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  1. #1
    RIP 2011 Zilch's Avatar
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    Case Study - LE175

    O.K., Hornies, what do the impedance curves tell us about these vintage LE175's on various horns other than, in spite of saying "8 Ohms" on the foilcal, they're 16 Ohms by contemporary standards and 10 Ohms is the appropriate L-Pad to use with them?

    1) Driver only

    2) Driver on 1217-1290 "Potato Masher" horn

    3) Driver on HL 91 horn/lens

    Is that the driver resonance down there at 400 Hz?

    What's the peak above 1 kHz, then?

    What's the usable LF cutoff?

    They're "loading" how low?

    They've rolled off -3 dB at what frequency?

    Anything else useful or important revealed here?

    I can provide Fs and Fmin for each, if that's helpful....
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    RIP 2011 Zilch's Avatar
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    And on these constant directivity horns:

    4) Driver on 1" thread-on throat adapter

    5) With throat adapter, on 2342 horn (4425 "Mini-Butt")

    6) With throat adapters, on PT-H1010 Progressive Transition waveguide.

    Clearly different, no?
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    Hi Zilch ( I'll play )

    Quote Originally Posted by Zilch
    O.K., Hornies, what do the impedance curves tell us about these vintage LE175's on various horns other than, in spite of saying "8 Ohms" on the foilcal, they're 16 Ohms by contemporary standards and 10 Ohms is the appropriate L-Pad to use with them?
    - I disagree with the 10ohm "Lpad" assessment ( mid circuit , I presume ) .

    - It's wiser to measure the impedance curve of the driver as it's typically used. That means to me ;

    (a) A 20 ohm conjugate resistor strapped across the driver .
    (b) A 16 ohm variable Lpad put in place .
    (c) Some assumption of attenuation "dialed-in" on the 16 ohm variable Lpad .
    (d) FWIW ; I assume a mid circuit padding of around 6 db with another 9 db "dialed-in" on the 16 ohm variable Lpad .

    So ;

    How about following these "typical-usage" conventions and then running 2 new impedance curves ( & of course posting them ) ;

    #1 ; 16 ohm Lpad fully "open" ( plus driver & 20 ohm conjugate ) .
    #2 ; 16 ohm Lpad with 9 db of attenuation ( plus driver & 20 ohm conjugate ) .




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    Quote Originally Posted by Zilch
    Is that the driver resonance down there at 400 Hz?

    What's the peak above 1 kHz, then?

    What's the usable LF cutoff?

    They're "loading" how low?

    They've rolled off -3 dB at what frequency?

    Anything else useful or important revealed here?
    (a) With many exponential horns ;
    - There's really no point trying to blindly determine much of anything from just impedance curves / when the impedance curves under review are contaminated or dominated with so-called mouth reflections ( caused by the overly small bell/mouth opening ).

    (b) The CD style horns or waveguides offer an easier "read" to spot the "loading".



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    RIP 2011 Zilch's Avatar
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    If 400 Hz is the driver resonance, then the -3 dB is at Fmin, where phase crosses zero, no?

    ~470 Hz for the exponentials and

    ~650 Hz for the CD's?

    [I'll do the other measurements, of course.... ]

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    Hi Zilch ,

    - ( those F3 points you've quoted seem too low )

    - FWIW; I always insist on a (eh) actual FR plot to arrive at any conclusion for horn/driver loading and the F3 point .

    - I suspect ( actually a little more than that ) each "class" of horn has its' own typical range of impedance signatures that require different visual interpretation. There is likely some formula ( that I haven't yet seen ) that accounts for the minamus & maximus of impedance peaks ( along with all the shifts ) that is also correlated to zero-phase crossings / but I don't know the formula .

    - Having said that; & based on my own visual criteria for interpreting loading strickly through impedance & phase plots ; I'd say the exponential is about 800 hz . Our discrepency is probably based on what each of us is "reading" as the primary resonance for the driver. ( I'm going with the largest/highest impedance peak for the driver with no horn loading ).

    - I'll get back to you with my opinion on the waveguides . 1100hz & 1000hz look right ( obviously, I'm using different criteria for the two horn families when accounting for the phase-wave & the zero-crossing points ) . False readings are easy enough to make when other "contaminating/complicating" visuals are introduced ( like; conical/hypex throat sections ). I need to view a couple of typical impedance signatures for the conical"planar" family &/or cylindrical family, of horn/throats .


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