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Old 14th July 2006, 09:19 AM   #1
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Default Active Crossovers

I want to talk about circuit topologies for Analog Active Crossovers. Of course this applies most commonly to subwoofers, and this is why I am putting it here.

But it also can apply more broadly to multi-amped systems.

Now, we know about the Sallen-Key type circuits.

I'm interested in the full range of approaches being used. I'm particularly curious to know if anyone is using a subtractive approach, so that one filter, with one set of frequency determining components, does the job. Then you should get flat summation.

I'm also interested in general issues for multi-amping, if you can make the power amp itself part of the active filter circuit.

Multiamping and powered speakers seem to be one of the audio thresholds not yet crossed, even in very pricey stuff.
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Old 14th July 2006, 09:43 AM   #2
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You're new here aren't you?

Active crossovers are talked about often on here, I think just about everybody is aware of their benefits. There has been a lot of good discussion on subtractive crossovers as well. Use the search function and start contributing to the threads

As for making power active filters with amps, this is possible, but hard to do in practice because of the amp stability issues when you start messing around with their gain.

Personally I would have put this in loudspeakers not subwoofers, but it's no big deal and I'm sure a friendly mod can move it.
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Old 14th July 2006, 01:29 PM   #3
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I'm interested in pursuing the active crossovers issue as it applies to subwoofers, and to multiamping in general.

Of course changing amp gain is a problem. I'm looking at it more as a full custom design.

I've never seen anything published about subtractive crossovers. Have you? Do you know some threads where that comes up?
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Old 14th July 2006, 01:34 PM   #4
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This started specifically in regards to subwoofers. Most of the examples are going to be subwoofer examples. Except for that, multi-amping is still fairly new.
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Old 14th July 2006, 01:56 PM   #5
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Quote:
I've never seen anything published about subtractive crossovers. Have you? Do you know some threads where that comes up?
Did you use the search button ?

Regards

Charles
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Old 14th July 2006, 02:04 PM   #6
SY is offline SY  United States
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An "exact phrase" search of "subtractive crossover" brings up 27 threads, including one with that title (started, appropriately, by phase_accurate).
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Old 14th July 2006, 02:14 PM   #7
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Interesting. I'd never seen "subtractive crossover" in print. I was just looking for words to describe it. Every printed or online source I've ever seen uses a high pass and a low pass filter. I've never really liked this. I'm glad people are exploring this other way, "subtractive crossover"
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Old 14th July 2006, 02:46 PM   #8
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Quote:
Originally posted by zenmasterbrian
Interesting. I'd never seen "subtractive crossover" in print. I was just looking for words to describe it. Every printed or online source I've ever seen uses a high pass and a low pass filter. I've never really liked this. I'm glad people are exploring this other way, "subtractive crossover"

(JPK) Try looking for Constant Voltage or CV crossovers. Small published a paper on this back in the 70's or 80's. They are subtractive. and the summed response is minimum phase. However, you might also want to look at state-variable filters. These are also referred to a subtractive but do not sum to a minimum phase repsponse.
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Old 14th July 2006, 03:04 PM   #9
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Forgot to mention: There is also some guy called John Kreskovsky who shows some alternative methods (to subtractive crossovers) for transient-improved crossover solutions on his homepage.

Unfortunately a search on the web mostly finds allpass crossovers (like LR) if one is searching for constant voltage crossovers.

Regards

Charles
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Old 14th July 2006, 03:14 PM   #10
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Hi,

Using the power amplifier as part of an active high pass filter up
to 3rd order is relatively straightforward, the stability issues come
with lowpass filters, these need unity gain stable stages.

(You drive an unity gain op-amp filter circuit from the junction
of the gain setting resistors of the power amplifier, here the
power amplifier gain appears to be 1, like the op-amp output.)

Subtractive filters are not very power efficient because allthough
they sum to all pass they exhibit poor phase matching in the c/o
region. Whatever the order of the filter one roll-off is always 6dB
per octave, it also peaks, the higher the order the bigger the peak.

One advantage of the L/R is its power efficiency, i.e. phase matching.

/sreten.
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