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12th November 2007, 08:37 PM  #1 
diyAudio Member
Join Date: Dec 2005
Location: Bologna

BIB impedance?
Hello,
anyone have a graph of the impedance of the Fostex FF225K driver in the BIB cabinet raccomended on zillaspeak site? thak you 
13th November 2007, 12:44 PM  #2 
diyAudio Member

here's a simulation of Z in halfspace using AJHorn plus a comparison of that BIB vs the same volume but shorter (2.5M) concial horn to illustrate the tradeoffs. I think measured impedance may be off from predictions. I use halfspace so frequency response can be checked with outdoor measurements to see how close a model might come.
note keeping the pipehorns volume fixed, the shorter horn has the upper bass advantage and vice versa with the longer horn. RCAFan (Bill Woods of AH!) ditched a larger/longer horn in favor of the 2.5M as the shorter horn was more accurate with cello. power input for sine has dropped to less than 1/10 of a watt on the BIB to reach 0.3mm xmax (we don't know the 70% BL figure  any guesses?) FF225K Z prediction http://img220.imageshack.us/img220/3...25kbib1jw8.gif FF225K BIB vs concial like RCAFan's kslot blh http://img218.imageshack.us/img218/7...vskslotsq7.gif 
13th November 2007, 01:59 PM  #3 
diyAudio Member
Join Date: Dec 2005
Location: Bologna

meanwhile I tried to simulate with lspcad demo:
http://img440.imageshack.us/img440/8...5khrbibvk9.png looks very different. Did I set somethin wrong? I want to know the impedance to make the crossover. 
13th November 2007, 03:06 PM  #4 
diyAudio Member

at high frequencies the pipe's impedance effects will drop out and you'll just have the same Z as an unmounted driver so can use the published inpedance graph in Fostex's FFl225K literature. Voice coil nductance is very low and there's not much rise even at 10Khz. You could do a tweeter crossover by "ear". typical highpass capacitor values with an 8 ohm tweeter might be 0.68uF >1uF.
at low frequencies a passive crossover to subwoofer could be tricky due to impedance interaction effects and parts expensive. AJHorn and MJK's Mathcad sheets will work for tapered pipes   I don't know LspCAD 
13th November 2007, 03:14 PM  #5 
diyAudio Member
Join Date: Jun 2003
Location: Chamblee, Ga.

Greets!
Neither LspCad or any of the other standard box programs can sim a BIB nor can they predict the impedance without an inductance value (Le), so you're left with the factory's published plot to approximately calculate it: Le(mH) = [(Zxo^2DCR^2)^0.5/(2*pi*Fxo)]*1000 Where: Fxo = some frequency point away from Fs Zxo = impedance at Fxo Dcr = Re GM
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13th November 2007, 05:26 PM  #6 
diyAudio Member
Join Date: Dec 2005
Location: Bologna

I also found this to calculate the LE
http://img444.imageshack.us/img444/8250/ff225klesu1.png So I use the impedance on the fostex graph to cross the tweeter. 
13th November 2007, 06:32 PM  #7 
diyAudio Member
Join Date: Jun 2003
Location: Chamblee, Ga.

Greets!
Since I get 0.226 mH using the formulas I posted, I don't recommend using this online calculator without multiplying its 'Henry' results by 1000 to get the labeled mH. Indeed, if you can, email them and ask them to review/correct it. GM
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14th November 2007, 10:47 PM  #8 
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Join Date: Dec 2005
Location: Bologna

So to cross the FF225K I use the published impedance.
Obviously for FT17H I use again the published impedance since it is a tweeter. And if later, a lot later, if I need more SPL I would like to add a woofer making a 3way I must use the impedance of AJ horn but using the LE value into simulation, right? Now I made a sim for a cross. It's a 2nd order cross with inverted polarity on the tweeter With diffraction sim schema without diff What do you think about? 
15th November 2007, 05:24 AM  #9  
diyAudio Member
Join Date: Jun 2003
Location: Chamblee, Ga.

Quote:
Looks good on paper! GM
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