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Tweakers' Asylum Tweaks for systems, rooms and Do It Yourself (DIY) help. FAQ. |
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Tweakers' Asylum Tweaks for systems, rooms and Do It Yourself (DIY) help. FAQ. |
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In Reply to: Active vs. Passive Crossovers? posted by John K on April 14, 2000 at 06:58:10:
Hi John, you are asking two questions here. The first answer is that an active circuit contains electronics eg tubes or transistors, whereas a passive circuit contains only passive components ie components with no gain such as resistors, caps and inductors. Passive circuits are always lossy, active circuits have gain. (OK, so transformers can have voltage or current gain but not power, I am generalizing a bit here.) Active circuits are capable of more sophisticated and tunable frequency responses, partly because filtering components are inserted into the feedback network of an amplification stage, and partly because active circuits also provide buffering and hence remove or reduce impedance matching problems between multiple filtering stages. Passive circuits are, on the other hand, less likely to color the signal.The second answer is that some people claim that active crossovers are better than passive crossovers for loudspeakers. What they are most likely comparing is (i) an active crossover at line levels, which drives multiple power amps, each of which individually drives a driver (speaker unit), and (ii) a passive crossover at speaker levels, which is driven by a single amplifier per channel, and which uses passive components to split the frequency range for each of the drivers (this is what most systems will be doing). This might be claimed to be better because:
- An active filter provides more accurate and/or tunable crossover characteristics
- Line-level filters run at high impedances relative to speaker-level filters and are hence more likely to be smaller and cheaper
- Line-level filters do not dissipate power between the amplifier and the speaker
- Passive speaker crossovers have non-linear impedance, alter the damping factor seen by the amplifier, and so on
- Passive crossovers interact with the drivers and are hard to design
- Drivers sound more dynamic when driven directly by an amplifier
However, there are also arguments against this arrangement (which I don't think I have heard very often, so perhaps I am in the minority):
- Speakers are designed with a crossover network matched to the particular characteristics of the drivers. (I am talking about good speakers here of course.) If you take the crossover out of the speaker designer's control then you have removed his/her ability to truly match the drivers, cabinet, and crossover together.
- Anything you put in the signal path alters the signal in some way. Passive components do too (a cable is a passive component, for example), but they are less likely to add coloration than active components. A typical active crossover will insert three or four gain stages into your audio path, and unless these are of very high quality then something is going to suffer.
- You are marching to the beat of a different drum :-) Nothing wrong with that, but most components are designed to work with other components, and making these kinds of changes in one part of the system will necessarily require "recalibration" in other parts of the system. This is why (hare-brained-theory alert!) minimalistic passive "pre-amps" can make you wonder why your amp/speakers sound both incredibly clear and in-your-face at the same time.
Of course in the end any arguments for or against are academic, the proof is in the eating, or rather the listening :-) If I were putting together a bi-amped or tri-amped system with amps directly driving the speakers, I would try using a passive line-level crossover. I would put a single very good buffer stage before it, so I could set the component impedances of the filters low relative to the input impedance of the amps. And I would be prepared to junk it if it didn't work :-)
I expect that is more than you wanted to know, good luck and happy listening! :-)
JohnR
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Follow Ups
- Re: Active vs. Passive Crossovers? - JohnR 23:27:50 04/14/00 (2)
- Re: Active vs. Passive Crossovers? - John K 07:55:06 04/15/00 (1)
- Re: Active vs. Passive Crossovers? - WT 15:12:19 04/29/00 (0)
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