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RS125P-8 + XT25SC40-04 Mini Monitor

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  • RS125P-8 + XT25SC40-04 Mini Monitor

    The XT25 is loaded into a waveguide from a buyout tweeter. I was hoping it would move the acoustic center of the tweeter far enough back to time align with the woofer, but testing shows there is still a 0.29 inch difference between them. Regardless it makes for a nice visual appeal. The OD of the waveguide is very close to the woofer.

    Directivity curves show that off-axis behavior of the woofer and tweeter match up pretty nicely between 1750 Hz and 3 kHz.

    Click image for larger version

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    Woofer HD testing seems to show that the breakup at 7.5 kHz might be HD related, but maybe not enough to warrant throwing a bottomless notch at it.

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    Tweeter HD testing lines up pretty well with other ring radiators.

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    All this suggests in my mind that a crossover point between 1750 Hz and 3 kHz would work well.

  • #2
    Back when I spent more time on this forum, the XT25 was known for having good HD performance, but a huge resonance at Fs that needed to be damped with an LCR and still forced a high crossover point. It seems to me that these new versions have addressed that. This tweeter appears to have ferrofluid from the data sheet. My impedance sweeps show a peak at 26 ohms.

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    Has anybody tracked this issue? Can anyone confirm if these new ring radiators exhibit the same ringing characteristic as the old ones? In my crossover simulations a single cap rolls off the response very nicely with no signs of an impedance hump in the low end roll off. Assuming that is the case, HD performance should be the only thing limiting the bottom crossover point.

    Comment


    • #3
      gregrueff - I'm finishing a build now with an XT25SC90-04 tweeter that is a few years old (2015 maybe?) -
      It shows exactly the same ZMA curve as yours. Planning on HP around 3.1Khz 3rd order, no waveguide per se.

      I think I hear a difference - wow, it's amazing!" Ethan Winer: audio myths
      "As God is my witness I'll never be without a good pair of speakers!" Scarlett O'Hara

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      • #4
        Cool.
        I took some more distortion sweeps of the tweeter with two different crossovers in place. This first is an approximate crossover point of 1700 Hz LR2:

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        This next one is LR2 around 2100 Hz.

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        Both sounded pretty smooth to me, but it looks like 2100 Hz takes the cake. I couldn't hear any honking, so I'm guessing these new FF ring radiators can go as low as you can bear the HD.

        Comment


        • #5
          I would listen to it, Greg. It will buzz with nasal quality if the Fs is not sufficiently suppressed. When I did the Xenums, and Xenoliths, I just popped a 30 ohm resistor across it to damp the peak. That as all it needed when xoverd at 2.6-2.7kHz. I am in agreement with Don as my impedance info looks similar to yours.

          Later,
          Wolf
          "Wolf, you shall now be known as "King of the Zip ties." -Pete00t
          "Wolf and speakers equivalent to Picasso and 'Blue'" -dantheman
          "He is a true ambassador for this forum and speaker DIY in general." -Ed Froste
          "We're all in this together, so keep your stick on the ice!" - Red Green aka Steve Smith

          *InDIYana event website*

          Photobucket pages:
          http://photobucket.com/Wolf-Speakers_and_more

          My blog/writeups/thoughts here at PE:
          http://techtalk.parts-express.com/blog.php?u=4102

          Comment


          • #6
            Thanks everybody.
            I think I need some help with some fundamentals though.

            What is the impact on my amp if there is a large impedance hump at the tweeter Fs, or say at around 2 kHz? It is the relative same magnitude as the woofer impedance spike, but just higher in frequency. How high a hump is too high? I'm hitting my target slope with a single cap...
            How can damping the impedance in the electrical domain keep from exiting the resonance in the frequency domain when I don't see an immediate impact on the FR curve when I use a LCR circuit targeted on the Fs? I guess this resonance doesn't show up in the FR and HD tests, so maybe that makes sense?

            Let's use the below snapshot as an example. Why is this bad, apart from not damping the tweeter resonance any more than with a simple L-pad?

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            Comment


            • #7
              I'm not really sure what to tell you except it can have a factor on a lot.
              The amp won't care one iota. The magnitude of the resonance in the tweeter can do a few things:
              1- buzz in the male vocal range.
              2- cause a suckout or cancellation in the woofer response if magnitude is high enough, or even help to create a peak in response.
              3- cause the HD measurement to be higher at said Fs.

              The impedance peak due to xover really isn't so much of an issue unless you're using a tube-amp, or a SS amp that oscillates with a reactive load. Most will be fine.

              I'm really surprised you say your XT has FF, as if it did, the Fs magnitude would not be that high and approach more of a 10 ohm Zmax instead. This means they are similar to the units I used, with a Zmax as you have so did mine, and therefore I think you should listen to it carefully to see if you hear any issues. I recommend using James Taylor for this, as he was easily discernible in this tweeter's outcome. I also know that different filters or topologies are more prone to allowing the Fs to come through at least in modeling. I'm just advising you to be careful, as that buzzing gets annoying.

              Later,
              Wolf
              "Wolf, you shall now be known as "King of the Zip ties." -Pete00t
              "Wolf and speakers equivalent to Picasso and 'Blue'" -dantheman
              "He is a true ambassador for this forum and speaker DIY in general." -Ed Froste
              "We're all in this together, so keep your stick on the ice!" - Red Green aka Steve Smith

              *InDIYana event website*

              Photobucket pages:
              http://photobucket.com/Wolf-Speakers_and_more

              My blog/writeups/thoughts here at PE:
              http://techtalk.parts-express.com/blog.php?u=4102

              Comment


              • #8
                No, you're absolutely right. I'm still learning here. The tweeter clearly does not have FF and has a huge spike . I was confused because the datasheet has a ferrofluid field, so I thought they may have added it.

                I know I'm sounding hard-headed, but I'm just trying to understand the impact of everything. It's clear to me that the impedance spike needs to be damped, so now I'm trying to figure out how much. It sounds like maybe the only way to know is to listen to music since I couldn't see any evidence of it in HD sweeps or FR plots.

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                • #9
                  I'm continuing my learning process.
                  I put together two different iterations of crossovers, focusing on the tweeter.

                  This one is second order electrical with an L-pad.
                  The roll-off tracks at 1750 Hz with 4th order slopes, but the reverse null shows the crossover point around 2250 Hz. The shape of the knee has been sacrificed for the sake of low component count.
                  The L-pad seems to be taming the impedance spike, but causes a top end rise that isn't offensive, especially with distortion so low up top.
                  I listened carefully for a nasally tone and couldn't detect anything.

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                  This second iteration is also second order electrical with an L-pad.
                  Same characteristics as before, but the slopes track at 1500 Hz 4th order. The knee more closely tracks with an accepted transfer function.
                  The reverse null shows the crossover point around 1750 Hz.

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                  Conventional wisdom would say this is too low a crossover point for the XT25, so I took a combined system sweep of this second iteration:

                  Click image for larger version

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                  This looks really good to me?
                  Again, with music listening I could detect no harshness that would be expected from crossing low or insufficiently damping the tweeter's impedance peak.

                  Finally, I put together a crossover the way I thought I probably should. I used a 12.5 ohm resister shunted across the tweeter to damp the impedance peak (it seemed to accomplish the same thing as a full blown LCR) and used a zobel to hit the target slope at 2500 Hz. But it's third order electrical and ups the component count on the tweeter from 4 to 7.

                  Click image for larger version

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                  Feedback and comments welcome, but I'm struggling to see why I shouldn't stick with the simpler crossovers.

                  Comment


                  • #10
                    Your first one there looks like an Fc closer to 3.2k, because if you are using an in-phase LR method, it should be -6dB at the summation/xover point. I agree that an L-pad is sufficient to damp the Fs, as that is what I did, and yours does seem to be doing the job. You are also correct that I would think the second is too low of a xover point. I have heard the SC90 used to about 2k vs my 2.6K, and you can just hear that the tweeter is a hair more uncomfortable. Some people do not find this offensive, and it suits them fine. I still feel that 12dB electrical slopes yielding 4th order acoustic rolloff of 2.5k+ is really the sweet spot as long as you damp the Fs to least 40% of it's initial magnitude. It doesn't have to be nulled out completely to reduce the nasality.

                    Later,
                    Wolf
                    "Wolf, you shall now be known as "King of the Zip ties." -Pete00t
                    "Wolf and speakers equivalent to Picasso and 'Blue'" -dantheman
                    "He is a true ambassador for this forum and speaker DIY in general." -Ed Froste
                    "We're all in this together, so keep your stick on the ice!" - Red Green aka Steve Smith

                    *InDIYana event website*

                    Photobucket pages:
                    http://photobucket.com/Wolf-Speakers_and_more

                    My blog/writeups/thoughts here at PE:
                    http://techtalk.parts-express.com/blog.php?u=4102

                    Comment

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