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/img220/3...25kbib1jw8.gif

FF225K BIB vs concial like RCA-Fan's k-slot blh
http://img218.imageshack.us/img218/7...vskslotsq7.gif

[testarossa2k]

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.

[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

schema

without diff


What do you think about?

[GM]

Originally posted by testarossa2k

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