
        Isiris Mk II updates- in final test stage!
        Obi-Wan
        Saint-Saëns Symphonique/AKA SMJ-40
        Modula PWB
        Calliope CC Supreme
        Natalie P Ultra
        Natalie P Supreme
        Janus BP1 Sub


        Resistance is not futile, it is Volts divided by Amperes...
        Just ask Mr. Ohm....

        Comment

        • tverry
          Junior Member
          • May 2002
          • 2

          #5
          Jon, count me in as interested in the MTM! This sounds like just the thing to upgrade to from my vifa PL18 and SS 9500 ported floorstander MTM. The pistonic nature of the HiVi has me drooling in anticipation of great resolution, and at 2/3 the price of the Seas W18E001... Although at 2/3 the freq response as well! I guess you guys have that figured out w/ the low, steep crossovers.

          Did you ever get a chance to play w/ the Hiquphon?

          I know you haven't had long to listen to it, but does the xt25 resolve detail as well as the Focal, or as well as the ss 9500? If you've put a few months on the Focal, how hard is it to design the crossover for an MTM w/ the Focal? I'm not very knowledgeable about crossover design but suspect it is a bit more complicated to add the second woofer than doubling a few values and dividing a few values...

          Have you heard any drawbacks to the MTM design in this case, or in general?

          Comment

          • JonMarsh
            Mad Max Moderator
            • Aug 2000
            • 15259

            #6
            Originally posted by tverry
            Jon, count me in as interested in the MTM! This sounds like just the thing to upgrade to from my vifa PL18 and SS 9500 ported floorstander MTM. The pistonic nature of the HiVi has me drooling in anticipation of great resolution, and at 2/3 the price of the Seas W18E001... Although at 2/3 the freq response as well! I guess you guys have that figured out w/ the low, steep crossovers.
            A lot of interesting questions, here- let's see if I can do them justice this early in the AM, before my cupa java....

            I think the W18 Excel is a great driver, and plan to retrofit my X1's with them. I'm not as keen about the W22, because of it's combination of roll off above 1500, followed by the most awful peak in the upper end known to mankind. The HiVi is attractive because of price, a lower Q breakup mode at 2.8 kHz, and good Xmax with a motor they've taken some trouble to optimize for symmetric behavior.

            Did you ever get a chance to play w/ the Hiquphon?
            I'm still doing some detailed driver level testing on tweeters, and as I don't own the Hiqaphon's I've temporary custody of, I've held off on doing them until I'm 100% sure of my procedures- "Keep your hands on the computer, and the level control turned down, and the tweeter won't get hurt!" :B Since I do have a destroyed Audax DTI01, as well as an MBTTR1, this is more than an academic consideration. The other reason I haven't been quite as keen on the Hiqaphon is the likelihood it won't be happy down low, due to Xmax and SD limitation. It could make a superb tweeter paired with something like the W18 Excel, or in a three way system.


            I know you haven't had long to listen to it, but does the xt25 resolve detail as well as the Focal, or as well as the ss 9500?
            I can't fully answer that question, becuase of two points. One, all my listening with the Focal version was done using a combo of PR41 passive preamp, Ayre V5, and the MP-DAC II which I built. The latter is in Denver; a second one I built is on it's way back to me from Denver. So, I've only been able to listen with a "secondary" quality source, my Sony SCD777ES SACD player.

            Also, the test box has no acoustical treatments for diffraction control, and isn't reinforced in the manner of the final enclosures. So, though the "test box" is good for evaluating crossovers and "macro" frequency response issues, I've never used it for listening evaluations and "micro" response evaluations- things more related to resolution.

            That said, I have to say the test box has never sounded this good before, so preliminary results are very encouraging. The XT25 doesn't seem to sound fizzy in a way that I can often identify with soft dome tweeters, and some of the things that I'm hearing that I don't like are almost certainly the SACD player. (which is stock, complete with lots of op amps and electrolytic caps in signal path).
            I'll have the other DAC back, and I'm building a third now, too, so soon I'll have a better reference for evaluation. I hope to have the MkIV proto cabinets done this weekend- but that may be optomistic, given the work involved.

            If you've put a few months on the Focal, how hard is it to design the crossover for an MTM w/ the Focal? I'm not very knowledgeable about crossover design but suspect it is a bit more complicated to add the second woofer than doubling a few values and dividing a few values...
            Actaully, it isn't really much harder than than, given that with LspCAD I could do a pretty close crossover just by doubling the driver (which ups the LF sensitivity, and halves the impedance). However, in the real world, the acoustical interaction between the drivers is more complex than that, including ways which it affects the upper end vertical polar roll off of the combined drivers.


            Have you heard any drawbacks to the MTM design in this case, or in general?
            The thing you have to look out for is the polar response vertically of the combined MTM array. This is often overlooked. With two seven or eight inch drivers, with the drivers closely spaced, the upper crossover really shouldn't be above about 1500- 1250 is more ideal, especially if you put the tweeters in between. This also ties into some rules of thumb for driver distance and crossover frequency. Ideally, you'd like an MTM array to behave like a conincident driver array that is taller than it is wide. The crossover can work very well, IF the polar resonse of the double midwoofers is still decent for at least a plus and minus 15 degree vertical window, through the crossover region and beyond- how much beyond depends on the slope. With the steep slope (acoustical target of 8th order L-R) crossover, a half octave beyond the crossover seems to be enough. So, having good polar behavior out to about 1750 is adequate, for a 1250 Hz crossover. You can just meet that criteria with dual 7's or 8's; do up to 2000 or 2500 and things fall apart off axis from time delays between the two woofers. In my opinion, any higher crossover than 1500 becomes somewhat fundamentally flawed, especially if more conventional crossovers are used. That puts a lot of onus on the crossover and the tweeter, or suggests a 3 way would be a better approach (which I've also done, but I like the elegance and relative ease of optimization of a two way crossover- hence, four versions of this speaker to date). As Bullwinkle J. Moose would say, "This time for sure!"

            I'm sure many may not agree with me, but that's what makes life interesting.

            In this case, the MTM design, in the same cabinet, but sealed, won't have the LF extension that the single driver ported version has, but for the person who wants these for an HT with a big sub, bass below 75Hz is a non-issue. With a working voume of about 44L, the system Q will be 0.65, with an F3 (anechoic) of about 58 Hz, with maximum SPL in the mid bass (75 Hz and up) of nearly 110 dB per cabinet. Sealed, anechoic, a single cabinet will still be able to do about 100 dB clean at 40 Hz, and with room lift would be pretty flat to 40 with more like 106 dB max output. Rock and Roll, hootchi koo? :E

            I'll keep everyone posted on how this develops. I'm encourage by the results so far. The dual HiVi *'s plus XT25 tweeter is the midrange/treble elements of the Arvo Pärt dipole speaker in the works. I hope to finish cutting the front baffle of the first test panel this weekend, too.

            Regards,

            Jon




            Earth First!
            _______________________________
            We'll screw up the other planets later....
            the AudioWorx
            Natalie P
            M8ta
            Modula Neo DCC
            Modula MT XE
            Modula Xtreme
            Isiris
            Wavecor Ardent

            SMJ
            Minerva Monitor
            Calliope
            Ardent D

            In Development...
            Isiris Mk II updates- in final test stage!
            Obi-Wan
            Saint-Saëns Symphonique/AKA SMJ-40
            Modula PWB
            Calliope CC Supreme
            Natalie P Ultra
            Natalie P Supreme
            Janus BP1 Sub


            Resistance is not futile, it is Volts divided by Amperes...
            Just ask Mr. Ohm....

            Comment

            • tverry
              Junior Member
              • May 2002
              • 2

              #7
              Thanks Jon! Geez now I just have to pick which configuration.... hmmm.

              Keep me posted, baby needs a new pair of MTMs!!

              Comment

              • JonMarsh
                Mad Max Moderator
                • Aug 2000
                • 15259

                #8
                BTW, in case anyone hasn't figured it out, these parts come from Parts Express.

                Your Audio Solutions HQ Since 1986. Shop Audio Parts from Speakers and Subwoofers to Home Theater and Pro Audio. Top Gear at Low Prices and FREE Shipping


                The HiVi woofers are distributed exclusively in the US by Parts Express.

                -Jon




                Earth First!
                _______________________________
                We'll screw up the other planets later....
                the AudioWorx
                Natalie P
                M8ta
                Modula Neo DCC
                Modula MT XE
                Modula Xtreme
                Isiris
                Wavecor Ardent

                SMJ
                Minerva Monitor
                Calliope
                Ardent D

                In Development...
                Isiris Mk II updates- in final test stage!
                Obi-Wan
                Saint-Saëns Symphonique/AKA SMJ-40
                Modula PWB
                Calliope CC Supreme
                Natalie P Ultra
                Natalie P Supreme
                Janus BP1 Sub


                Resistance is not futile, it is Volts divided by Amperes...
                Just ask Mr. Ohm....

                Comment

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