Active vs. Passive X-overs - diyAudio
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Old 4th June 2005, 04:19 PM   #1
Pete H is offline Pete H  United States
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Question Active vs. Passive X-overs

I just read through a few dozen threads from this forum after doing a search on cross-overs. Now I have more questions then when I first began my search. I have always used passive X-overs for my projects and I was looking for some information regarding their use. So that's where I'll start.
1. In a 3-way home speaker I have the typical passive X-over dividing up the frequencies between the woofer, mid and tweeter. Given that each driver has an impedence of 8 ohms, how do I know what the final impedence of the entire system is? Do I apply the formula for calculating the ohms of resistors connected in parallel? Or do the X-overs play some part in the impedence via the fact that each driver only sees it's portion of the overall bandwidth? Or, is there some way I can measure the impedence myself? It would be better if the impedence could be calculated before-hand so that I can design it to fit the specifications of my amp.
2. Ok, so much for my original question. Now, after reading through those threads I am wondering about the benefits/tradeoffs between active and passive X-overs. I'm building a new system in my car and I've always used passive X-overs in the past. But now I'm wondering if active X-overs might be the better choice.
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Old 4th June 2005, 04:29 PM   #2
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The amp sees 8 ohms still as the crossover effectively only makes each drive unit connected to the amp in it's bandwidth. Don't worry about ohms too much, it's only a nominal figure anyway.

If you can spare an amp per drive unit active is best, but just slapping together a textbook filter is unlikely to give anything like what an active crossover can achieve.
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Old 4th June 2005, 04:39 PM   #3
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Default Re: Active vs. Passive X-overs

Originally posted by Pete H
I'm wondering if active X-overs might be the better choice.
Benefits of active XO:
- increased dynamic range
- improved transient response & damping factor
- reduced IM distortion
- easier amp load (constant impedance)
- easier control of sensitivity differences, phase, time delay and frequency tailoring

Cost (including more amps)

Given a competent application of each, active sounds better than passive.

I'm not sure why you're trying to figure out the impedance of your speaker system. If you're going active, the speaker impedance probably won't matter.
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Old 4th June 2005, 04:39 PM   #4
soongsc is offline soongsc  Taiwan
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The main reason one would go to active XO would be power efficiency. Using passive XO, sometimes you have to sacrifice power to gain balanced performance. Passive crossovers are easier to built. Active crossovers take some more design and building skills, but if you get off-the-shelf ones, they might work well enough.

The fact about passive crossovers is that there are many design options, and the driver impedance varies as much as up to twice the nominal impedance.

The good thing is that there are many tools out there that simplify the process enought that the results would be quite good if you use them correctly.
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Old 4th June 2005, 07:03 PM   #5
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You mentioned that this will be for a car.....

In a car, active is far superior to passive IMO, no questions asked. There's a reason just about every successful SQ competiter uses an active setup. Passive crossovers just aren't powerful enough to deal with the complicated acoustics of a car's interior (unless you design some of your own for your specific car and driver placement, generics just can't hack it). One of the most important things you can do with an active setup is time align each individual driver to account for pathlength differences. It is possible to place each driver so that the pathlength differences are minimal, but that takes a lot of work and quite often you have to hack apart the car's interior to accomplish it. Going active is an easy to get around that, and it's much simpler to set up an active crossover network for your install and your tastes than to set up an equivalent passive crossover network for the same, especially considering the amount of trial and error necessary to get it right.
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Old 4th June 2005, 08:32 PM   #6
Mr Evil is offline Mr Evil  United Kingdom
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Changing from a passive to active crossover is, in my opinion, the biggest improvement you can get on the electronics side of things. Since I first tried them I have never gone back, unless I'm building something that absolutely must be as cheap as possible.

It isn't necessarily more expensive though: Good passive crossover components are expensive, but decent amps can be cheap (e.g. chip amps).
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Old 4th June 2005, 09:17 PM   #7
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Changing from a passive to active crossover is, in my opinion, the biggest improvement you can get on the electronics side of things.
Completely agree with this statement.

Also - the much vaunted damping factor of the amp only comes into play when the amp sees the driver directly.
With the l/r/c network of the passive xover you also might introduce some nasty distortions introduced through the back emf from the driver.
Since I started building speakers in '86, within 3 years I went to active xover and never looked back.
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Old 4th June 2005, 10:43 PM   #8
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Well, why might the passive crossovers be better?
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Old 4th June 2005, 11:13 PM   #9
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perhaps more then a "yes it is statement" would be more useful . Maybe some technical white papers comparing the 2? Most home speakers and car components are still passive and would expect the costs to be about the same for active so they must have some reason. perhaps thats just the standard like how cars use unbalanced rca's when they are poorer then balanced. I think active would be harder to implement for the diy with standard parts. resistors 5% and caps 10% sometimes 20% tolerance. so that would make a larger difference then passive?
I think Panasonic uses active on some of their mini/micro systems and some car components can be bi-amped(memphis) but I see passive is more common except for sub/low frequencies where its cost prohibitive.
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Old 4th June 2005, 11:31 PM   #10
Mr Evil is offline Mr Evil  United Kingdom
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The main benefits have already been outlined, but if you want an in-depth analysis of the differences, then you will be wanting to read the ESP articles "Benefits of Bi-Amplification" (parts 1 and 2) and "Active Vs. Passive Crossovers".
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