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Old 16th March 2012, 08:22 PM   #1
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Default Impedance question

Hello guys!

I'm finally starting to build a new pair of speakers.

I bought 4 SB Acoustics SB22, 2 SB17, and 2 SB29 tweeters. Going on 50l sealed for the bass drivers. I simulated a crossover, and the frequency and impedance response looks like the attached picture.

Now... I haven't thought about the impedance when I bought the drivers (the SB22 has a 5,6 ohms RDC), so, as you can notice, the impedance goes REALLY low in the 70-150 Hz area (like 2.5 ohms), and then stays slightly above 3 ohms (that is for a high sensitivity and linearity)...

My amplifier is a Cambridge Audio Azur 640A (the original, not the V2)
Specifications / Azur 640A The ultimate award winning integrated amplifier / Azur / Products / Cambridge Audio

I saw in its manual that the DC protection comes in at 2 ohms or lower... Do you think I could use this crossover? If not, what can I do to increase the impedance in that area? (Over 300-400 Hz it's not that difficult, but in that area I have no idea).
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Old 16th March 2012, 11:56 PM   #2
PLB is offline PLB  United Kingdom
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Hi cosmindaniel12,

I would suggest that you first test the accuracy of your measurement system by measuring a close tolerance resistor of say 4 ohms. Was the graph you posted made with the Xover in circuit and all drivers installed or was it just the driver in free air or was it the results of a simulation?

Regards

Peter
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Old 17th March 2012, 05:50 AM   #3
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It's the result of a simulation, with the Xover and all the drivers (final system impedance).
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Old 17th March 2012, 11:58 AM   #4
AllenB is offline AllenB  Australia
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Did you measure the impedance yourself, including phase?

It might be worth seeing if you can't manage the same filtering with a few changes. If you had a good reason and a good amp then this might be a reasonable compromise but for the most part you should probably work on it a little further.
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Old 17th March 2012, 01:27 PM   #5
PLB is offline PLB  United Kingdom
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I think you may want to check the SPL of your simulated response as well. Your graph shows this to be about 95dBSPL over most of the band, which is very high for the normal 2.83Vrms/1meter test conditions. Also, did you account for the baffle step compensation (Baffle Step Compensation) in your crossover design or is your simulation based on half space environment?
I mention this because BSC can make a noticeable difference to the impedance in the frequency range you mentioned.

Regards

Peter
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Old 18th March 2012, 11:34 AM   #6
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The graph is simulated in Boxsim. It takes into account the baffle step and the difraction. The SPL is so high mainly because of the small impedance. But I don't think I can lower it and still keep a LR2 400 Hz crossover freq.

Crossover attached (400 Hz LR2 and 2200 Hz LR4 crossover points).
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Old 18th March 2012, 10:00 PM   #7
AllenB is offline AllenB  Australia
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Quote:
Originally Posted by cosmindaniel12 View Post
400 Hz LR2
Electrical or acoustic? Your filters are 300Hz and higher Q. Are the other drivers 8 ohm?
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Old 19th March 2012, 02:19 PM   #8
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The crossover points and slopes are acoustic.
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Old 20th March 2012, 03:33 AM   #9
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Quote:
Originally Posted by cosmindaniel12 View Post
Now... I haven't thought about the impedance when I bought the drivers (the SB22 has a 5,6 ohms RDC), so, as you can notice, the impedance goes REALLY low in the 70-150 Hz area (like 2.5 ohms), and then stays slightly above 3 ohms (that is for a high sensitivity and linearity)...

My amplifier is a Cambridge Audio Azur 640A (the original, not the V2)
Specifications / Azur 640A The ultimate award winning integrated amplifier / Azur / Products / Cambridge Audio

I saw in its manual that the DC protection comes in at 2 ohms or lower... Do you think I could use this crossover? If not, what can I do to increase the impedance in that area? (Over 300-400 Hz it's not that difficult, but in that area I have no idea).
First of all, the DC protection does not really "come in at 2 ohms"-it kicks in at some current draw which is more easily triggered by 2 ohms. So really you should be fine except if you start to clip the amp.

As for the impedance itself, 70-150 Hz seems like the natural low impedance of the woofers. If you try to increase the impedance by lowering the crossover, I suspect you'll start having more and more problems due to the impedance peak at the resonance frequency.
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