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Old 16th October 2012, 10:18 PM   #31
4Torr is offline 4Torr  United States
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A passive crossover stands between the amplifier and the driver. Feedback from the driver is attenuated and phase shifted by crossover components and the amps's damping factor is greatly reduced.
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Old 16th October 2012, 10:58 PM   #32
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I made these posts to a different thread but the same issues keep coming up. There are things that can be done with active analog and active digital that can not be done with passive, but for conventional crossovers thee is no reason a passive system can not perform as well or better than an active. He are my previous posts:


Some simple truths can sum it up. Taken as system the combination of active crossover/amp/driver and amp/passive crossover/driver represent predominately linear systems. Since linear systems with the same frequency response have the same transient response it follows that if the active and passive crossovers systems are designed to have identical frequency response then they must have the same transient response. Any difference is damping due to series impedance in the passive crossover simply means that the transfer function of the passive crossover will have to be somewhat different that that of the active system to account for the difference in damping.

The real difference between passive and active systems is insertion loss and the way the response to changes in VC temperature.

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I read all the time that active filter are better because the transfer function doesn't change with changes in the driver's Z due to a variety of dynamic effects. Again, that is not an accurate statement. The thing that is overlooked is that this is limited to the voltage transfer function of the filter. But in reality, what determines the acoustic output of a driver is the current flow through the VC. Even in that case where the amplifiers output Z is zero applying a fixed voltage transfer function across the driver terminals does not mean the acoustic output remains constant under dynamic conditions. The driver VC Z changes with time (heating) and excursion and these factors will affect the current and therefore the radiated SPL. Active can make a lot of things easier, and I and am fan of actives. But many proponents of active make arguments for them which are overly simplistic and ignore many of the real issues resulting form the dynamically changing VC Z.
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So when you start talking about comparing active and passive, start at the beginning. That is not the amplifier output terminals in an active case. Remember that a simple, text book active HP filter has a impulse that decays exponentially or with oscillations, depending on Q of the filter. The impulse tail will past through and be amplified by the power amplifier and feed to the driver. So it isn't just the amp that is controlling the driver, it's the output of the active crossover and it's particular damping characteristics.
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Old 16th October 2012, 11:10 PM   #33
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I'm guessing that in the active crossover, levels were not matched the same as the original passive and there was more bass output.
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Old 16th October 2012, 11:26 PM   #34
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Quote:
Originally Posted by john k... View Post
no reason a passive system can not perform as well or better than an active.
With a damping factor (at the driver terminals) that varies according to frequency, insertion loss, and all the additional nasties inductors bring along, how could a passive ever be better than an active?

I can see that they can be good enough in many cases, but better than active?

I'm still unconvinced.
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Old 16th October 2012, 11:31 PM   #35
Bare is offline Bare  Canada
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Well intentioned perhaps but a waste of typing effort IMO.
IF he knew the subject, the poster Would Not have made the initial statement.
This is an open mouth forum after all :-0
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Old 17th October 2012, 12:00 AM   #36
Remlab is offline Remlab  United States
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Quote:
Originally Posted by john k... View Post
I made these posts to a different thread but the same issues keep coming up. There are things that can be done with active analog and active digital that can not be done with passive, but for conventional crossovers thee is no reason a passive system can not perform as well or better than an active. He are my previous posts:


Some simple truths can sum it up. Taken as system the combination of active crossover/amp/driver and amp/passive crossover/driver represent predominately linear systems. Since linear systems with the same frequency response have the same transient response it follows that if the active and passive crossovers systems are designed to have identical frequency response then they must have the same transient response. Any difference is damping due to series impedance in the passive crossover simply means that the transfer function of the passive crossover will have to be somewhat different that that of the active system to account for the difference in damping.

The real difference between passive and active systems is insertion loss and the way the response to changes in VC temperature.

------

I read all the time that active filter are better because the transfer function doesn't change with changes in the driver's Z due to a variety of dynamic effects. Again, that is not an accurate statement. The thing that is overlooked is that this is limited to the voltage transfer function of the filter. But in reality, what determines the acoustic output of a driver is the current flow through the VC. Even in that case where the amplifiers output Z is zero applying a fixed voltage transfer function across the driver terminals does not mean the acoustic output remains constant under dynamic conditions. The driver VC Z changes with time (heating) and excursion and these factors will affect the current and therefore the radiated SPL. Active can make a lot of things easier, and I and am fan of actives. But many proponents of active make arguments for them which are overly simplistic and ignore many of the real issues resulting form the dynamically changing VC Z.
----

So when you start talking about comparing active and passive, start at the beginning. That is not the amplifier output terminals in an active case. Remember that a simple, text book active HP filter has a impulse that decays exponentially or with oscillations, depending on Q of the filter. The impulse tail will past through and be amplified by the power amplifier and feed to the driver. So it isn't just the amp that is controlling the driver, it's the output of the active crossover and it's particular damping characteristics.
Understood about active"ringing"(An active implementation is not putting real stress on an amplifier though, while a relatively similar passive implementation does). But being able to test an an almost limitless amount of variables in real time is not just a small advantage over passive implementations.
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Last edited by Remlab; 17th October 2012 at 12:10 AM.
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Old 17th October 2012, 12:08 AM   #37
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Originally Posted by chris661 View Post
I can see that they can be good enough in many cases, but better than active?
I think this phrase sums it up fairly well and is how I think of it.

We can debate the pros and cons of each all day, but if we look at it from the perspective of the best passive implementations vs the best active solutions then the active version will win every time. Yes you can argue that going active in certain situations is overkill, will end up costing more and probably wont sound better, so it obviously makes far less sense then a well implemented passive design, but that's not really the point imo. Given that situation the active version wont sound worse then the passive design, it's just that going active wont give you any worthwhile gains.
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Old 17th October 2012, 12:34 AM   #38
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Has anyone here converted existing passive speakers to active?

A while ago I was given some old Mission 702es, and I wasn't keen on them, but they did have good, if 'warm', bass. I ripped out the passive crossovers and connected the drivers directly to a pair of 30 ebay amps driven from a PC with surround sound card and digital crossover software. I didn't do anything except set the amp volume controls to match the tweeter and woofer levels at the crossover. The phases of the woofer and tweeter seem pretty well aligned at crossover, and I've done no measurements.

I am still thrilled every time I listen to them, and I've hardly changed anything about the setup in months. I don't think I'm exaggerating when I say my setup is the best I've heard anywhere. And it looks pretty good too, with the multiple amps and speaker cables; you might almost mistake it for 'high end', except it that sounds so much better.

If the DIY enthusiast can't indulge in a bit of 'overkill' then who can? All those passive speaker designers with their first or second order filters, impedance matching, and power-sapping 'pads' seem like masochists to me.
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Old 17th October 2012, 12:35 AM   #39
Pano is offline Pano  United States
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Active is easier to get right, so for many it's "better", and that's really important. Active can also be hyper-detailed, if you like that sort of thing (it's seductive).

But they best systems I've ever heard have all been with passive filters. All world class, all passive. Really, once you get a really great crossover you shouldn't be hearing it anyway, so what does it matter?
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Old 17th October 2012, 12:54 AM   #40
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I think getting it 'right' has a lot of definitions to different people. If by arriving at something that pleases you in a DIY effort is 'right' for you doesn't mean it's right for someone else.

In my opinion getting a design right is just as hard, or easy, whether you go active or passive, that is if 'right' for you is a perfectly implemented crossover for a certain set of drivers where everything has been put together with a lot of measurements and a large chunk of CAD work.

Coppertop's idea of right in this situation is a fine example. If you're going to put together a system with minimal or next to no measurements then you are far more likely to arrive at something good then if the design were passive. This is one of the reasons why active xovers are very popular. But in that situation neither the active or the passive designs would be my definition of what could be considered 'right'. And in that situation if a proper passive xover were to be designed using measurements etc, then it almost certainly would out perform the active version put together without.

Now if you are putting together a system using measurements and CAD software, then going active is a little easier because it gives you access to building blocks that you just don't have when going passive. But if you were to limit the selection of active to filters to only the ones that can be replicated passively, then neither active nor passive is any easier to design for then the other, they are just different.
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Last edited by 5th element; 17th October 2012 at 12:57 AM.
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