Using passive and active crossover at the same time. - Home Theater Forum and Systems - HomeTheaterShack.com

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post #1 of 13 Old 09-09-14, 12:28 PM Thread Starter
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Using passive and active crossover at the same time.

I recently went out with a friend I haven't seen in a long time. he picked me up, and was proudly describing his car audio system to me. What really threw a monkey wrench into my understanding of electronics and audio was that, by recommendation from an audio expert, he is using an active crossover, but retaining the passive crossover for his front separates. He told me that the sole purpose of keeping the passive crossover was to protect the speakers from blowing.

What?? Really?? I have never heard of anybody doing this. Is this really a viable way to protect speakers? The only way I can see this as protecting speakers is to keep from exceeding the frequency limits of the individual speakers. But I think educating yourself to understand the limits of the speakers, then not setting you crossover past these limits, far outweighs keeping the passive crossover in as a "for dummies" limiter. Am I wrong?

It seems to me that keeping the passive crossover in circuit will retain any phase distortion that would eliminate one of the main advantages an active crossover; phase distortion between speakers. Again, am I wrong here?
it would also keep from adjusting the active crossover any further past what is already limited by the hardware.
Never mind the fact that there aren't many places to hide the hardware in an Audi TT.

I know that this is car audio stuff in a home theater forum, but it's the "use-the-crossover-as-a-limiter" theory in question, not really it's specific application. Is this actually an accepted practice? It doesn't sound right to me, but my friend says he got the info from an audio professional, so it's driving me a little nuts.

Any input is appreciated!
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post #2 of 13 Old 09-09-14, 07:54 PM
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Re: Using passive and active crossover at the same time.

Yeah he's throwing away any of the benefits he would receive by going active by continuing to use the passives too. He's basically doing bi-wiring or passive bi-amping.

http://www.chuckhawks.com/bi-wire_bi-amp.htm

I understand being worried about blowing the drivers but the only driver to worry about is the tweeter. He can wire a cap with the tweeter to protect it. I'm not sure on values so Google helps there.
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post #3 of 13 Old 09-09-14, 11:52 PM
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Re: Using passive and active crossover at the same time.

I agree totally, the crossover is going to add an undesired filtering for the drivers and would likely stand out if the system was measured (and is probably audible as well).
As stated above, a simple cap inline with the tweeter is all that is required to protect it. The value of which cannot be determined without knowing the crossover frequency used and the tweeters parameters.

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post #4 of 13 Old 09-10-14, 12:40 AM Thread Starter
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Re: Using passive and active crossover at the same time.

Just to clarify, the OEM passive crossover isn't really protecting anything, though. The high pass filter will route anything above the woofer frequencies to the tweeter, not limit the upper end of what the speaker is designed to handle. I'm pretty sure this is the case, right?
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post #5 of 13 Old 09-10-14, 04:30 AM
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Re: Using passive and active crossover at the same time.

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mattmc61 wrote: View Post
Just to clarify, the OEM passive crossover isn't really protecting anything, though. The high pass filter will route anything above the woofer frequencies to the tweeter, not limit the upper end of what the speaker is designed to handle. I'm pretty sure this is the case, right?
Low frequencies are much more energetic. You're not likely to blow a tweeter by sending a 10kHz tone through it...it would have to be unbearably loud. The same energy into a lower frequency is much more likely to push it past its limit. Or, the same energy as distortion as you clip your amp, which is also more likely at lower frequencies.
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post #6 of 13 Old 09-10-14, 06:07 AM Thread Starter
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Re: Using passive and active crossover at the same time.

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Low frequencies are much more energetic. You're not likely to blow a tweeter by sending a 10kHz tone through it...it would have to be unbearably loud. The same energy into a lower frequency is much more likely to push it past its limit. Or, the same energy as distortion as you clip your amp, which is also more likely at lower frequencies.
I just used the tweeter as an example. the same applies for the low pass woofer scenario. What I am getting at, is that the passive crossover isn't really protecting anything, and I'm trying to make sure my train of thought is correct.
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post #7 of 13 Old 09-10-14, 07:58 AM
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Re: Using passive and active crossover at the same time.

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mattmc61 wrote: View Post
He told me that the sole purpose of keeping the passive crossover was to protect the speakers from blowing.
What?? Really?? I have never heard of anybody doing this. Is this really a viable way to protect speakers? The only way I can see this as protecting speakers is to keep from exceeding the frequency limits of the individual speakers. But I think educating yourself to understand the limits of the speakers, then not setting you crossover past these limits, far outweighs keeping the passive crossover in as a "for dummies" limiter. Am I wrong?
By leaving the passive network in place, specifically, at least a series capacitor on the tweeter, he is indeed providing added protection. The woofer, not really, unless there is also a series capacitance to prevent very LF, though that would be a bit unusual.
With an active XO, the amplifiers are (usually) connected directly to the drivers. Any upstream failure, mishap, etc. such as in the XO, could result in a full bandwidth signal being sent through the amp...and to the drivers. A woofer might be able to survive that, but very few tweeters would.
By having (at minimum) the series cap on the tweeter, the harmful bass frequencies will always be blocked.

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mattmc61 wrote: View Post
It seems to me that keeping the passive crossover in circuit will retain any phase distortion that would eliminate one of the main advantages an active crossover; phase distortion between speakers. Again, am I wrong here?
No, but "phase distortion" is not an inherent quality of passive networks, only poorly designed ones.

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it would also keep from adjusting the active crossover any further past what is already limited by the hardware.
I have no idea how your friend actually implemented his, but it is quite possible to cascade active and passive filters beneficially. Implementation, as always, would be the key.

cheers,

AJ

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post #8 of 13 Old 09-10-14, 03:09 PM
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Re: Using passive and active crossover at the same time.

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ajinfla wrote: View Post
By leaving the passive network in place, specifically, at least a series capacitor on the tweeter, he is indeed providing added protection. The woofer, not really, unless there is also a series capacitance to prevent very LF, though that would be a bit unusual.
With an active XO, the amplifiers are (usually) connected directly to the drivers. Any upstream failure, mishap, etc. such as in the XO, could result in a full bandwidth signal being sent through the amp...and to the drivers. A woofer might be able to survive that, but very few tweeters would.
By having (at minimum) the series cap on the tweeter, the harmful bass frequencies will always be blocked.
Excellent point, AJ. but on the "protection" issue I had always operated under the belief the main danger to tweeters was clipping on the low freq end. The passive crossover elements would just pass the (very high level) high freq components created during clipping straight through to the tweeters and let all the magic smoke out.

Have I been mistaken all this time?

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post #9 of 13 Old 09-10-14, 07:32 PM
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Re: Using passive and active crossover at the same time.

In Matt's scenario above, there is an active filter in front of the amp, which means the tweeter amp will be bandwidth limited and less likely to clip. Yes, if clipped, it might still be possible to damage with tweeter even with the passive components in front of it, but the likelihood is reduced.
Plus by upstream "failure", I did not mean clipping, but something like the XO passing DC to the amp and driving it full throttle into the tweeter, which would certainly result in its demise.
Again, I don't know the exact implementation used, but on paper, what he is claiming has merit.

cheers

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post #10 of 13 Old 09-10-14, 07:49 PM
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Re: Using passive and active crossover at the same time.

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robbo266317 wrote: View Post
I agree totally, the crossover is going to add an undesired filtering for the drivers and would likely stand out if the system was measured (and is probably audible as well).
That is true only if the filter frequencies are close (within a few octaves) to the passives. But there is another way to be beneficial (with that scenario). Set the active filters well away from the passives. Lets say the passive is at 2k. With the active, you set the HP at< 400hz and the LP at> 8 Khz. You are far enough away to have minimal effect on the 2k filter function, but now your tweeter amp is bandwidth limited (>400hz) and less like to clip if/when the woofer amp does, as power density spectrum tends towards bass frequencies.
This of course assumes, like my previous answers, that the legs of the passive XO are separately/independently driven.

cheers

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