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.
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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.
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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.
 
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.
 
Pete H said:
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

Disadvantages:
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.
 

soongsc

Member
2005-03-26 2:31 pm
Taiwan
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. :)
 
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.
 
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).
 
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.
 

ocool_15

Member
2004-11-26 3:15 am
sk
perhaps more then a "yes it is statement" would be more useful:D . 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.
 
Well, I installed my amp in my car and decided to take out the passive low pass X-over for the rear speakers and instead use the low pass filter built into the amp. The system sounds more balanced and I have more control over the setup because the filter in the amp is variable. I'm also concerned about the varying impedence of a driver vs. the fixed impedence used to calculate a passive X-over. So I think I'm sold on the active X-over argument. Only problem is I would need a 6 channel amp to set up my car stereo properly, or, buy a second 2 channel amp to drive the subs. Perhaps that's not such an expensive option.
However, in order to use active X-overs on my home theatre setup I would need to buy mutiple amps and X-overs. I suppose multi-channel X-Overs and Amps are available for home theatre systems? Any recomendations? Fortunately, the new home theatre receiver/amplifier I purchased has pre-amp outputs for all 7 channels.
Hmm. Do you think it would make a big difference to use a two channel amp and active X-over to power a single 2-way loudspeaker in place of a passive X-over on a single channel? Seems kind of strange to have a tweeter powered by one channel and the mid powered by a another. But I'm willing to try new things.
Thanks for all the helpful advice.
 

soongsc

Member
2005-03-26 2:31 pm
Taiwan
I have a set of three way active XO's second order butterworth using LM379S chip amps (I think) which was built in the 80's. A friend of mine also built the same ones around that time after I did. They sounded good at that time, but not as good as I would like.

The current passives that I built sound better because I am able to design -> listen -> test -> tune one cycle a day until I get a satisfactory result. If I did the same with active crossovers, the cycle would be longer if it involves design structure changes instead of just value changes even if you have a circuit board milling machine.

I'm sure I will be going back to active XO some day if I can get more precise in the design stage through accurage modeling of drivers, and buy a circuit milling maching.

So if you want good commercial quality in a sorter time regardless of power efficiency, then go passive. If you want only the best, then take the time to go active.
 

moamps

diyAudio Member
2002-10-11 8:15 am
Croatia
www.moxtone.com
Re: beginner

uli said:
An active design is not necessarily better than a
passive one, or vice versa.

For a beginner i is definitely easier to use active circuitry.

If the above were true, then statistically speaking there would be much more diy-based active designs than passive ones, which is not the case.

I am not aware that there are any diyers around here who chose an active design for their very first speaker project.

Designing passive Xovers needs much experience and knowledge.

Would you please elaborate what extra knowledge is needed to design a passive xover?

Regards,
Milan
 
Re: beginner

uli said:
For a beginner i is definitely easier to use active circuitry.

Designing passive Xovers needs much experience and knowledge.

There is no better way per se, the point is what a designer is
able to do. An active design is not necessarily better than a
passive one, or vice versa.

It is not ok if participants of this forum caused by lack of knowledge
and lack of education be that rude to others <edited>

None of the above refutes any of the points I raised regarding the superiority of an active filter:
- 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

I then said: Given a competent application of each, active sounds better than passive.

Your response above does not address a competent application. You're saying that a less than competent active filter design is not as good as a competent passive filter design. I would agree with that, but I'm waiting for you to tell me how a competently designed passive filter can sound better than a competently designed active filter. As far as I know, the points I raised in favour of active filters are not controversial. I have never heard anyone state a technical reason supporting the superiority of a passive filter.

Regarding your claim that an active filter is too difficult for a beginner, I disagree. It's far easier to get an active filter right than a passive. There are plenty of DIY and off-the-shelf designs available, which allow anyone with even a little knowledge to design a superior filter. Not having to contend with the impedances and varying sensitivities of the drivers makes an active filter far easier to work with.

I don't think you have the right to call anyone rude, when you dismissed two posts with lines like "That is simply not true" and no explanation.
 
@datubie - I run the behringer for a year now - and this unit is completely quiet. Had the marchand 48 dbLR before - no problem with the unit, but she definetely had an audible hiss - when you came close to the speakers without an audio signal present.

One tip - make sure those tabs that loch the xlrs are bent down a little before inserting the plugs. They tend to be pushed up through the weight of the plugs after some time - and then noise gets introduced because the xlr's do not connect properly any more and are not locked.
This is my only complaint that I have about the dcx.