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Old 28th August 2004, 11:36 AM   #1
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Default Designing passive tri-mode crossovers?

In a "tri-mode" setup, with a stereo pair of drivers and a mono bridged subwoofer, all three the same impedance (4 ohms in my case,) are there any different rules for calculating crossover component values?

The subwoofer is actually 4 ohms but because the amp is bridged, the amp sees it as the equivalent of a 2 ohm load on each channel. Is that right?

Do I select high-pass values as normal for the 4 ohm stereo drivers, or does the bridged subwoofer load change the formula? (Since I don't know whether it should be considered 2 or 4 ohm.)

For the low-pass filter for the sub, do I count it as 2 ohm or 4 ohm, and how does the stereo pair affect the calculations?

If anybody knows an online calculator that can handle this, it would be helpful. I didn't find one.
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Old 28th August 2004, 01:13 PM   #2
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Well, the best answer is.... None of the above!

With the system you have described, your best bet is to use an active crossover for the mid/sub section. Going passive for this application means you end up with huge, heavy, expensive inductor(s) that will not perform as well as $5.00 of electronics. Search the site for active crossovers and see what you think...
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Old 30th August 2004, 10:27 AM   #3
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Quote:
Originally posted by pinkmouse
Well, the best answer is.... None of the above!

With the system you have described, your best bet is to use an active crossover for the mid/sub section.
Technically not an answer to the question, but thanks.

An active crossover would require a separate amplifier, right? I may add a dedicated sub amp in the future, but for now I want to try a tri-mode crossover and see how it works. I was asking about component selection so I could build one myself rather than buying one of the premade ones I've seen on crutchfield.com.
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Old 30th August 2004, 11:07 AM   #4
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Hi mate

I assumed when you were said you were using a bridged mono sub that you meant a seperate amp...

Can you clarify a litte more about your setup?
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Old 30th August 2004, 03:41 PM   #5
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Well, it's alternately called multi-mode, tri-mode, tri-way, or whatever the amp manufacturer wants to call it. Many amps that are capable of running bridged are also capable of running "tri-mode".

It basically means, with a single left/right set of speaker outputs, that are capable of being bridged, you put a stereo pair on them as normal, and then a third speaker across the bridged terminals.

It could be three subwoofers with no crossover (although I'm not sure why you would do that) or it can be a single subwoofer "bridged" and mid/high speakers for the stereo pair. In this case, you need a crossover that has two high-pass filters and one low-pass filter.

Crutchfield.com sells one called the PAC TM-100.

My question was about the low-pass filter. From what I understand, in bridged mode, an amplifier "sees" half the speaker's impedance on each channel, so I didn't know whether that meant you should select the low-pass components based on 4 ohms (the speaker's actual impedance) or 2 ohms (what the amplifier "sees".)
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Old 30th August 2004, 04:08 PM   #6
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Hmm, car audio, that explains why I have never heard of the "tri-mode" system before...

Ok, the amp needs to be able to handle 2ohm loads properly for a start.

Crossover components should be selected on the speakers' impedance, but if you don't have the facility to measure the impedance curve of the driver in the enclosure, then using any formula is fairly pointless. I'm not trying to put you off, but be prepared to spend a while fiddling with component values to get the best out of your system.

Good luck
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Old 30th August 2004, 05:05 PM   #7
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Sorry, I thought being a crossover question it was more general than car audio.

Check this out:

http://www.mmxpress.com/technical/multi.htm

I found that a while back. Uppon reading it again, I noticed that the author selected crossover components based on the ohm rating of the speakers, which was 4 ohm, even though he mentioned that the amp must be 2 ohm stable (just like with a regular bridged 4 ohm subwoofer.) So I guess that is the answer, although it didn't sink in the first time I read it. I think that article is one I found accidently, and it's how I first heard about multimode operation.
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Old 1st September 2004, 05:09 AM   #8
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The good news: the crossover is designed using the speaker's impedance. The amp is irrelevant.

That bad news: using a textbook formula for a "4 ohm driver" is unlikely to give good results. That nominal 4 ohm driver may vary between 2 and 50 ohms at different frequencies. Calling it 4 ohms is okay for figuring power requirements but crossovers are much more complex. You need measurement equipment and simulation software to get it right.
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Old 1st September 2004, 05:45 PM   #9
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So, that would mean the pre-made one is probably not optimal for a particular speaker, either, right?
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Old 4th September 2004, 12:33 PM   #10
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Yup, and even if you had the same driver, it would need to be in an identical enclosure for the crossover to work properly.
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