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Old 8th April 2003, 05:28 PM   #1
massive is offline massive  Netherlands
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Default Some questions about active crossovers

First, I want to let you know that I already read much about electronic XO's. But there is a lot that I don't understand, mostly technical stuff. What I do understand is that there are major advantages to electronic XO's.
What I want to know is if I should go active. Soldering is a bit of a problem, I am not very good at it, but as long as the XO is not too complex it should be okay. Two poweramps is no problem either. I already have a pre/power setup (Quad 34+405). And last but not least, because it is my first experiment, the active crossover should be reasonably priced i.e. 20-30 euro's/dollars max.

Questions:
I read here that there is no interaction whatsoever between your drivers and XO. Does that mean that if you use drivers with a very linear frequency response, a "textbook" active XO is all you need (with gain control)? For example this one by Rod Elliott. I can hardly imagine. Passive XO's get complex quite easily.

I am thinking of building for example the Tempo. (Seas KT27F + Vifa PLW 18 340/8).
Or I would like to activate the Vifa MPR2 MXT . (Vifa XT300 + Vifa PLW18/225/8). Both drivers have a very linear response.

What are the odds that this project can succeed? What could go wrong? Without measuring equipment. And without really understanding the technical details of the XO.
I do have all relevant data of the drivers mentioned. Thanks in advance.
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Old 8th April 2003, 08:02 PM   #2
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Default Re: Some questions about active crossovers

Quote:
Originally posted by massive

What are the odds that this project can succeed? What could go wrong? Without measuring equipment. And without really understanding the technical details of the XO.
I do have all relevant data of the drivers mentioned. Thanks in advance.
It can succeed pretty well. You have the most important measuring equipment in your ears. Problem is not how the crossover is implemented but at which frequency it crosses over in relation to the directivity of the drivers.

If you just take a midwoofer and tweeter and build a crossover something like 24 dB/oct Linkwitz-Riley (or better yet 48 dB/oct) and adjust the levels for the two drivers then everything else is upto the choice of the drivers and the design and quality of the speaker cabinet with all details such as internal damping, reflex tuning frequency etc.

A good speaker has slightly dropping power response (power response ~ the sum of frequency response measured in 360 o circle around the speaker). Usually the frequency response at 45 - 60 degrees pretty well estimates the power response. This means that the directivity of the elements should be similar from the midbass to tweeter over the crossover frequency.

With usual drivers, lets say 6.5" midwoofer and 1" tweeter the directivity is such that at crossover frequency of lets say 2500 Hz (quite usual) the tweeter is quite omnidirectional whereas the woofer is more like cardioidic. So the power response has a dip just below the xover frequency. As well the *egde diffraction* will make similar dip in little bit upper frequencies. If your speaker design and construction is free from all of these caveats then your pure 24 dB/oct xover will work just fine, *if* your elements are in the same vertical line, which means you must either have the from panel slanted backwards or you must use a waveguide (~horn-like device) with tweeter mounted in the back of it. This is also called the *time alignment* of the drivers. The last and also the least thing to notice is the baffle step diffraction, which means that below certain frequency which depends on the cabinet width the sound from the midwoofer will radiate into full space instead of the half space which makes the response drop by 6 dB/oct. So this is at least something that simple stock active crossovers usually miss. Without it the response from mids to lows will not be straight, though the room response will effectively correct is at some frequencies.
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Old 8th April 2003, 11:22 PM   #3
Kanga is offline Kanga  Australia
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Quote:
The last and also the least thing to notice is the baffle step diffraction, which means that below certain frequency which depends on the cabinet width the sound from the midwoofer will radiate into full space instead of the half space which makes the response drop by 6 dB/oct. So this is at least something that simple stock active crossovers usually miss.
If you are using an active XO, it is relatively simple to correct for baffle step, and much easier than with a passive XO. Also there is no loss in efficiency over the midrange region like there is with a passive circuit. See http://sound.westhost.com/bafflestep.htm

I have built an active XO with Rod Elliot's board, and while its not quite up and working yet (hopefully in a week I'll be able to test it) that has been relatively straight forward, even with my basic understanding of electronics. Its also one of the cheapest available options. An alternative is the Marchand Electronics boards or kits.

A basic sound level meter would be a very useful tool for basic setup, and is not that expensive (maybe US$40).

Mick
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Old 9th April 2003, 01:38 AM   #4
f4ier is offline f4ier  Australia
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While an active filter takes away the not-usually wanted filter-driver impedance interaction, one still has to deal with acoustic equalization to remain true to the target transfer function. So a textbook Sallen-Key or Multiple Feedback filter, for example, would still need modification -- just as textbook passive filters need modification. The filter could be modified and/or assisted by external passive or active equalization.

Cheers

Isaac
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Old 10th April 2003, 08:03 PM   #5
massive is offline massive  Netherlands
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Thanks!
What if I have a readymade design, where baffle-step and edge diffraction is already accounted for? Could that be translated into an active filter? For example the Vifa MPR2 MXT that i mentioned earlier uses a simple 12dB/oct filter, there is only one correction for the midwoofer. A LCR filter, probably baffle step correction.

Can you use two different amps in an active setup?
What would Elliott's crossover cost in Europe?
f4ier: what do you mean by "acoustic equalization to remain true to the target transfer function." ??
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Old 11th April 2003, 01:43 AM   #6
OMNIFEX is offline OMNIFEX  Jamaica
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Quote:
Originally posted by massive

Can you use two different amps in an active setup?

You can use any two amps you like.


Here's how it works (The equal signs, are the cables)


Preamp = Crossover


Crossover High Section = Tube Amp

Crossover Low Section = Transistor Amp


Tube Amp = High Frequency Drivers.

Transistor Amp = Low Frequency Drivers.


Wait I have a crossover lying around, I'll take a pic.
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Old 11th April 2003, 02:00 AM   #7
OMNIFEX is offline OMNIFEX  Jamaica
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Now, Keep in mind, I'm not a Photographer
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Old 11th April 2003, 02:03 AM   #8
OMNIFEX is offline OMNIFEX  Jamaica
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Old 11th April 2003, 04:01 AM   #9
Kanga is offline Kanga  Australia
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Quote:
What if I have a readymade design, where baffle-step and edge diffraction is already accounted for? Could that be translated into an active filter?
Hi Massive

It may be difficult to convert a passive filter design to an active one, because the passive one will probably be taking into account driver/XO interactions. The best you could do is to take notice of the XO frequency and slopes to get you started in designing an active XO.

However it is a lot easier to design an active XO for a couple of reasons:
*No driver/XO interactions, so you just measure each driver response on its own, choose the XO frequency and type, and the resultant response should be the sum of the two driver responses with the XO applied.
*Its a lot easier to change XO frequency by changing values of relatively small components. On the Marchand XO you can do this really easily by changing resistors only. On the ESP XO you can change the frequency to a certain extent just by changing 1/4W resistors, however for larger changes adjusting the capacitor values is recommended.
*Various fancy equalisation functions are easy to implement (eg baffle step, shelf filter etc).

So if you pick a couple of drivers (with some thought of course) and have basic measument ability (eg sound level meter) then generating and active XO should be straight forward.

If my current biamping exercise turns out OK, then I may build a 10W class A amp to power the tweeter only. Sweeeet.

Mick

ps Omnifex - does that camera have focus by any chance....
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Old 11th April 2003, 12:52 PM   #10
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Quote:
Originally posted by Kanga


It may be difficult to convert a passive filter design to an active one, because the passive one will probably be taking into account driver/XO interactions. The best you could do is to take notice of the XO frequency and slopes to get you started in designing an active XO.

ps Omnifex - does that camera have focus by any chance....
Going from passive to active components -- if you go to the thread discussing "Frequency Dependent Negative Resistance" there is a reference to a Burr Brown application note -- Low Noise Low Distortion Deisgn for Antialiasing and Anti Imaging Filters -- (hint -- plug FDNR into Texas Instruments search engine to find the article.)

The note goes through the transform from passive to active -- by multiplying the values of resistors, inductors and capacitors by 1/s all inductors become capacitors, all resistors become capacitors and all capacitors become Frequency Dependent Negative Resistors. Now whether this helps with the interactions I don't know, but it's the first step.

and Omni -- with respect to the image quality. try NOT to fill the screen with the entire image, step back a bit and let "depth of field" do it's job. I do a lot of macro-photography, everything from 35mm to 4X5 and digital in between.
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