Do any Kappa owner have the means to generate a Polydome impedance curve

Discussion in 'Infinity Loudspeakers' started by swechsler, Feb 10, 2018.

  1. swechsler

    swechsler Frog Whisperer

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    As we know, actual impedance is more than just a single number. I was browsing midranges at Parts Express and noticed that some mids that actually were rated at nominally 8Ω were actually significantly lower at the frequencies we care about.

    So, does anyone out there have the equipment to do this? Or, does anyone know of a graph that's already been created (a brief Google search yielded nothing)?
     

     

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  2. dlucy

    dlucy dlucy67 (Doug) Subscriber

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    Search for Kappa 9 and you'll find a famous one showing ~1 ohm around 50 Hz. That's what kills unsuspecting amps.

    I, too, would love to see some frequency response and impedance curves from the lower-numbered models. I have a pair of 8's that I love and expect to collect more.

    The lower-numbered models don't dip as low as the 9's do, from all I can gather anecdotally.
     
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  3. swechsler

    swechsler Frog Whisperer

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    Yeah, I've seen the curve for the Kappa 9. I'm looking for a curve for the standalone driver. Since it's affected by the crossover and the other drivers, a curve for the entire system won't be of any help.
     
  4. dlucy

    dlucy dlucy67 (Doug) Subscriber

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    Ah. I'm not even sure what piece of test gear would do that: plot impedance over the audio frequency range. Must be a few pieces of specialized gear. Beyond that, manually testing and recording a value every X Hz while the speaker is open so you can get to the driver.
     
  5. dlucy

    dlucy dlucy67 (Doug) Subscriber

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  6. elnaldo

    elnaldo Addicted Member

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    you can do it with a computer and a sound card, some resistors, and a free (or demo) software like ARTA.

    But you need a working driver to do it yourself. Do you have a working Polydome to do the measurements, or do you need somebody with a driver, to measure it?
     

     

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  7. swechsler

    swechsler Frog Whisperer

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    I have quite a few working polydomes. If you can point me to a site on how to do it, I might give it a try.
     
  8. BrianLW

    BrianLW New 'Hood' Subscriber

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    I downloaded the ARTA software, found a 100 ohm resistor (measured as 99.8, meter calibration unknown), and cobbled the necessary hardware together.
    http://www.artalabs.hr/download.htm

    LimpHardware.jpg

    I then tested a pair of unmounted 902-5793s, the ones with the metal mesh behind the dome, and merged the results, as seen below.

    Polydome902-5793Pair.jpg

    You can see a difference in the 80-200 range, probably due to differences in the compliance of the surrounds (both are yellowed, but came from separate sources), but they are fairly consistent from 800 Hz to 4500 Hz, the range in which they are used in a Kappa 9.

    Here are the separate plots. Driver1.jpg Driver2.jpg

    Hope this helps.

    UPDATE:

    Just for fun I pulled a Millersound rebuilt 902-3075 polydome from one of my Kappas, and got this plot.

    Polydome902-3075alaLeGall.jpg

    Edit: Corrected the crossover points and a typo. Thanks swechsler!
     
    Last edited: Feb 24, 2018
  9. swechsler

    swechsler Frog Whisperer

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    Awesome, thanks so much! This will make the search a lot easier.

    Just for reference, the Kappa 6-9 polydome crossover points are 800 and 4500 hz.
    The Kappa 6.x and 7.x use 500 and 4500 hz, the 8.x is 700 and 4700 hz, and the 9.x is 750 and 3500 hz.
    All this info is from infinity-classics.de, which has been known to be somewhat unreliable, but it's all we've got AFAIK.

    Interesting that the older polydomes appear to have a slightly higher impedance. Also, it seems that the polydome used in the Kappa 9 is different from the lower models (3075 vs 3076, assuming the spec sheets on infinity classics are correct. I just looked at all my bad polydomes, 6 of them, and they're all 3075, but I don't know where most of them came from except for two which definitely came from Kappa 8s).

    To be continued...
     
    Last edited: Feb 24, 2018
  10. BrianLW

    BrianLW New 'Hood' Subscriber

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    Thanks for the correction, 800 and 4500 agree with the tech sheet and owners manual. I'm not sure where I got the other numbers :dunno:

    I'll update my post for posterity. I'll also fix the typo. It was a 902-3075.
     
    Last edited: Feb 24, 2018
  11. Listnnthdark

    Listnnthdark Active Member

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    Very Interesting . This question will reveal my ignorance . When you compare the graphs for Series 2 polydome with those from the millersound repair , What do the graphs say ( or your interpretation ) about and difference in output or sound ? They look very close to me with a few wiggles difference ( yep i said wiggles ) :(
     

     

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  12. BrianLW

    BrianLW New 'Hood' Subscriber

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    I had on hand the capability to do the plots, so I plotted what I have. Interpretation of the results I will leave to those with more experience.
     
  13. swechsler

    swechsler Frog Whisperer

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    It looks like the Millersound polydomes have a slightly higher impedance. Would be nice if we had either a Millersound-modified 2nd gen Polydome, or an original unmodified Polydome, so we could see if the difference is due to the Millersound mods or the different generations, but I suspect the difference isn't that significant either way.

    Now the search, to see if we can find some good replacements.
     
  14. Listnnthdark

    Listnnthdark Active Member

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    Oh Ok Good point .. The graph uses 2nd gen polydome ( Series2 w mesh ) compared to first gen w millersound treatment . I bet you are right that any difference in measurement would = no apparent diffence in sound character .. At least to those with mortal ears like mine . And they are approved by Bills ear ! ( I realized that the graph is not a "response curve " just shows impedance , which :no: (I don't get ) The whole discussion is based on fact that we all appreciate what we have ( Thank You Arnie ) and want to keep it . Thanks to those of you who have with your knowledge sought to solve the issue .
    Ive done some sculpture , mold making and casting and it would (seem) to be an easy thing to make a mold that has no "undercuts " ( which as far as I can tell it would not ( a" slip cast" ) to match the Polydome . But what material and how to make it the right and uniform thickness is a :crazy: I guess that is what the guy who "sells" the silicone domes on eb did .. but Silicone ? Yeah its clear and looks right so I guess that's enough to "sell " them but not to me ..
     
  15. Listnnthdark

    Listnnthdark Active Member

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    So " slightly higher impedance" would mean (on paper ) slightly higher output ?
     
  16. swechsler

    swechsler Frog Whisperer

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    Actually a higher impedance will lead to a lower output, but that can be compensated for with the level controls. The more significant issue is the change it will have on the crossover frequency, but we're talking about less than 1 Ω difference.I suspect neither effect will be audible (might be if you were comparing left to right, but I would never recommend running two different versions of a driver in a single pair of speakers).
     

     

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  17. brettMc

    brettMc New Member

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    Hi All,

    This is an excellent thread! I have several polydomes in my man cave: one still functioning original, the Millersound refirb, two silicon covered and two Western Speaker Repair versions. The silicone is total junk, you can see the dome material collapse as the coil moves forward reproducing vocals. The silicone is muddy and really unclear. The Western Speaker is a Chinese made drop in replacement that is way to loud, it CANNOT be rebalanced in using the attenuation controls. The Millersound is excellent, however the material used ‘rubs’ and has its own sound, so it’s a hint grainy. The original polydome material makes no sound (as you depress the voice coil gently) and so if you want vocals more forward you can really turn the originals up without any additional treble frequencies added.

    Does anyone want to go in on paying for 3D scans of original polydomes? With some basic measurements we should be able to replicate the originals with a contemporary material that will not degrade. We need overal shape, mass and thickness.

    For some reason I am under the impression that the original material was chosen because it mimics ‘skin’ or vocal chord...

    Lemmeknow.
     
  18. BrianLW

    BrianLW New 'Hood' Subscriber

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    Mine makes no sound when I press it gently in. I would contact Bill LeGall @ Millersound and see what he thinks is going on.
     
  19. Listnnthdark

    Listnnthdark Active Member

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    Appreciate the last 2 responses .:thumbsup: Both own Millersound "Poly"domes . Just the folks we want to hear from :thumbsup: Brett , Thanks for that review of all options you tried . I have to think if clear silicone ( as in tube at home depot ) ( And breast implants ) were a good material for loudspeakrs SOMEONE ( and their brother ) would have used it . Fool the eye but not the ear . After reading your post I was almost ready to PANIC:yikes: ( If the millersound repair has sonic issues Im sunk )( My originals are ok ( now ) I have to think Bill would want to know and correct it . Sure glad you chimed in after that Brian .. Whew
     
  20. okeeteekid

    okeeteekid Well-Known Member

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    Could the slightly higher impedance be from the speakers resonance frequency?
     
    Last edited: Mar 6, 2018

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