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Attenuate compression driver output.

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_djk_ View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote _djk_ Quote  Post ReplyReply Direct Link To This Post Posted: 22 September 2007 at 2:48am
"There shouldnt be a problem adding a serial and parallel resistor right before the tweeter, conserving the correct impedance.. ?? "
 
Didn't look at the schematic for the networks, did you?
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Post Options Post Options   Thanks (0) Thanks(0)   Quote HansA Quote  Post ReplyReply Direct Link To This Post Posted: 22 September 2007 at 1:16pm
_djk_ no. Do you have a link to the schematics?
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Post Options Post Options   Thanks (0) Thanks(0)   Quote godathunder Quote  Post ReplyReply Direct Link To This Post Posted: 22 September 2007 at 1:32pm
http://www.jblproservice.com/pdf/SRX700%20Series/SRX722F.pdf
 
Im certainly no expert but I dont understand why its not possible to put an L pad at j2, 1+2, maintaining the same impedence seen by the crossover but diverting some of the power from the driver
 
Id be grateful for an explanation djk
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Post Options Post Options   Thanks (0) Thanks(0)   Quote jsg mashed Quote  Post ReplyReply Direct Link To This Post Posted: 22 September 2007 at 5:46pm
Originally posted by godathunder godathunder wrote:

I dont understand why its not possible


I agree. Looking at the schmatic for the 2*12, I see a couple of notch filters (to get rid of response peaks), a 3rd-order HP and a 1st order LP in the path to the tweeter.

There's no driver Z correction that I can see, so it's fair to assume the xover is designed assuming the driver is a constant resistance. So the L pad should be fine.

...because Good is Dumb.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote csg Quote  Post ReplyReply Direct Link To This Post Posted: 22 September 2007 at 8:31pm
right, lets get this clear. An L pad, designed correctly, and correctly wired across the comp driver will NOT effect the load the crossover network sees, and will therefore NOT cause any problems or damage to the speaker.
It will however have the following effects:
1) it will increase the apparent power handling of the comp driver as some power is burnt in the l pad
2) It will effect the spectral balance of the speaker as a whole.
 
Now, the increase in power handling is an advantage, but the effect on spectral balance ( sound)  is debatable. If your preference is for less treble, then i suggest you go for it - at worst it will cost you a few quid in resistors and a bit of time.
 
You will do no damage to the speaker providing you do the job properly.
 
I have designed many many passive filter networks over the years and can confirm that every passive network can be modified in this way as the nature of the l pad, keeping the load impedence the same from the viewpoint of the crossover network in essence makes the l pad invisible to the crossover. Electrically, the crossover simply doesent know it is there. Anybody who says that you cant add an l pads to this network is incorrect.
 
let me know how you get on
 
chris
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Post Options Post Options   Thanks (0) Thanks(0)   Quote csg Quote  Post ReplyReply Direct Link To This Post Posted: 22 September 2007 at 8:36pm
me again.
 
I just have seen the post above mine.
 
There is no such thing as a constant resistance driver, therefore the crossover can not be designed that way.
 
Drivers only have a constant resistance at DC ( 0Hz), which also happens to be very effective at cooking voice coils. Drivers are rated with a nominal impedence ( impedence being dependant on frequency and effected by the inductance of the voice coil)
 
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Post Options Post Options   Thanks (0) Thanks(0)   Quote _djk_ Quote  Post ReplyReply Direct Link To This Post Posted: 23 September 2007 at 9:02am
Of course the connection dot drawn linking L+ and IN- is an error.
 
The notch formed by the 16µF and 0.20mH is at about 2.8Khz, and the notch formed by the 3µF and 0.3mH is at about 5.3Khz


Edited by _djk_ - 23 September 2007 at 12:42pm
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Post Options Post Options   Thanks (0) Thanks(0)   Quote _djk_ Quote  Post ReplyReply Direct Link To This Post Posted: 23 September 2007 at 12:16pm
"Id be grateful for an explanation"
 
A horn woks by adding radiation resistance to the resistance of the driver by itself. With a real horn (finite mouth) we will see a series of cyclical peaks in the impedance due to reflections from the mouth. Corresponding to these impedance peaks we will also see peaks in the acoustic output of the horn, most noticable above the 1/2W point (the finite mouth generally masks the lower frequency peaks).
 
JBL does not show the impedance of the horns and drivers used in the speakers in question here, but they do show the 2451H (used in the earlier version of the SRX722), but on a bigger horn. The bigger horn will have its peaks at different frequencies, but will serve as an example. The second peak is in the middle of the operating point of the horn (2352 in this example), and the impedance rises from about 12R either side of the peak to about 30R right at the peak (the plot is actually of a J, not an H as specified in the graph). If you were to add an L-pad with some attenuation the impedance peak will be swamped by the parallel resistor in the L-pad. This will now cause excess attenuation in the notch filters and interfere with the crossover point too.
 
At a bare minimum we will have to adjust the values of the notch attenuation resistors (this will change with the setting of the L-pad), and we will also have to increase the value of the 2.2µF in the 18dB filter (this will also change with the setting of the L-pad). With the L-pad set to about 6dB of loss "pulling everything after the 1.2Khz crossover point back by 6-12db" you will probably need to increase the 2.2µF to 5.6µF (or more). Keep in mind that the textbook 18dB crossover for an 8R resistor would have 10µF, 0.75mH, and 30µF for 1.3Khz (the SRX722 nominal crossover frequency is 1.2Khz).
 


Edited by _djk_ - 23 September 2007 at 1:51pm
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Post Options Post Options   Thanks (0) Thanks(0)   Quote HansA Quote  Post ReplyReply Direct Link To This Post Posted: 23 September 2007 at 12:30pm
Funny. My fifth edition "loudspeaker design cookbook" mentions nothing about using driver attenuation circuits in any filter.. I don't get it. Why cant you do it in this one? One more time, more detailed :P
further, the schematics are realy bad drawn imho. :P
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_djk_ View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote _djk_ Quote  Post ReplyReply Direct Link To This Post Posted: 23 September 2007 at 12:55pm
I was editing while you were posting.
 
Also consider the 0.3mH in series with the 5.6µF, what is it actually doing? It has a pole zero at about 3.9Khz and about 30R of reactance at 16Khz. It's difficult to calculate because the reactive component of the load (driver + horn) is not known. If it was about 30R as shown on the data sheet for the 2451H (which is not correct, and not our horn), and purely resistive (which it is not), then this inductor would only cause about a 3dB loss at 16Khz. We do know that if we use an 8R L-pad set to 6dB of attenuation that the driver which is about 6R mid-band and around 15R in the high end will drop to 9.5R, so the 0.3mH inductor will cause the high end to roll off even more than the design calls for.
 


Edited by _djk_ - 23 September 2007 at 1:43pm
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Post Options Post Options   Thanks (0) Thanks(0)   Quote HansA Quote  Post ReplyReply Direct Link To This Post Posted: 23 September 2007 at 1:06pm
yes. It even start to make sense in my onion brain.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote rezsbc Quote  Post ReplyReply Direct Link To This Post Posted: 24 September 2007 at 11:18am
 
ok so will just any sock in the horn do the trick or what???
 
(interesting thread though thanks for everyones input learning a little although it is above my head...)
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