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Old 26th August 2012, 03:54 PM   #1
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Default Choosing an active 3-way crossover

I would like to try switching over to an active crossover. My speakers are nothing particularly special at the moment, Polk RT55 speakers with a Polk PSW12 powered subwoofer. I may get better speakers at some point in the future, but these are good. I recently heard them used with a very fancy biamped setup and they sounded much, much better, so I am going to try going this route.

So the first thing I need is a crossover. It will take some balls to cut the wiring for the passive crossovers in the speakers, but no big. Because of the subwoofer, I believe I want a 3-way stereo crossover. Sub crossing over to mids should be about 100 hz, and mids to tweeters in the RT55s are, I think, set up by the Polk factory at 2.5khz.

I've researched crossovers and the Marchand
XM9-3AA would work, although it's quite expensive and changing crossover points entails opening the unit and inserting new circuit cards. Not the end of the world, but I'm wondering if I could just us a less costly pro-/DJ-oriented unit such as a DBX 234s. Obviously you need 1/4"-to-RCA adapters, but that's easy. That unit allows you to change the crossover points right from the front. Is there any reason why it would be a poor choice? ... would it not perform up to the level of a Marchand, and if so in what way? What might prove a better choice?

Many thanks for any advice.
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Old 27th August 2012, 11:14 PM   #2
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No replies yet, so I'll give it a shot.

There are some major advantages to using active crossovers, but there is also a lot of complexity to convert a speaker designed and built for passive xovers to use of actives.

Most passive xovers in commercial speakers, and many in DIY designs, are not a straightforward transfer function. They usually invlolve some other frequency adjusting circuitry. Some of it can be eliminated when switching to active; some of it can't. So be advised that simply unplugging the existing passives and installing an active might not yield better overall results.

First of all, don't cut any wires, and don't throw anything away, and don't change anything that you won't be able to change back if the actives don't work out.

Secondly, you need more information about the passive xover than just its frequency. Do you know the slope? Is there any other kind of equalization in the passive xover, such as baffle step compensation?

Don't spend a lot of money on a fancy active xover. Marchand makes excellent products, but I guarantee that you will not get this right on the first try. It's just not that simple. Buy the cheapest active you can find. Those DBX 234's are okay for experimenting, but they sound like crap. I owned one. I hated it. There are no better cheap ones, so you must accept poor fidelity initially. Mini-DSP (if you don't know what that means, find out NOW) is probably a better choice to begin with, as slopes and xover points are flexible. The DBX uses LR4 (if you don't know what that means, find out NOW) fixed slopes, which work for many speakers but not for all.

Do you have another amp? You need two if you're using a two way active xover, three amps if you're doing 3-way. I recommend starting between woofer and midrange, leaving the tweeter passive.

These are some of the challenges. It can be very rewarding to make the switch, but it can also be a disaster. Not trying to scare you off, but it seems that you need to learn a lot more about crossovers before taking the plunge.

Peace,
Tom E
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Old 27th August 2012, 11:43 PM   #3
ra7 is offline ra7  United States
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The above is good advice. I would suggest a lot of reading before doing anything to the passive crossovers.

Also note that going active has certain advantages, but fundamentally, passive can be just as good as active. Active wins in certain areas like crossing to a sub, where crossover components become large and sometimes, cost prohibitive. But a passive crossover for the tweeter is going to be just fine, and in fact, has the advantage of protecting the tweeter from DC.
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Old 28th August 2012, 12:09 AM   #4
Remlab is offline Remlab  United States
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Linkwitz states..
I have a strong preference for line level active dividing networks ahead of the power amplifiers (Ref. 2, 12, 17). In this approach the power amplifier output is connected directly - except for a very low resistance speaker cable - to the voice coil of the driver. The amplifier takes maximum control over the motion of the speaker cone which gives a greater sense of clarity and dynamism compared to a passive dividing network between amplifier and driver. Active crossovers make much more effective use of amplifier power. A clipping woofer amplifier is not seen by the tweeter, which has its own amplifier. The clipping of the woofer amplifier may not even be noticed in this case. It would surely be heard with a passive crossover, where it might even overheat and damage the tweeter, because of the large amount of high frequency energy in the clipped signal.

Crossover filters for a speaker usually incorporate frequency response corrections for the individual drivers to obtain a desired overall response. The active network has the advantage of correcting easily for different sensitivities of drivers and equalizing not only the individual drivers but the combined response as well. Not having to deal with the interaction between driver impedance and passive filter network gives the designer of an active crossover/equalizer much greater freedom and control to develop a superior product.
The best crossovers ...
The best electrical crossover filter is one that maintains the acoustic polar response of a loudspeaker throughout the crossover frequency range as output shifts from one driver to the next. The sum of acoustic lowpass and highpass outputs must have allpass behavior without high Q peaks in the group delay. The highpass filter section must attenuate out-of-band driver terminal voltages at a sufficiently high rate so that cone excursion decreases with decreasing frequency and nonlinear distortion is minimized.

The crossover must be inaudible

The crossover must be inaudible on program material. This also implies that the power response of the two drivers must be similar in the crossover region, and that requires special attention during the loudspeaker's concept and design phases.

Crossovers may be implemented either as passive RLC networks, as active filters with operational amplifier circuits or with DSP engines and software. The only excuse for passive crossovers is their low cost. Their behavior changes with the signal level dependent dynamics of the drivers. They block the power amplifier from taking maximum control over the voice coil motion. They are a waste of time, if accuracy of reproduction is the goal.

Accurate Stereo performance tests SL - October 2009
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Last edited by Remlab; 28th August 2012 at 12:20 AM.
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Old 28th August 2012, 12:53 AM   #5
sreten is offline sreten  United Kingdom
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Hi,

Whilst the above is true, its a truism, proper active x/o's done properly.

its not easy to do it properly, and if you can its not necessarily cost
effective, e.g. an expensive stereo power amplifier for tweeters
compared to active active biamping for bass to mid/treble and
a passive x/o for mid/treble.

For most two ways active is a waste of money unless you have
a nice dinky little treble amplifier to use with the main amplifier.

rgds, sreten.
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Old 28th August 2012, 02:12 AM   #6
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I doubt the original poster can interpret all that techno mumbo jumbo. I admire Linkwitz, but he's not for the novice. He also happens to sell plans and parts to build active crossovers, so he's hardly impartial.

Here is some simple advice: Polk makes pretty nice speakers for the mainstream consumer such as you. Active crossovers are not a simple way to improve the quality of sound, and they're not for mainstream consumers. They can be complex and expensive and difficult to implement properly. There is no suitably cheap or simple solution, no magic bullet. And then you need amps.

Passive crossovers are not poison, and definitely not a "waste of time". The vast majority of speakers use them, and some are quite stunning. If you are really interested in improving your existing speakers and willing to learn a few things about how they make good or bad sound, read some of the forums about crossovers and parts quality, especially capacitors. You can probably improve the sound of your speakers by merely substituting better quality parts in the existing passive crossovers. It's not difficult, not complicated, can be as cheap or expensive as you want to make it, and can pay modest to considerable dividends in better sound.

Most speaker manufacturers, even Polk, make compromises in the quality of components they use in their crossovers because few people ever see them and don't know or care that better parts might sound better.

Peace,
Tom E
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Old 28th August 2012, 09:40 AM   #7
AndrewT is offline AndrewT  Scotland
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Quote:
Originally Posted by madisonears View Post
.......................... If you are really interested in improving your existing speakers and willing to learn ..................You can probably improve the sound of your speakers by merely substituting better quality parts in the existing passive crossovers. It's not difficult, not complicated, can be as cheap or expensive as you want to make it, and can pay modest to considerable dividends in better sound..............
I cannot recommend this to beginners.
If the speaker designer has done his job properly then he will have "tuned" the passive crossover components to take account of the parasitics in the components used.
Swap to a component with different parasitics and the beginner ends up with an "untuned" speaker. He should then adjust some or all of the other components and maybe introduce new or additional components to bring the passive speaker back to spec. Then you might get better sound or you might not. It will certainly be different.
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Old 28th August 2012, 11:55 AM   #8
Remlab is offline Remlab  United States
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As far as Polk is concerned, they do an amazing job with their passive crossovers. There really isn't much left to improve upon. Amazing value at any price point..
http://www.stereophile.com/content/p...a3-loudspeaker
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Last edited by Remlab; 28th August 2012 at 12:06 PM.
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Old 28th August 2012, 02:43 PM   #9
Davey is online now Davey  United States
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foodchain,

As the fellas have alluded to, it's not quite straightforward to "switch to an active crossover" with an existing set of speakers. Passive crossovers usually perform a lot of shaping, non-textbook filtering, etc, etc, to achieve the final design.
Generic, active crossovers will most likely have completely different electrical responses than the crossoovers now inside your RT55's. That may be a good thing or it may be a bad thing.

Anyways, you mentioned "I recently heard them used with a very fancy biamped setup and they sounded much, much better." What are the details of that setup? Can you copy it? Did the person who implemented it know what he was doing?

Cheers,

Dave.
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Old 28th August 2012, 03:02 PM   #10
AndrewT is offline AndrewT  Scotland
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Bi-amped could have meant he connected a separate amplifier to each pair of speaker terminals.
That is Bi-amping. He did not say active speakers.
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