BIB impedance?

[testarossa2k]

Hello,

anyone have a graph of the impedance of the Fostex FF225K driver in the BIB cabinet raccomended on zillaspeak site?

thak you

 

[freddi]

here's a simulation of Z in half-space using AJ-Horn 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 half-space 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. RCA-Fan (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/img220/3236/ff225kbib1jw8.gif

FF225K BIB vs concial like RCA-Fan's k-slot blh
http://img218.imageshack.us/img218/7930/ff225kbib1vskslotsq7.gif

 

[testarossa2k]

meanwhile I tried to simulate with lspcad demo:
http://img440.imageshack.us/img440/8053/impff225khrbibvk9.png

looks very different. Did I set somethin wrong?

I want to know the impedance to make the crossover.

 

[freddi]

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.

AJ-Horn and MJK's Mathcad sheets will work for tapered pipes - - I don't know LspCAD

 

[GM]

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^2-DCR^2)^0.5/(2*pi*Fxo)]*1000

Where:

Fxo = some frequency point away from Fs
Zxo = impedance at Fxo
Dcr = Re

GM

 

[testarossa2k]

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.

 

[GM]

Greets!

Since I get 0.226 mH using the formulas I posted, I don't recommend using this on-line 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

 

[testarossa2k]

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

An externally hosted image should be here but it was not working when we last tested it.

schema

An externally hosted image should be here but it was not working when we last tested it.

without diff

An externally hosted image should be here but it was not working when we last tested it.

What do you think about?

 

[GM]

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?

What do you think about?

Well, you will need to design a parallel notch filter to flatten the F225K's Fs peak as well as designing a zobel for the woofer based on whatever 'Le' you use, whether calculated or simmed.

Looks good on paper!

GM