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Network Design Advice Sought - What Load Impedence Value to Use For Cornwall Bass Bin ?


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Al:

Could you try again to reword what is confusing you. Having nearly 35 years designing L-C filters makes it difficult for me to understand the thinking of a new-comer to the field. I take too may very basic things for granted.

I am also quite overwhelmed trying to understand/refresh everything too soon. So I am going to ignore the context of my previous post and start off with something very basic. In that process I maybe overstating the obvious in the following post, but it is only to give you an idea and the thought process from where I come from and what logical flow I followed (or illogical as may may preceive and prove me so). So please do not mistake me for dwelling on the obvious.

As much outrageous (sharing your humour!!) maybe Gill’s acceptance of oversight and further publically correcting himself, fortunately it reveals that I and Gill are on the same page. Unfortunately this does not place me in the same page as you in the following context:

From Gill’s revision, both Gill and I seem to model the woofer as Re+Le and consequently Z=Re+JXL where Re is indepedent of freq.

If the woofer were purely resistive then we need to deal only with Re. But we know that the woofer has a voice coil inductance and it contributes to Reactance and by virtue of that the woofer’s ultimate Ohms is no longer just resistive but it is a combination of resistance and reactive resistance that ultimately becomes impedance. We further know that the DC resistance part does not change with freq but the reactive part is one that changes with frequency thus rendering the Z of the woofer as one that is not constant but one that varies with freq and thus represented as Re+jXl.

The nominal impedance of the woofer is not Re, but an average in the bandwidth of interest and that average is based on the value Z(f). So a speaker may have a DC Resistance of 4 Ohms, but its nominal impedance turns out to be say 7 Ohms.

Now, how did the 4 Ohm Re now suddenly become a nominal Z of 7? That is precisely due to the contribution of JXl - the AC factor.

So where I fail to be on the same page with you is you seem to use Z(nominal) + jXl to model the woofer instead of Re+JXL. The nominal impedance in itself is due to the contribution of XL adding to Re. For e.g for my cornwall bass bin you use the woofer model as 7+JXL. The 7 Ohm in itself is because of Re(=3.7)+JXl.

So it is confusing to me that you model the woofer as Z(nominal) + jXl instead of Re+JXl when the nominal 7 Ohm already is a result of the Xl’s reactive contribution.

Thanks.

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Gram,

Ok.. I think what happening is that you are limiting the parameters to only two (Rdc + Xl). There are actually THREE factors. I'm going to call them Rdc + Rac and XLe. When you put an ohmmeter on the voice coil you are measuring the DC resistance of the wire. There is also additional resistive loading provided by the enclosure and all the mechanical factors involved with the acoustics. This shows up only when the speaker is seeing AC within the woofer range. It's what I call Rac. The total simple impedance would then be Square_root ((Rdc+Rac)^2 + XLx^2). If you "tune out" the inductive component with a series capacitor © and measure the resistive loss that is left at the resonance of C and Le, it will be Rdc + Rac, it will be about 6 to 7 Ohms in the K33 woofer. This is the impedance you use to design the woofer filter.

Does that make sense?

Al k.

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Al:

Does that make sense?

Thanks for the detailed and patient explanation. Reading this in conjunction with re-reading your website article on measuring complex impedance, makes it much clearer now. This has also made me wary of my simplified measurement process and I doubt the accuracy of those data. I am going going to get a WT3 Tester as suggested by Lee and redo all the measurements and I will repost them.

BTW, as much wonderful the internet is for informstion, it sure is a double edged sword. So much for many formulas and design advice based on just Re alone!

Thanks & Regards

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Gram,

I'm happy that I was finally able to make it understandable. That's not always easy for an egg-head type like me to do!

Your doubts about the measurement method is quite justified. It also points up the fact that the manufacturers of woofer drivers never give you the correct information. They will tell you it's 4 Ohms or 8 Ohms and that's it. They are telling you a lot of NOTHING! The only exception I have seen is a driver by TAD that gives you values for the required Zobel in the spec sheet. It also demonstrates that fact that loudspeaker people think they know filters but few of them actually do!

Lee is getting the test software too and he will be able to evaluate it properly. I would wait until he looks it over before I would buy it. In my case I actually use an HP3563a dual-channel FFT analyzer with the simple simple resistor bridge. The A and B channel display the simple Zo and polar phase directly over any frequency range I need. From Zo and Phase I convert to R +-Jx and then compute L from the Jx for any frequency I need. The dual trace oscilloscope method I describe in the on-line procedure is simply a way to do it with equipment that a person is more likely to have lying around.

Al K.

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Al:

In your zobel network recommendation you had mentioned to use non polarized electrolytic capacitor. Why does it have to be electrolytic and not say polypropelene? Similarly in woofer bypass caps or a N order T section filter to woofer, does it necessarily have to be electrolytic?

Thanks

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Caps in a zobel have a resistor in sereis with them. A low Q cap has a resistance internally. That is what makes them low Q by definition. That being the case, why spend extra money for a cap with high Q and then stick a resistor in series with it? You might as well save some money and go with an electrolytic! You could use a polypropylene, but why?

In a woofer filter, the caps are all operating at low frequency. This is where a cap has it's best Q (Q=X/R) so mylar caps work quite well in woofer filters. You can even use electrolytic caps, but I think it's a bit "tacky"to do so.

Al K.

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