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Old 26th December 2012, 10:46 PM   #1
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Default Qu re: Tweeter LCR resonance compensation

I was looking at a VIFA application sheet for one of their old tweeters. It was very comprehensive and included a suggestion for an optional LCR network to place across the tweeter to compensate for the resonance (approx 1,500 c/s).
Now I understand that this will reduce the normal res' peak and allow the crossover to "see" a flatter impedance at the drive unit and so perform more closely to its theoretical design goals. It was the next bit that I didn't understand. The sheet goes on to say:

"The load provided by the cross over should be as low as possible at the tweeter resonance frequency. To ensure maximum electrical damping and consequently minimum excursion, it is also recommended to apply a parallel compensation circuit. This is essential for high power output."


So I am clear on its purpose in optimizing the cross over termination but not on how the network reduces the excursion.......
any thoughts would be appreciated.....

And a very "Merry Summer Solstice Situation"* to everyone.
Jonathan

* I am in the Southern hemisphere!
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Old 26th December 2012, 11:15 PM   #2
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The D27 35-06??
The 45 -06 says the same thing but I am using it first order with the P-13
I think it really depends on how much power you are going to put through the system. I too would like to understand exactly how the LCR parallel circuit works and whether or not they are the optimum way to proceed
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Old 26th December 2012, 11:23 PM   #3
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It all sounds very mystical but the real truth of the matter is that it's all misunderstood. The reality is that a tweeter connected through a passive crossover has an acoustic output which can be measured. The response of the tweeter, including the damping of its motion, is directly related to that acoustic output. If the acoustic output is that of a 2nd order Q = .5 high pass response the system composed of the tweeter, x-o and amp is that of a 2nd order HP response and the tweeter's excursion and damping will be that of the 2nd order response. Similarly if it is a 4th order, or what ever it is.

This is where the argument of direct connection to the amplifier yields better control falls a part because if the tweeter is connected to an amplifier through an active crossover that has the identical response as that of a passive system then both systems (tweeter + crossover + amp) will behave the same.

The major difference between active and passive is that the voltage transfer function of the active system is not affected by the changes in the tweeter's voice coil impedance. But since it is the current flowing through the voice coil that generates the driving force this current is still altered by changes in Zvc.
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Old 26th December 2012, 11:26 PM   #4
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Quote:
Originally Posted by Moondog55 View Post
The D27 35-06??
The 45 -06 says the same thing but I am using it first order with the P-13
I think it really depends on how much power you are going to put through the system. I too would like to understand exactly how the LCR parallel circuit works and whether or not they are the optimum way to proceed
The purpose of the LRC network is just to flatted the impedance so that the crossover sees a more constant and purely resistive load. If you are using a single cap in your crossover the LRC network would be advantageous.
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Old 26th December 2012, 11:35 PM   #5
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Thanx John
Sorry for my abstruseness but how does an LCR parallel network differ from a Zobel / CR parallel network
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Old 26th December 2012, 11:40 PM   #6
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Hi Moondog55,
Close but no cigar! Mine was a D26NC-15-06 (discount from Jaycar a few years ago). I notice if you try to get Vifa D26 data or spec's via Google nothing turns up but punch in the full model number and we get the official sheet.

Cheers, Jonathan
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Old 26th December 2012, 11:42 PM   #7
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Hi john k....
The flattening of the impedance curve is not the question...
We (me and fellow Aussie Moondog55) are both are interested in the effect of the LCR network on dome excursion and power handling.
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Old 26th December 2012, 11:50 PM   #8
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Hi Moondog55,
Re #5. I can do that one. The Zobel network compensates for the gradual rise in impedance as the frequency rises. So we have the DC value of the coil (say about 6 ohms in an 8 ohms system) then we get a rise due to the fact that the coil is around the pole piece and is an iron cored inductor. Philips and others would occasionally place a copper piece over the centre pole piece to minimise the rise. So an appropriate Zobel will have an "R" equal to the DC value of the coil and the "C" of the capacitor will reduce the impedance seen by the x-over and compensate for the corresponding rise in "L" of the coil.
That will flatten out the curve above the resonance to make it more or less purely resistive.......
But we both wait eagerly for the effect of the LCR at resonance......
(BTW I hope you saw my subsequent post on that multi-way speaker thread we were both on the other day. My rant was not aimed at you at all but largely triggered by the patronizing attitude of one of our old "Colonial Masters" who......Anyway I won't go on ha ha.)

Jonathan
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Last edited by Jonathan Bright; 27th December 2012 at 12:09 AM.
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Old 27th December 2012, 12:19 AM   #9
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Hi john k....
The flattening of the impedance curve is not the question...
We (me and fellow Aussie Moondog55) are both are interested in the effect of the LCR network on dome excursion and power handling.
That is the point. All the LRC does is flatten Z. It has no effect on excursion at all. It just makes the crossover filter design a little easier. For example, if you have two filter designs that yield an acoustic LR4 high pass response at 2k and one design has an LRC network and the other doesn't, they will still both have the same excursion and power handling. All that matters is the final acoustic response.

Now there may be cases (like a single series cap crossover) where it is not possible to achieve the same acoustic output with and without the LRC. That is a different issue which is the result of how the cap reacts to the Z it sees since the cap in series with a flattened Z will yield a different acoustic output than a cap in series with the tweeter natural Z with impedance peak.
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Old 27th December 2012, 12:21 AM   #10
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Do go on; I enjoy the verbal tussle LOL
I have a couple of pairs of that D25NC left; great tweeter even if routing out the hole is a little hard.
I guess the LCR parallel network becomes more important the lower you cross?

Sometimes you can be totally right in the totally wrong context
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