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Introducing the New RST Tweeters From Dayton Audio

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  • #31
    I'm thrilled and will be ordering a pair of the A version pronto. But is anyone else a little worried by the impedance 'blip' of both models around 2kHz? Or is it likely benign?

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    • #32
      Originally posted by 50 watt head View Post
      I'm thrilled and will be ordering a pair of the A version pronto. But is anyone else a little worried by the impedance 'blip' of both models around 2kHz? Or is it likely benign?
      I'm hoping for someone to put up some distortion plots to see if it shows up. Sent from my SM-G960U using Tapatalk
      Electronics engineer, woofer enthusiast, and musician.
      Wogg Music

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      • #33
        Originally posted by 50 watt head View Post
        I'm thrilled and will be ordering a pair of the A version pronto. But is anyone else a little worried by the impedance 'blip' of both models around 2kHz? Or is it likely benign?
        I noticed that, too. I suspect that it has to do with the opening into the pole piece vent and/or damping material. There may be a way to tweak that if so.

        dlr
        WinPCD - Windows .NET Passive Crossover Designer

        Dave's Speaker Pages

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        • #34
          The "blip" at 2K is a classic trait of a tweeter that has had little thought given to the pole piece venting and/or rear chamber design. I wonder if they are omitting the nice domed felt that covers the pole vent in order to save money, or if the pole vent is no longer chamfered. Also seems strange that they would change from the old 'scanspeak' style chamber that worked so well on the previous drivers.

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          • #35
            I do see the little blip in the impedance, but I don't really see much in the way of any movement in the frequency response because of it. I'm still somewhat new at interpreting graphs... what would the consequences of that blip likely be crossover-wise? Would it likely be audible in any way?

            And I know TMM didn't say this... but, I kind of doubt that any part of this RS tweeter replacement had "little" thought given to it, seeing how it is the replacement of one of the best 'nicer' tweeters in DIY audio history, and PE knows that... he just mentioned that the blip is a 'trait' of little thought being put into the pole venting... probably a good observation that many - including me - may not know.

            I don't even remember seeing a similar 'blip' on a tweeter that I can remember...Actually I just looked... saw a tiny 'ridge' on the impedance on the 'silkie' tweeter... could be a clue to it's existence on the RST line?

            TomZ
            *Veneering curves, seams, using heat-lock iron on method *Trimming veneer & tips *Curved Sides glue-up video
            *Part 2 *Gluing multiple curved laminations of HDF *Cello's Speaker Project Page

            *Building the "Micro-B 2.1 Plate Amplifier -- Part 1 * Part 2 * Part 3 * Part 4 * * Part 5 'Review' * -- Assembly Instructions PDF

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            • #36
              Quite a few Morel tweeters have poorly designed backsides, to the extent their impedance profiles resemble ported cabinets.*

              I wouldn't sweat it on this tweeter, though.*
              Don't listen to me - I have not sold any $150,000 speakers.

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              • #37
                This problem is that I tested fairly extensively going back to 2001. Tweater Tweaks was one of my earliest pages at my site in the way of testing. Many of the earlier tweeters had an issue with this. Even some better qualities ones did. The ones without were those such as the Scan-Speak line that had a quality felt dome over the pole piece vent. Without one the tweeter has a resonance set up by the distance between the dome and the rear of the chamber. They also had some serious impedance peaks at Fs.

                Morel MDT-20:
                Click image for larger version

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                Corresponding SPL:
                Click image for larger version

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                Note the similarity in the impedance that looks like a resonance at about 1900Hz. That is raw untreated (red & blue). Then see how the treatment in the two drivers (yellow & light blue) smooths both the SPL and impedance while drastically reducing the impedance peak at Fs. The two drivers are then very closely matched, especially in SPL. What this shows is not what appears to the a resonance at 1900Hz, it's more like destructive inteference from a resonance at about 1500-1600Hz that reduces the SPL in and around that area and causes the impedance to drop.

                dlr
                WinPCD - Windows .NET Passive Crossover Designer

                Dave's Speaker Pages

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                • #38
                  For clarity, frequency response anomalies aren't always reflected in the impedance curve, but impedance anomalies are always related to something in the frequency response. It's due to back-emf and is almost always due to some kind of resonance. Impedance peaks at Fs are prime examples. Compare the difference between the same driver type, one with a full rear chamber and one without.

                  dlr
                  WinPCD - Windows .NET Passive Crossover Designer

                  Dave's Speaker Pages

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                  • #39
                    A certain 2 inch dome has this impedance peak frenquency response glitch. Mods shown below in a German magazine. I have done a similar mod on this driver with 1/4 felt piece the hole in the poll piece.
                    Attached Files

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                    • #40
                      That's the DC50F if i'm not wrong, I've played around with that also. I believe the problem there was that the reflection off the back of the rear chamber wasn't sufficiently suppressed - the resulting dip-peak combination in the frequency response makes it apparent that this is more of a reflection than a resonance. I attempted to fix it by adding poly stuffing to the rear chamber and pole vent. Adding too much stuffing in the chamber or the vent resulted in the Fs increasing and consequently the non-linear distortion increasing below 1kHz when you worked it into flat target response. At what I determined to be an optimal amount the reflection and impedance bump were still there, although much reduced in severity compared to standard. This amounted to doubling or tripling the amount of fill in the rear chamber and adding just a small wisp of fill in the vent behind the existing foam piece.

                      I'm not a speaker driver engineer but I believe there can be two impedance peaks without a significant glitch in the frequency response. My understanding is that if the pole vent is too restrictive there are actually two resonant chambers - the area directly behind the dome in front of the pole vent and the rear chamber. If you have a tweeter with a big impedance peak at 700Hz and a small one at 2kHz, what you usually see in the frequency response is a gradual roll off from 2kHz turning into a steep one below 700Hz.
                      Stuffing the pole vent can alleviate the double resonance but can also reduce the effectiveness of the rear chamber if there is too much stuffing. This may leave you with a friendlier impedance curve to work with, but a more significant roll off in frequency response and therefore a tweeter which has to work even harder to cross at a low frequency, giving higher non-linear distortion. If there is no significant problems with frequency response or non-linear distortion out of the box it may actually be better to just work around the extra impedance bump in the crossover.

                      A better solution seems to be to fill the area directly in front of the pole vent with a domed felt/wool piece so there isn't a resonant chamber created directly behind the dome any more. This seems to alleviate the double resonance while still using the rear chamber effectively.

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                      • #41
                        TMM, I just found the link to your web site in your member info page. You should add a link to that in your signature that will show when you post. Sounds like you followed a path similar to mine, though I started out as an EE and morphed into a software engineer. My large test baffle is 2m x 2m in my basement that is attached to a stairway when I swing it down into position.

                        dlr
                        WinPCD - Windows .NET Passive Crossover Designer

                        Dave's Speaker Pages

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                        • #42
                          I really wonder if it is not the vent in the pole that is the resonance source. I used a piece of felt across the diameter and extending past the length of the pole piece. There is seldom enough damping in the rear chamber of tweeters or dome mids and the chambers are usually to small to allow much volume to be used for damping material but that’s the economics. There is some serious competition in the 30 to 60 dollar tweeter market and designers must have to make some daunting performance vas.cost decisions. As audiophile Dundee would say “That’s not a rear chamber, THIS is a rear chamber”

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                          • #43
                            My DIY rear chambers for Dayton DC50 and Dynaudio D52. The are open back and dampied along their length and I guess would be transmission line loading.
                            Attached Files

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                            • #44
                              Originally posted by fdieck View Post
                              I really wonder if it is not the vent in the pole that is the resonance source. I used a piece of felt across the diameter and extending past the length of the pole piece. There is seldom enough damping in the rear chamber of tweeters or dome mids and the chambers are usually to small to allow much volume to be used for damping material but that’s the economics. There is some serious competition in the 30 to 60 dollar tweeter market and designers must have to make some daunting performance vas.cost decisions. As audiophile Dundee would say “That’s not a rear chamber, THIS is a rear chamber”
                              The chamber (if there is one) if used properly is fully stuffed to damp the rear wave. The larger the chamber, the more successful the damping. But that damping material must press firmly against the vent opening into the chamber. I tested one tweeter years ago that had some felt in the chamber, but it left a gap. This allowed a serious pipe resonance to form, separate from the Fs and rear-of-chamber reflection resonance. It disappeared with the thin felt pad pressed tightly against the opening.

                              dlr
                              WinPCD - Windows .NET Passive Crossover Designer

                              Dave's Speaker Pages

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                              • #45
                                Sold out already, it seems.

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