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Old 5th April 2006, 09:42 PM   #1
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Default Need help with impedance curves and FR plots...

Hi all!

I've been reading the forum for some time, and now I'm finally attempting to build my first loudspeaker using crossovers. I really need some help... I think I may have finished my design, and I'd be most grateful for your input!

I settled on a 2.5 way tower speaker, with 2 Tang-Band W4-657SB mid / woofers and a Neo3 PDR tweeter each, time aligned.

Anyway, I must have logged 24 hours or more in LSPCad 6 by now. I have designed 2 crossovers. One of them gives me the flattest frequency response (Third order on the tweeter; second order on the woofer with an added RC shunt circuit); the other one has a pronounced dip (Third order on the tweeter; second order on the woofer). - but it stays above 4 Ohms all the time.

I'll be using the speaker with a Rotel separate amp and a tube amp with a 4 Ohm tap. But I wonder if they can survive a dip to 2 Ohms like the one the "flattest" design has.

I really don't know if the "flattest" design has a good overall impedance, or if the depression on the 4 Ohm min. design will be audible. I can build any of the two crossovers... and I need your help!

This first post has the graph for the "flattest" design...
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Old 5th April 2006, 09:43 PM   #2
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And this one is the 4 Ohm minimium design...

Which one would you choose?
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Old 7th April 2006, 12:02 AM   #3
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Hi...

No opinions? I really need your help I'm afraid I may blow up my amp by building the "flattest" crossover (my first choice...)
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Old 7th April 2006, 12:09 AM   #4
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Well, you will have to be very unlucky to kill an amp with an increased impedance, so I would give the flat version a go. It's actually quite hard to tell exactly how much of an issue you might have though, as the legend on the graphs covers up the peak that we need to see!
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Old 7th April 2006, 12:24 AM   #5
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Hi Al, thanks for your answer!

I'm not worried at all about the peak on the impedance curve, but the sub-4 Ohm impedance from 500 Hz to 2000 Hz! It reaches it's minimum point at around 1,200 Hz, with some 2.1 Ohms of impedance.

I'd love to build the "Flattest" crossover, but I'm worried about that dip on the impedance.

On my first post, look at the first graph... that's the one that worries me (the frequency response looks fine to me).
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Old 7th April 2006, 12:29 AM   #6
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Ahh. Well, I've seen worse in commercial speakers. The main thing is that it's relatively smooth, so as long as your amp isn't underpowered then you should be ok. What are you thinking of using?
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Old 7th April 2006, 12:33 AM   #7
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Quote:
Originally posted by pinkmouse
Ahh. Well, I've seen worse in commercial speakers. The main thing is that it's relatively smooth, so as long as your amp isn't underpowered then you should be ok. What are you thinking of using?

That's good to know

I'll be using a Rotel RB-1070 with them, or a Margules Audio ACRH-1 (the designer told me it would drive a pair of MartinLogans without issues, so I'm not worried too much).

The thing is, I'd love to try the speakers with a DIY tube amp I have, which is a clone of the Leak TL-12+'s. It has a 4 Ohm tap, but I'm not sure it can survive the dip. I think it could be a non-issue, since it's rather high in frequency, but since this is my first effort, I'm not really sure.

Thank you very much for your help. Hopefully, I won't have any issues with impedance, and I can build the crossover this weekend! I'm happy.
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Old 7th April 2006, 07:34 AM   #8
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I would try another xover - I wouldnt try the low impedance one, you will run into problems driving it.
Are these drivers 4ohm? Have you tried adding a resistor tothe mid driver to try & bring the impedance back up?
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Old 7th April 2006, 08:10 AM   #9
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I personally would be doing everything to avoid the 2 ohm drop. Do you have a schematic of your crossover? You may need to get more creative in your topology.
Also, is the low frequency performance into half or full space (i.e do you have baffle step compensation?).
What kind of response are you after ? Fourth order Linkwitz Riley?
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Old 7th April 2006, 09:11 AM   #10
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Quote:
Originally posted by fjhuerta
Hi Al, thanks for your answer!

I'm not worried at all about the peak on the impedance curve, but the sub-4 Ohm impedance from 500 Hz to 2000 Hz! It reaches it's minimum point at around 1,200 Hz, with some 2.1 Ohms of impedance.

I'd love to build the "Flattest" crossover, but I'm worried about that dip on the impedance.

On my first post, look at the first graph... that's the one that worries me (the frequency response looks fine to me).

Only a suggestion, the plot shows the crossover frequency is alittle bit too low, try set it to about 2.2k hz. And the plot also shows that the cap of mid-woofer filter need to be abated a little.
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