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!
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!
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 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
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.
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.
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.
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
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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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.
Hello Australia,
I think the statement in the vifa sheet simply means:
If we assume a Vifa D25AG-05-06 (Fs=1.5kHz) with a simple first order crossover (C=6.8uF), the resulting driver voltage is:
We get a 4dB hump at the resonance frequency. And this means 4dB more excursion and 4dB more power in comparison to a filter with LCR added.
Of course John K is right, the hump is also visible in the acoustic response. In practice a notch is often not necessary, because ferrofluid, L-pad and 2nd or 3rd order filters serve the same purpose.
I think the statement in the vifa sheet simply means:
If we assume a Vifa D25AG-05-06 (Fs=1.5kHz) with a simple first order crossover (C=6.8uF), the resulting driver voltage is:
We get a 4dB hump at the resonance frequency. And this means 4dB more excursion and 4dB more power in comparison to a filter with LCR added.
Of course John K is right, the hump is also visible in the acoustic response. In practice a notch is often not necessary, because ferrofluid, L-pad and 2nd or 3rd order filters serve the same purpose.
Oooooooooooooooookaaaaaaaaaaaay!
Let's try just once more.
The quote in the second paragraph of the first post is lifted straight from the Vifa application notes. I know typos can creep in so I read it several times so that I reproduced exactly what they said. Now as far as I can tell (i) no one has disputed that the LCR network flattens the impedance. (ii) I think we all agree that this allows for a more or less theoretically accurate termination for the preceding cross over.
It is THEIR claim that it also reduces the dome excursion and increases power handling that I am curious about.
Now my first thoughts were the same as john k's; i.e. I didn't see how that would occur but Vifa make the unit (and many others) and they do it for a living and so I did give them the benefit of the doubt that they might know something that I didn't.........and I do that with most people as it is almost always true, ha ha...
So anyone want to address the original question pleeeeeeeeeeeeeeease?
Either there (i) is an answer based in physics (ii) there is a typo in their own notes or (iii) Vifa don't know what they are talking about......
Cheers Jonathan
Let's try just once more.
The quote in the second paragraph of the first post is lifted straight from the Vifa application notes. I know typos can creep in so I read it several times so that I reproduced exactly what they said. Now as far as I can tell (i) no one has disputed that the LCR network flattens the impedance. (ii) I think we all agree that this allows for a more or less theoretically accurate termination for the preceding cross over.
It is THEIR claim that it also reduces the dome excursion and increases power handling that I am curious about.
Now my first thoughts were the same as john k's; i.e. I didn't see how that would occur but Vifa make the unit (and many others) and they do it for a living and so I did give them the benefit of the doubt that they might know something that I didn't.........and I do that with most people as it is almost always true, ha ha...
So anyone want to address the original question pleeeeeeeeeeeeeeease?
Either there (i) is an answer based in physics (ii) there is a typo in their own notes or (iii) Vifa don't know what they are talking about......
Cheers Jonathan
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http://www.d-s-t.com.au/data/Vifa/D26NC-15-06.pdf
Using a .22mH coil from Jaycar 0.8mm wire( 20AWG) whose nominal DCR is 0.3 Ohms seems reasonable as the DCR of the tweeter is nominally 4.6 so about 6.5%
What happens when you use a coil with bigger wire/lower DCR such as the Jantzen 16AWG ( 1.29mm) at 0.16ohms resistance??
Using a .22mH coil from Jaycar 0.8mm wire( 20AWG) whose nominal DCR is 0.3 Ohms seems reasonable as the DCR of the tweeter is nominally 4.6 so about 6.5%
What happens when you use a coil with bigger wire/lower DCR such as the Jantzen 16AWG ( 1.29mm) at 0.16ohms resistance??
Yup. That was one my thoughts. I have a lot of data sheets from 20 yrs ago and they seem to like relatively big coils across the tweeters. (I mean keeping the same time constant but reducing the capacitor in proportion. If that is policy there must be a theoretical basis for it as it would be more expensive.) I don't think we can completely discount a linguistic issue. If you make their first sentence run from the present beginning up to the word "excursion" I wouldn't have thought much more about it. But then that would still leave their assertion that a compensation network is essential for high power use.
And the whole flattening of the impedance curve is not THAT crucial with second order series networks and that is what they illustrate, give values for and recommend as a minimum....
Yours, Jonathan
And the whole flattening of the impedance curve is not THAT crucial with second order series networks and that is what they illustrate, give values for and recommend as a minimum....
Yours, Jonathan
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If you have a high Q tweeter and a low order crossover you van get far more than a 4 dB hump. I have seen were the crossover has been at 6-8 kHz 6 dB/octave and were the 1 kHz resonance of the dome has resulted in an output not far from the level of the output above the crossover not supressed 12-18 dB as it should be.
Hi Johnathan, I think that Dissi did just that
what he is saying is that it is not the LCR circuit that gives less excursion itself, it is the fact that the crossover works as it should (because there isn't a big impeadance bump stuffing it up) that reduces the excursion!! ie without the LCR the crossover is ineffective around the resonance frequency causing higher excursion than you want. With the LCR the crossover works as intended and you get more attenuation at resonance and hence less excursion.
Tony.
Thanks DrBoar, I can see how the conjugate net work aids the response with low order networks but I am still not sure we have addressed why such an LCR network would increase power handling and reduces excursion and that seems to be what Vifa are claiming.
What we really want is someone au fait with network analysis.....and as yet they haven't surfaced......
What we really want is someone au fait with network analysis.....and as yet they haven't surfaced......
Hi Tony, sorry our posts crossed. I saw Dissi's post and I understand it. I didn't include all the material from the Vifa App Note. They (Vifa) won't even countenance a first order network for this unit. And I think most posters here are aware that second order networks (at least in series configuration) are largely immune from interactions with the res' peak. I should have posted the whole section so as to avoid the distraction of 1st order cross over// res' peak effects.
You may be right. I'll have a longer think. Just never did enough work on resonance networks..... I sort of feel that the LCR components must be shunting power away from the driver at that frequency for Vifa to insist they be included for high power usage. I mean if they are in parallel and if they end up producing a flat response then at resonance the network must offer a substantially lower impedance and so reducing the power to the terminals.....
The accompanying graphs show an 8 ohm peak with a DC resistance of less than 6. Rough calculations mean that the LCR series resistance must be about 3 ohms to get that sort of result.......if that is the case then it would seriously increase power handling and reduce excursion......and their comments would then make sense. Capeche?
Jonathan
You may be right. I'll have a longer think. Just never did enough work on resonance networks..... I sort of feel that the LCR components must be shunting power away from the driver at that frequency for Vifa to insist they be included for high power usage. I mean if they are in parallel and if they end up producing a flat response then at resonance the network must offer a substantially lower impedance and so reducing the power to the terminals.....
The accompanying graphs show an 8 ohm peak with a DC resistance of less than 6. Rough calculations mean that the LCR series resistance must be about 3 ohms to get that sort of result.......if that is the case then it would seriously increase power handling and reduce excursion......and their comments would then make sense. Capeche?
Jonathan
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I personally find the whole "it only flattens the impedance, but does nothing for the power handling" thing counter intuitive...
I really haven't gone into it at an electro mechanical level, but just in laymans terms I envision it (most probably incorrectly) as the LCR network effectively bypassing the tweeter at the resonance frequency (or at the very least shunting off a certain proportion of the signal to earth at the resonant frequency), which I would have thought would reduce the output of the tweeter at that frequency.
I'd be very interested to see the spl graph of a tweeter with only an LCR filter compared to just the raw driver.
Certainly it should be easy enough to simulate...
Tony.
I really haven't gone into it at an electro mechanical level, but just in laymans terms I envision it (most probably incorrectly) as the LCR network effectively bypassing the tweeter at the resonance frequency (or at the very least shunting off a certain proportion of the signal to earth at the resonant frequency), which I would have thought would reduce the output of the tweeter at that frequency.
I'd be very interested to see the spl graph of a tweeter with only an LCR filter compared to just the raw driver.
Certainly it should be easy enough to simulate...
Tony.
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