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Old 6th October 2004, 11:09 AM   #1
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Unhappy How much do you focus on impedance correction?

There I was last night, playing with XoverSim and getting to an acceptable result. I had a pretty flat SPL response and nice looking impedance curve.

Then I thought, "let's simulate a xover from a good commercial enclosure using these drivers". Shock and horror. I wasted many hours tweaking a xover that was getting nowhere. The SPL response was way flatter in the commercial xover, but there's an pretty scary impedance rise around 5 KHz.

Is it me or even serious manufacturers don't focus too much on impedance correction?

After seeing how difficult it is to obtain an acceptable SPL response, I don't even dare to think about flattening the impedance curve.

Now I'm wondering, is it worth the effort, flattening the impedance?

How can I calculate how much attenuation I will have based on amp parameters and impedance?
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Old 6th October 2004, 02:16 PM   #2
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Manufacturers are very pragmatic when it comes to using parts that add to expense but don't necessarily help the sound, so instead of automatically using impedance correction they tend to be sure it is required. Amateurs tend to be more goal and less cost oriented when it comes to the search for perfection and will often use impedance correction before making sure it is either required or desired.

Also beware of crossover sims. They are quite useful for deciding on topology and potential component value ranges but, like box programs, are not 100% accurate; there is no substitute for measuring the finished product and tweaking it out.
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Old 6th October 2004, 02:39 PM   #3
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Impedance compensation is very important. It makes the amplifier's life easier since it sees a resistive load above 300Hz. This means lack of phase shift between voltage and current and the reproduction can be loud and clear up to maximum level without signs of compression.

Its not that difficult to implement, it just needs some trial and error after making your calculations with your favorite software. Crossover simulators are not that accurate in practice, even if its just about a zobel. Start with software simulation but dont count on it, measuring and fine tuning must follow but its quite easy if you find the trick.
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Old 7th October 2004, 10:31 AM   #4
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Just to make myself clearer, I attached pictures of my xover + resulting SPL & impedance graphs, and the ones of a xover recommended by Visaton (after all I found one for my combination of drivers).

The pictures speak for themselves, and I hope you understand my dilemma now, so to speak.

The Visaton filter seems to be a lower Q one (forgive me if I'm making some confusions here and please correct me if so) and it flattens the SPL response an octave or so lower by attenuating it. My LP filter is a 3rd order Linkwitz alignment and it sure doesn't flatten the response that much, like it should, but it has a great looking impedance curve. But flat impedance is of no good if there's that early roll-off beginning at approx 1KHz, I think I won't have any bass!

Please tell me, how can I get rid of that impedance rise in the Visaton xover? I tried to add a zobel to it but it doesn't seem to do any good, in fact it gets worse.


The drivers are these:
woofer: http://www.visaton.com/english/artik..._457_1_19.html
tweeter: http://www.visaton.com/english/artikel/art_266_1_8.html

My xover:
Click the image to open in full size.

Graphs for my xover:
Click the image to open in full size.

Xover suggested by Visaton:
Click the image to open in full size.

Graphs for my the Visaton-suggested xover:
Click the image to open in full size.
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Old 7th October 2004, 11:06 AM   #5
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There's no such thing as a 3rd-order Linkwitz-Riley filter
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Old 7th October 2004, 11:38 AM   #6
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Quote:
Originally posted by richie00boy
There's no such thing as a 3rd-order Linkwitz-Riley filter
Sorry, Butterworth (or is it Bessel?) The first filter I tried was 2nd order Linkwitz and I hadn't refreshed my memory
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Old 7th October 2004, 11:49 AM   #7
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from here it looks like your woofer zoebel did a nice job of flattening the upper impedance rise and smoothing out the response in the crossover region.

As for the low end, are these simulations or your own measurements? Did you splice a nearfield measurement onto the farfield? what is the baffle width/enclosure alignment? It looks like you may either be tuned too low for that driver or in need of some baffle step compensation, or both.
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Old 7th October 2004, 12:05 PM   #8
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Quote:
Originally posted by mr_push_pull
Please tell me, how can I get rid of that impedance rise in the Visaton xover?

The impedance peak can easily be corrected by a LCR network parallel to the inputs terminals of the total crossover, therefore correcting the system impedance of both drivers together. So for example the first parallel leg is the LCR-network, the second one is the woofer low-pass and the third parallel leg is the tweeter high-pass.

Nearly all two-way speakers have an impedance peak just below fx, it is more a coincidence that the first Visaton crossover doesn't show a peak.
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Old 7th October 2004, 02:05 PM   #9
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I see two problems with your crossover apart from the the impedance problems. First of there is no BSC in your LP section. You should use bigger inductor instead of the 0,33mH, like in the Visaton one, so about 1mH. The capacitor then should be about 6,8uF to keep you XO point.
The second problem is with the HP filter. I think you should use a 3rd order filter as well, because of phase problems. Not necessarily, but IMO it gives better integration with the LP.
Try first modify the LP filter.

Zozo
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Old 7th October 2004, 02:06 PM   #10
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Quote:
Originally posted by BobEllis
As for the low end, are these simulations or your own measurements? Did you splice a nearfield measurement onto the farfield? what is the baffle width/enclosure alignment? It looks like you may either be tuned too low for that driver or in need of some baffle step compensation, or both.
Those sims use the published data. I have been assured and reassured that it is trustworthy with Visaton as they have a very strict measurement procedure. Now I don't even have the drivers, my order should arrive each moment (I hope).
I once stumbled accross a page on the Visaton website where they describe the measurement procedure, but I can't seem to find it now, unfortunately. The data is far-field free air response in infinite baffle (this is what I remember from that page).

Quote:
Originally posted by Zozo
I see two problems with your crossover apart from the the impedance problems. First of there is no BSC in your LP section. You should use bigger inductor instead of the 0,33mH, like in the Visaton one, so about 1mH. The capacitor then should be about 6,8uF to keep you XO point.
The second problem is with the HP filter. I think you should use a 3rd order filter as well, because of phase problems. Not necessarily, but IMO it gives better integration with the LP.
Try first modify the LP filter.

Zozo
I used 3rd order for LP because this way the impedance is flatter than with my 2nd order attempts. It has something to do with the second series inductor I think. Anyway, I'll try using the suggestion with the series RLC for the whole xover with 2nd order LP (the Visaton XO).
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