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Old 10th March 2013, 07:44 AM   #91
Stevenn is offline Stevenn  Australia
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Gents
Attached are graphs of my working of PCD crossover. XO 300Hz and 3000Hz
Woofer has Zobel and parallel contour filter.
Tweeter has Zobel, parallel resistor and series RLC
Mid has Zobel and parallel resistor.
I had great difficulty getting a smoooth flat response, the drivers I have are not that compatible, unfortunately.

I am a bit concerned as cost of components and drivers is getting up there with price of good pair of proven speakers, and that is before I have butchered everything with a soldering iron.
All comments welcome...
Attached Images
File Type: gif Graph1.gif (28.2 KB, 44 views)
File Type: gif Graph2.gif (21.9 KB, 42 views)
File Type: gif Graph3.gif (21.6 KB, 40 views)
File Type: gif Graph4.gif (26.1 KB, 38 views)
File Type: gif Graph5.gif (26.6 KB, 38 views)
File Type: gif Graph6.gif (26.7 KB, 11 views)

Last edited by Stevenn; 10th March 2013 at 07:45 AM. Reason: Spelling
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Old 10th March 2013, 09:22 AM   #92
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Wintermute,
Try this and I think that you will be happy in the end and will achieve the results that you are after. Remove the crossover from you bass mid. Check the impedance curve with the speaker in the box. Build yourself a conjugate network, an LCR network that goes across the terminals of the speaker as close as you can get. What you are trying to do is make an inverse relationship to the impedance reactance curve of the driver. The resistor value will be a total of the resistance of the capacitor and the inductor and whatever you have to add to be the total. None of the components need to be very large as you are only trying to match the properties of the voicecoils impedance values. The coil can be very small gauge wire. First is the capacitor and Inductor values and finally the resistor to match the level of the voicecoil. When you get this right the impedance curve will be flat, you will have a perfectly resistive load on both the crossover and the amplifier when you are done. My friend who is expert at this does it quickly with a set of inductance and capacitor switchable boxes so he can change values very quickly. Your polar response of the device will be something you will not see any other way. He would kick my *** if he knew I was posting this information. Try it, you will get somewhere you will never get with your speakers any other way. You can do this on the tweeter also, just be careful and do the testing at very low power as you will not have a crossover to cut the low frequencies going to the tweeter. When you get them both to have flat impedance curves then do your crossovers, you will be pleasantly surprise in the results. Best polar response you will ever get and the amplifier will function with the lowest distortion levels as the feedback circuit does not have to fight the external pole of the speaker that it has no way of correcting.
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Old 10th March 2013, 10:57 AM   #93
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I think that was directed at Steve Steve, have you purchased the drivers yet?

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Old 10th March 2013, 02:53 PM   #94
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Yes,
That's what happens when we write in the middle of the night! That was for Steven

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Old 10th March 2013, 10:00 PM   #95
Stevenn is offline Stevenn  Australia
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Thanks yr replies.
Kindhornman - You have not said why this is necessary. There are Zobel ciricuits for all 3 drivers. What is it specifically in graphs I posted that you are concerned about?

Wintermute - I already have mid and tweeters. A Peerless 8" 831868 doesn't have the peak in SPL response like SB 23 but is more than double the cost.

Cheers
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Old 11th March 2013, 02:34 AM   #96
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Stevenn,
A Zobel is not a conjugate network, It does not have an inverse relationship to the moving coil in a gap. Yes it does flatten the impedance curve but it does not do the same thing as the conjugate network that eliminate both impedance, phase shift and the back emf wave in the same manner. Ignore what I said, it doesn't matter, but I was trying to tell you something that you will not get from the simplicity of the Zobel network.
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Old 11th March 2013, 02:52 AM   #97
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Also Stevenn as the XO on the woofer is in an area where the impedance is probably already equal to the DCR there is probably no need for a Zobel network on it. A Zobel on the tweeter would be there to flatten the impedance for the amplifiers sake too, so if cost is becoming an issue that part of the network could be eliminated as well
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Old 11th March 2013, 03:22 AM   #98
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The most cogent explanation I can find to post here comes from another thread where this is discussed in detail. I think that this covers most of what is going on. It is another pole in the crossover and changes the transfer function of the speaker amplifier circuit. I wish I could explain it as well as my friend did to me and I know that there is more to it than meets the eye. I will just post the explanation that covers the basic of the conjugate network.

That all sounds very nice, but it is not exactly what happens. I've prepared a couple of sims to try and show what is actually going on. First of all, if the electrical damping of the driver changed, then it's impedance would have to change. Back in 2004 this was a topic of discussion on the Madisound board for the case of a series resistor changing the damping. My analysis showed that while the damping of the system (again note the word system, not driver) changes, the damping of the driver doesn't. Qes of the driver never changes. The impedance of the drivers is fixed.

Next, in the figures below, recognize that everything to the right of the capacitor represents some impedance load, ZL, that the capacitance sees. What happens without the LRC (which is a conjugate network) is that this load (the driver alone) has both resistive and reactive components (capacitance and inductance). The series capacitance of the 1st order crossover in combination with the inductive component of the driver's impedance at the impedance peak forms a LC resonance. You can see this in the first figure below where a cap is in series with a model of a driver's Z.
Next to it I have replace the model of the driver's Z with an inductor in series with a resistance. You can see similar behavior.

Click the image to open in full size.


In the next figure I have added a conjugate network which eliminates the the reactive components in the in the load, ZL, making it purely resistive. As you can see, there is no resonance peak in the response now because there is no inductive component in the load for the capacitor to resonate with.


Click the image to open in full size.

But the impedance of the driver model remains the same. Yes, the conjugate network has an impedance of 4.4 ohms at impedance peak of the driver model, and yes this does provide a current path to ground, but what suppresses the peak in the response curve is the elimination of the reactive nature of the load seen by the cap. This can be better observed by replacing the conjugate network with a 4.4 ohm resistor, the same value as that in the conjugate network. The last figure shows a comparison between the response with the conjugate network in red and when the shunt is just a 4.4 ohm resistor in blue. The resistor does add the same damping to the system at Fs as the conjugate network and therefore the LC resonance between the crossover cap and the inductive component of the driver's Z has a lower peak. But it is still present because the reactive components in the driver's Z have not be compensated for. (Note that the value of the series cap has been changed to compensate fro the difference impedance so as to keep the crossover point the same.)

Click the image to open in full size.

So it is not so simple as to say the LRC provides a current path to ground and therefore damps the resonance. There is more to it. Te correct conjugate network eliminates the cause of the resonance be removing the reactive components in the driver's Z around Fs. This is exactly the same as what a Zobel does to eliminate the effect of the rising impedance due to voice coil inductance. In neither case does the driver's impedance change. What changes is the impedance the upstream elements in the crossover network see, and how they interact with that impedance.
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Old 11th March 2013, 03:40 AM   #99
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Here is another link discussing the LRC conjugate tank circuit. Take a look, you can control the Q of the circuit in a way that you can not do with the Zobel RC circuit alone.

Introduction
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Old 12th March 2013, 10:56 AM   #100
Stevenn is offline Stevenn  Australia
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Kindhornman.
Thank you for your detailed post, which I just about understand.
The conjugate network would seem to be a great tool for use in advanced design and is worthy of a thread all of its own. Surely top brand speakers should incorporate this as a matter of routine?
However I have neither tools or instruments (switchbox?)to measure the parameters suggested, nor the where withall to determine sizes of the particular components involved.
I started this thread with a view to getting members' help as newbie in renovating a pair of speakers by using some freely available recognised software WinIsd and PCD and whilst I feel my understanding of Physics involved has improved, I am no closer to starting this and in particular have no confidence in my results which I have posted for critique.
A quality speaker kit is starting to look like a good option!
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