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on the reduction of Harmonic Distortion of AMT drivers in lower frequencies

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  • edited January 7
    And finally for tonight, I managed to take the heavily modified 5002 diaphragm and "turn it into" the lesser modified diaphragm that I smoked earlier.   So I took readings in the same location, same drive level.  The only one missing here is the original "X-thread" one.



    As you can see, the "lesser modified" diaphragm doesn't show as much "control" of the 3rd order HD below 2k, but that is expected, and the huge bump near 1kHz is all but gone.  I can see this one being a real winner once horn-loaded, and notching the response above 2kHz will further reduce the THD making it even cleaner.   Just have to wait to see how the X-thread one compares.  I think the best figure of merit is killing that huge peak near 1.4kHz in stock form, by ≈ -21dB with just a little enginuity.  :)

    I still want to get more diaphragms from Hygeia, but that will wait for summertime maybe.  

    6thplanetdcibelWolfrjj45
  • By the end of the week I hope to test out the RT-4001 modified diaphragms.
  • edited January 7
    Just to clarify, these tests are at ear-piercing levels with just one tweeter.  I have to wear both earplugs & earmuffs as the highs are deafeningly loud even with just a few watts on a single tweeter.   I do that to see the large-signal behavior, even though it may be unrealistic.  Two of these in a pair of speakers will without a doubt operate very, very cleanly; much cleaner than this impromptu test shows, where I just threw them on the couch just to get rough readings.  I may need to test these outside to get more accurate results, hopefully it doesn't drive the neighbor's dogs crazy.  
  • edited January 7
    RT-5002 including the original X-thread diaphragm.  Thoughts?  I really like the 3rd order reduction on the heavily modified version from 1,5kHz to 3kHz, but it would need a steep filter to really kill the output and HD near 1k.




    So I guess I was optimistic about totally removing all distortion, as nothing is totally free, but the new modifications did "beat out" the x-thread one from these preliminary findings, so all in all, I feel I'm headed in the right direction.  I may just have to devise a new modification that is a compromise between the newer "heavily modified" and the "lesser modified" versions.  To be continued, hopefully soon.
    dcibel
  • Four modded RT-50021's headed out to a customer in Los Angeles, CA.   : B)




    I can modify each diaphragm and reassemble the tweeter back in about 30min.   So, all in all not that bad.  While I was modifying these tweeters, I kept thinking about how this modification really kills the 3rd order HD in the lower frequencies (and a little bit in the upper frequencies).  But what about the 2nd order?  Where does it come from?  It doesn't seem to originate from the same degree-of-freedom that I am taming with this current modification.   I think I stumbled upon a possible culprit -- I had to think deeper.  But unfortunately taming this other degree-of-freedom of diaphragm movement (that might be responsible for 2nd order HD) is not possible with my lowly home-tools and materials/technology.  I know what I need to do to try it out, I just can't.   At least now right now.  Maybe in the future.  Stay tuned!  
    jholtz
  • I have decided not to worry too much about 2nd order HD, unless it is really too high.
    But 3rd order really bugs me. I strongly prefer it way, way down.
    Plus you are testing at 120dB - I would imagine distortion at 100 dB would be down by 5 dB, maybe more.
    Don't think you need to measure outside, standard pseudo-anechoic gated measurements work great for tweeters.

    Don, Donno, or "Hey you" all work for me, But never "Mr Johnson"
  • edited February 10
    Those are near-mic measurements of just a couple of inches, so they are not absolute to the scale that is showing on the graph.   There is no way they can produce 120 dB @ 1m with out blowing up.  That would be a 100W RMS sine wave sweep.   That is a lot of thermal power that would roast most any tweeter.  My tests are roughly 105ish dB @ 1m.  

    If I have a chance this week, I'll try to work on the 4001 diaphragms to see what I can improve on them.
    rjj45
  • edited February 10
    Don, you should take a look at the Purify 6.5inch'er if you haven't already.

    https://hificompass.com/en/reviews/purifi-audio-ptt65w04-01a-midwoofer

    look at that stupid low HD performance!   Only other commercially available cone transducer with harmonic distortion that low that I've ran into is the Beyma 8LW30 (save for Accuton's that are stupid expensive with those ceramic cones!)
  • Initial measurements of a stock RT-4001 taken, with initial conditions (relative drive level, mic distance) notated.  Diaphragm modified.  Patient is resting in ICU, expected to make a full recovery in 24 hours.  Next measurement tomorrow.  This one's for you Don!    B)


  • OscarJr said:
    Don, you should take a look at the Purify 6.5inch'er if you haven't already.

    https://hificompass.com/en/reviews/purifi-audio-ptt65w04-01a-midwoofer

    look at that stupid low HD performance!   Only other commercially available cone transducer with harmonic distortion that low that I've ran into is the Beyma 8LW30 (save for Accuton's that are stupid expensive with those ceramic cones!)
    Yepper, my friend - saving my pennies for the upcoming Purifi 4 inch mid/woofer.
    I think the Purifi guys are going to revolutionize the market.
    Don, Donno, or "Hey you" all work for me, But never "Mr Johnson"
  • OscarJr said:
    Initial measurements of a stock RT-4001 taken, with initial conditions (relative drive level, mic distance) notated.  Diaphragm modified.  Patient is resting in ICU, expected to make a full recovery in 24 hours.  Next measurement tomorrow.  This one's for you Don!    B)


    Sigh, I build so slow! Got a RS270/Morel Cam558 / Hygiea 3 way next in the queue. Looking forward to some extended listening of the 5002s. I might have to ship my 4001s down south for some mods...
    Don, Donno, or "Hey you" all work for me, But never "Mr Johnson"
  • The 4001 responded as well, but I think the "lesser" type modification that I used was perhaps "too much".  The energy of the resonance around 2,250 Hz ended up shifting into the 1kHz range, as well as a little bit more 2nd order near 10kHz.   Actually, I guess it works out, because this AMT would not be used anywhere near 1 kHz (unless you horn load the crap out of it), and the 2nd order increase that is near 10kHz, is technically near 20kHz sound, since it is a multiple of the fundamental, and it should not be noticeable that high up on the spectrum, me thinks.






    I tested it out late last night, so I don't know if the patient made the full recovery, but I will try to test again tonight when I get home from work.   All in all, I'd say this AMT is good down to 2kHz at it's reference sensitivity which IIRC is around 91ish dB @ 2.83V/1m.  
  • edited February 12
    Oh, come on dude, who plays at a polite level of 91 dB <grin>
    " Honey, I was playing  Motorhead last night at 85 dB and it didn't sound right!"
    6thplanet
    Don, Donno, or "Hey you" all work for me, But never "Mr Johnson"
  • I know, I was just saying with respect to matching a low-mid driver to it. :)

    Come to think of it, I think 2nd order went up just a tad with the 4001's if I had to guesstimate.  But there is a greater separation between 2nd and 3rd order.  More testing needs to be done.
  • Everyone needs to have a hobby! I think we all like creative problem solving. Add up just some of the time you have spent, and you could have bought those Beyma horn AMT tweeters several times over, but where's the challenge in that?
    Don, Donno, or "Hey you" all work for me, But never "Mr Johnson"
  • Oh I already have a pair of the Beyma TPL-150H's.  They're awesome.   I'm wondering if I can make those even better as well!  But those extra diaphragms are $120 a piece!  
    rjj45
  • Hey Oscar, so what's all involved in your modifications? Are you still just sticking a string across the back side of the diaphragm to provide an extra anchor point for the pleats? I've got a pair of AMT3-4 that may have some benefit from a modification, but maybe not so much as I should be crossing them around 3kHz.
    I'm not deaf, I'm just not listening.
  • Nope.   I'll tell you, only because you're gonna help me with crossover design.   ;)
    dcibelrjj45
  • edited February 14
    Hope you make that speaker soon, I don't know how much time I have left :p

    Is your driver selection pretty much nailed down, or are you still testing and deciding on bass drivers ?
    I'm not deaf, I'm just not listening.
  • Too many things have gotten in the way, due to work.  I never got the bass driver test off the ground unfortunately, lol.    Mostly just too swamped at work, as I am right now, lol.
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