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Passive crossover question

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malina View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote malina Quote  Post ReplyReply Direct Link To This Post Topic: Passive crossover question
    Posted: 24 July 2014 at 3:12pm
I am doing some dual 8" + 1" speakers and a a passive crossover for them.

The question is: if there are two 8"(each 8 ohm) with sensitivity of 95db and one 1" (8ohm) driver with sensitivity od 109db how should I calculate the attenuation for 1".

Should the attenuation for 1" be 8db in aspect to dual 8" or there are some ohm laws that say different?


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Post Options Post Options   Thanks (0) Thanks(0)   Quote RobinMatrix Quote  Post ReplyReply Direct Link To This Post Posted: 24 July 2014 at 3:36pm
It is not as easy as that.

My advice:  put a microphone (cheap Behringer ECM8000 is fine) in front of the speaker,  take a sweep of the bass, and a sweep of the HF, use the REW software (free) or something like it,  without moving the mic or moving the gain controls on the audio interface.  Then you have some curves you can make an estimate with.

Without a measurement, you are just guessing.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Conanski Quote  Post ReplyReply Direct Link To This Post Posted: 24 July 2014 at 3:41pm
You gain 3db in sensitivity every time you double the number of drivers so you can consider the dual 8 combo to have a 98db sensitivity which means there is an 11dB difference between them and the CD. But that is taking the ratings at face value, in practice the average sensitivity could be higher or lower, the horn the CD is mounted on will have an effect on it's response, and if you try to flatten the response of the 8's with notch or shelving filters that will lower their sensitivity. So bottom line is you're going to have to connect it all up and test it with an adjustable L-pad or with an active crossover before finalizing the passive network.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote mhtplsh Quote  Post ReplyReply Direct Link To This Post Posted: 24 July 2014 at 5:27pm
Originally posted by RobinMatrix RobinMatrix wrote:

It is not as easy as that.

My advice:  put a microphone (cheap Behringer ECM8000 is fine) in front of the speaker,  take a sweep of the bass, and a sweep of the HF, use the REW software (free) or something like it,  without moving the mic or moving the gain controls on the audio interface.  Then you have some curves you can make an estimate with.

Without a measurement, you are just guessing.


If there is 3/2 way speaker box, you will need to take 3/2 graphs.  So how will we take the measurement without moving the mic?
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Earplug Quote  Post ReplyReply Direct Link To This Post Posted: 24 July 2014 at 5:59pm
"If there is 3/2 way speaker box, you will need to take 3/2 graphs.  So how will we take the measurement without moving the mic?"


You want a general curve of the response at 2 to 3 meters from the box, not a 1W/1m thing. Also, real life response of the cabinet will be different to estimates from the sensitivity ratings of the components. You need to use your ears a bit there as well.

"So bottom line is you're going to have to connect it all up and test it with an adjustable L-pad or with an active crossover before finalizing the passive network.

+1


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Post Options Post Options   Thanks (0) Thanks(0)   Quote odc04r Quote  Post ReplyReply Direct Link To This Post Posted: 25 July 2014 at 9:58am
As I've said a few times before, it is not the most exact way of doing things but it you have an active xover then connect up the cab using that and some amp channels then play with the settings until you are happy. When you are then replicate those settings in a passive crossover.

As well as an SPL reading you will want to perform an impedance sweep which is easily done with just a soundcard and a few resistors + software like REW. That will help you figure out your l-pad values to get the relative gains you want at the crossover point.

This method takes no account of the phase shift a passive xover will induce but as you are building a fairly simple crossover then I think you will get decent results with it if you calculate the numbers correctly.

Edit - As a last check before you buy expensive coils and capacitors to make the boards, simulate your passive design using a program such as LTSpice or similar. You should see the expected relative gains and xover points.


Edited by odc04r - 25 July 2014 at 9:59am
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Marko Quote  Post ReplyReply Direct Link To This Post Posted: 25 July 2014 at 12:11pm
Malina if you allready designed the filters and Lpad is the only thing left to do you can connect cone drivers and CD on separate amp channels through the passive filters you have and set levels the way you like and see the difference in dB on amp gain pots.
Then measure CD impedance and with impedance and dB reduction calculate the Lpad. There is lots of Lpad calculators on line.

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Post Options Post Options   Thanks (0) Thanks(0)   Quote ceharden Quote  Post ReplyReply Direct Link To This Post Posted: 25 July 2014 at 7:50pm
To echo what has already been said, you can get a rough starting point by the theory but ultimately you will need to tune it by ear to what sounds right.

Sensitivity values do not at all accurately reflex the actual output of the drivers at the crossover point.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote malina Quote  Post ReplyReply Direct Link To This Post Posted: 26 July 2014 at 9:12am
Thanks.

Will do some measurements as soon as I get some proper USB sound card  with phantom powerSmile

Any suggestions? 
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Post Options Post Options   Thanks (0) Thanks(0)   Quote RobinMatrix Quote  Post ReplyReply Direct Link To This Post Posted: 26 July 2014 at 6:31pm
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Post Options Post Options   Thanks (0) Thanks(0)   Quote odc04r Quote  Post ReplyReply Direct Link To This Post Posted: 26 July 2014 at 11:05pm
I use an ART Dual Pre, no problems so far.
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