Hey Guys,
Im new to much of this and im just trying to get my head around all of these new concepts.
As i understand it, Linkwitz crossovers are a great active filter to split a signal into high/mid/bass. The result of the crossover being flat with no peaks/dips or phase reversals.
Specifically for subs, a linkwitz transform can be used as a bass boost to increase the amplitude of low frequencies? So you might be able to extend flat response down another 10Hz or so before rolling off?
Is my current mode of thinking correct? I have noted that the transform and the crossover are different circuits and perform different tasks. Im just trying to picture how it might fit together. Is it something like:
For a sub, you would use a low pass on the input signal, linkwitz or otherwise, and then use the linkwitz transform to extend the gain at lower frequencies if desired. All of this occuring before the power amplification stage....
The advantage of this being, you would be able to reduce distortion by putting the speaker into a larger box but also maintain the same or better amplification at low frequencies by applying the transform?
Cheers all
Im new to much of this and im just trying to get my head around all of these new concepts.
As i understand it, Linkwitz crossovers are a great active filter to split a signal into high/mid/bass. The result of the crossover being flat with no peaks/dips or phase reversals.
Specifically for subs, a linkwitz transform can be used as a bass boost to increase the amplitude of low frequencies? So you might be able to extend flat response down another 10Hz or so before rolling off?
Is my current mode of thinking correct? I have noted that the transform and the crossover are different circuits and perform different tasks. Im just trying to picture how it might fit together. Is it something like:
For a sub, you would use a low pass on the input signal, linkwitz or otherwise, and then use the linkwitz transform to extend the gain at lower frequencies if desired. All of this occuring before the power amplification stage....
The advantage of this being, you would be able to reduce distortion by putting the speaker into a larger box but also maintain the same or better amplification at low frequencies by applying the transform?
Cheers all
Guy Incognito said:
The irony of it is that you will probably want to put the driver in a smaller box, not a larger one.
1) LT ccts require a decrease in system Q to allow for a decrease in Fb, therefore it is desireable to start with a higher Q than what you want to end up with.
2) it is EXTREMELY desireable to size the box such that the power applied is incapable of bottoming the driver(s)... keep in mind that a LT cct will mean high levels of power and excursion are being applied to the sub. It is more desireable to have an amp clip than a driver bottom, IMHO.
The sequence I went thru when building the system i built?
1) decide on driver and amp
2) make sure driver(s) can handle power of amp (thermal), revisit point 1 as required
3) size box such that Q can be reduced to allow LT cct to work.
4) reduce size of box to limit excursion to less than Xmax at full power.
If design meets the above parameters, then you can calculate an LT for it.
The last point I want to make is that there is no greater "efficency" to be had by using a LT cct. In fact, overall, a LT makes the overall sub less efficient, if you think about it.
e.g. If you have a sealed sub which is capable of 110dbspl at 30hz (full power or full excursion) and you apply sufficient 'LT" to get flat extension to 15hz, but at the cost of db gain in the LT, what you've really done is reduce the overall gain allowable on the sub to 100db at 30hz. You cannot apply the same drive levels to the sub that would give you 110 db spl at 30hz, at 15hz, or either your amp will clip or you driver will bottom (depending on how you've design the sub system)
Still... these LT-based systems have their place. The system i built used a set of 4 tempest classic drivers and a LT-capable ADA-1200 amp. The system is 95db spl 1w/1meter, and is capable of 126 db spl at full power above rolloff. I "threw away" about 14 db of that with a rather agressive LT. But the system is still capable of 110-112 db (ignoring room gain) at 16hz.
So if your initial efficency is high enough, or spl requirements are low enough, an LT can work well for you.
GB
I'd like to add a couple of things.
The Linkwitz Transform uses zeros to cancel out the "natural" poles of the driver/box combination. It can do this whatever their Fc or Q-value is.
(Though, I recognise that you did say "LT ccts", which might mean that the specific circuit implementations that are floating around impose this limitation. I've only ever implemented my LTs digitally, so I don't know about that. My point is, the principle doesn't require the initial Q to be higher than the desired Q.)
Well, that depends on how you define efficiency. It's not going to change the power-in/SPL-out ratio of the driver at any frequency, so I would argue that efficiency is unchanged. Having large amounts of gain at LF will just limit the maximum input you're able to give the driver at higher frequencies, so limiting your total system SPL.
That should work. Using the larger box would mean you need less equalisation, reducing power input to the driver and so reducing distortion a little bit. However, I agree with Greybeard1 - this isn't the most common use of the LT. Most people employ it as a way to allow the use of a smaller-than-ideal box for a given driver - for cosmetic or logistic reasons, and/or to flatten a driver that would not reach a low enough cutoff in any box size.
I use the LT to equalise my woofers (2x8" per cabinet) flat to 20Hz in a box that's slightly too small (Q~0.85, IIRC). They also wouldn't reach 20Hz even if the box was dimensioned correctly. I ended up with around 15dB of gain at LF, which definitely needs bearing in mind. However, it works nicely and sounds great.
The Linkwitz Transform uses zeros to cancel out the "natural" poles of the driver/box combination. It can do this whatever their Fc or Q-value is.
(Though, I recognise that you did say "LT ccts", which might mean that the specific circuit implementations that are floating around impose this limitation. I've only ever implemented my LTs digitally, so I don't know about that. My point is, the principle doesn't require the initial Q to be higher than the desired Q.)
Well, that depends on how you define efficiency. It's not going to change the power-in/SPL-out ratio of the driver at any frequency, so I would argue that efficiency is unchanged. Having large amounts of gain at LF will just limit the maximum input you're able to give the driver at higher frequencies, so limiting your total system SPL.
That should work. Using the larger box would mean you need less equalisation, reducing power input to the driver and so reducing distortion a little bit. However, I agree with Greybeard1 - this isn't the most common use of the LT. Most people employ it as a way to allow the use of a smaller-than-ideal box for a given driver - for cosmetic or logistic reasons, and/or to flatten a driver that would not reach a low enough cutoff in any box size.
I use the LT to equalise my woofers (2x8" per cabinet) flat to 20Hz in a box that's slightly too small (Q~0.85, IIRC). They also wouldn't reach 20Hz even if the box was dimensioned correctly. I ended up with around 15dB of gain at LF, which definitely needs bearing in mind. However, it works nicely and sounds great.
Thanks guys!
The amount of knowledge floating around is amazing. Its great to be able to have access to it.
I must admit, as i have read more about it, i have become more drawn to the idea. Primarily because i do want to make a sub with a relatively small enclosure.
Ive been torn between two peerless speakers - the 10" (830843) and the 12" (830845). The application notes recommend a 62L box with 65% fill for both drivers. This is okay but i would prefer something a wee bit smaller.
As this is my first project its unlikely i will be jumping into equalising the input, i want to concentrate on building a good sealed sub. However, its becoming very tempting to at least leave room in the design for adding this as an option later.
Im going to put pen to paper and see what comes up. I know its all trade offs but i never liked making them!
Any recommendations or comments are welcome,
Thanks again
The amount of knowledge floating around is amazing. Its great to be able to have access to it.
I must admit, as i have read more about it, i have become more drawn to the idea. Primarily because i do want to make a sub with a relatively small enclosure.
Ive been torn between two peerless speakers - the 10" (830843) and the 12" (830845). The application notes recommend a 62L box with 65% fill for both drivers. This is okay but i would prefer something a wee bit smaller.
As this is my first project its unlikely i will be jumping into equalising the input, i want to concentrate on building a good sealed sub. However, its becoming very tempting to at least leave room in the design for adding this as an option later.
Im going to put pen to paper and see what comes up. I know its all trade offs but i never liked making them!
Any recommendations or comments are welcome,
Thanks again
Wingfeather said:
I'm not 100% sure about this...
There was a post over at the home theater forum where a fellow named Dave Alan posted the following, which was copied from a link at the Rythmic Audio site about their LT cct... the link no longer works, so I copied out the post... the part in quotation marks are from the Rythmic Audio site, the rest is Dave's comments.
Anyway... I think my original comment is wrong. You don't have to force Q lower to lower Fs. But based on the quote above, changes in system Q and Fx are related.
GB
Hi Greybeard,
I'm 99% certain the person who wrote that has simply misunderstood the meaning of the maths behind the LT.
---
A quick overview for anyone who hasn't seen it before (I apologise for the mathsey nature of this, and also for how horribly hard it is to read - if anyone knows a good way of doing equations in text form I'd be very interested!):
The driver in a sealed box has a complex conjugate pair of poles determining the frequency and Q of it's LF rolloff, shown as the denominator in the following equation (the numerator is just 1 for now, so I'm ignoring the driver/box combo's zeros - but they aren't important for this stuff anyway):
H(s)driver = 1 / (s^2 + (Wc/Q)s + Wc^2)
Where Wc is the cutoff frequency and Q is, well, the Q.
The Linkwitz Transform introduces an electronic filter with a numerator that matches the denominator of the driver/box, and a denominator with the Fc and Q that we actually want, like so:
H(s)filter = (s^2 + (Wc/Q)s + Wc^2) / (s^2 + (Wc'/Q')s + Wc'^2)
Where Wc' and Q' are the desired system parameters.
When you cascade this electronic filter with the driver/box combination you're multiplying the two transfer functions together. The denominator of the speaker and the numerator of the filter cancel out entirely, leaving simply:
H(s)system = 1 / (s^2 + (Wc'/Q')s + Wc'^2)
You can see that you can choose Wc' and Q' to be whatever you want - there's no interdependence between them at all. Moreover, the parameters of the original driver and box aren't even present anymore, so they don't contribute at all.
What I'm wondering is if the mentioned interdependence is some kind of limitation with a common implementation of the LT. It's been a while since I designed my last Sallen Key filter so I can't say offhand...
(damn typos...)
I'm 99% certain the person who wrote that has simply misunderstood the meaning of the maths behind the LT.
---
A quick overview for anyone who hasn't seen it before (I apologise for the mathsey nature of this, and also for how horribly hard it is to read - if anyone knows a good way of doing equations in text form I'd be very interested!):
The driver in a sealed box has a complex conjugate pair of poles determining the frequency and Q of it's LF rolloff, shown as the denominator in the following equation (the numerator is just 1 for now, so I'm ignoring the driver/box combo's zeros - but they aren't important for this stuff anyway):
H(s)driver = 1 / (s^2 + (Wc/Q)s + Wc^2)
Where Wc is the cutoff frequency and Q is, well, the Q.
The Linkwitz Transform introduces an electronic filter with a numerator that matches the denominator of the driver/box, and a denominator with the Fc and Q that we actually want, like so:
H(s)filter = (s^2 + (Wc/Q)s + Wc^2) / (s^2 + (Wc'/Q')s + Wc'^2)
Where Wc' and Q' are the desired system parameters.
When you cascade this electronic filter with the driver/box combination you're multiplying the two transfer functions together. The denominator of the speaker and the numerator of the filter cancel out entirely, leaving simply:
H(s)system = 1 / (s^2 + (Wc'/Q')s + Wc'^2)
You can see that you can choose Wc' and Q' to be whatever you want - there's no interdependence between them at all. Moreover, the parameters of the original driver and box aren't even present anymore, so they don't contribute at all.
What I'm wondering is if the mentioned interdependence is some kind of limitation with a common implementation of the LT. It's been a while since I designed my last Sallen Key filter so I can't say offhand...
(damn typos...)
Well, yes and no, I'd say.
Playing a sine wave at x Hz and y dBSPL out of a given driver/box system is going to require a certain amount of power and produce a certain amount of distortion. Whether you get up to y dBSPL at x Hz because of the LT or because you simply turned up the volume doesn't really matter.
Then again, if you're designing your system to use an LT from the outset (which you should be, really) then you may go with too small a box, reducing natural efficiency down low and thus requiring more power to actually reach y dBSPL at x Hz. In this case, the total system will probably have more distortion at LF than one without the LT, in a larger box.
The LT in itself is a low-level electronic filter (or a DSP filter) and isn't going to contribute very much (if indeed any) distortion of its own. The main worries with using one are either that input noise gets boosted a lot at LF or that you might end up with enough gain at the bottom end to clip things when playing back bass-heavy material.
Playing a sine wave at x Hz and y dBSPL out of a given driver/box system is going to require a certain amount of power and produce a certain amount of distortion. Whether you get up to y dBSPL at x Hz because of the LT or because you simply turned up the volume doesn't really matter.
Then again, if you're designing your system to use an LT from the outset (which you should be, really) then you may go with too small a box, reducing natural efficiency down low and thus requiring more power to actually reach y dBSPL at x Hz. In this case, the total system will probably have more distortion at LF than one without the LT, in a larger box.
The LT in itself is a low-level electronic filter (or a DSP filter) and isn't going to contribute very much (if indeed any) distortion of its own. The main worries with using one are either that input noise gets boosted a lot at LF or that you might end up with enough gain at the bottom end to clip things when playing back bass-heavy material.
Wingfeather,
I think you described it pretty well and linkwitz sums it up in a similar fashion:
from http://www.linkwitzlab.com/filters.htm#9
So it seems you can place the poles either side of Fs (as already pointed out). I couldnt see any interdependence other than the restraints of equaliser voltage clipping and Xmax.
I know you can model excursion in CAD software but do you guys know the maths to do? Ive googled a fair bit and havent found much information. Its it a hard property to model?
I would like to model power consumption and excursion with an LT in a spreadsheet. So far ive only been able to find general relations:
For ever octave lower you need four times the excursion and sixteen times the power.....
cheers all
I think you described it pretty well and linkwitz sums it up in a similar fashion:
from http://www.linkwitzlab.com/filters.htm#9
So it seems you can place the poles either side of Fs (as already pointed out). I couldnt see any interdependence other than the restraints of equaliser voltage clipping and Xmax.
I know you can model excursion in CAD software but do you guys know the maths to do? Ive googled a fair bit and havent found much information. Its it a hard property to model?
I would like to model power consumption and excursion with an LT in a spreadsheet. So far ive only been able to find general relations:
For ever octave lower you need four times the excursion and sixteen times the power.....
cheers all
I'm afraid I don't know the maths of it offhand. I'm pretty sure it's based on the T/S params of the drivers - from these you should be able to derive various aspects of performance. Presumably, the method is detailed in an AES paper from a while back. I'll have a look when I get the chance to see if I have it (I have access to a reasonably complete archive).
Alternatively, if the writers of such software as WinISD hang around here I'm sure they can give you far more information than me!
Alternatively, if the writers of such software as WinISD hang around here I'm sure they can give you far more information than me!
Fellas,
Go here http://www.linkwitzlab.com/thor_splmax.htm and download SL's spreadsheet http://www.linkwitzlab.com/closed-box1.xls and you can analyze the full system. This will allow you to determine excursion, voltage, and current limitations at different frequencies for any driver/box/power-amp combination.
Cheers,
Dave.
Go here http://www.linkwitzlab.com/thor_splmax.htm and download SL's spreadsheet http://www.linkwitzlab.com/closed-box1.xls and you can analyze the full system. This will allow you to determine excursion, voltage, and current limitations at different frequencies for any driver/box/power-amp combination.
Cheers,
Dave.
Thanks guys,
I gather you can build a model of the system using an equivalent circuit and use the T/S parameters to conjure up various performance curves of the driver. Dark magic!
Ive tried to reverse engineer the Linkwitz spreadsheet but its too hard to decompose and understand in detail, without another reference.
I'll stick to winISD/Linkwitz spreadsheet for now!
Any winISD authors in town
I gather you can build a model of the system using an equivalent circuit and use the T/S parameters to conjure up various performance curves of the driver. Dark magic!
Ive tried to reverse engineer the Linkwitz spreadsheet but its too hard to decompose and understand in detail, without another reference.
I'll stick to winISD/Linkwitz spreadsheet for now!
Any winISD authors in town
Ping for
richie00boy.http://www.readresearch.co.uk/
He's full of knowledge on LT. Also see Rod here
sound.westhost.com
He's helpful.
iUSERTLO72p
richie00boy.http://www.readresearch.co.uk/
He's full of knowledge on LT. Also see Rod here
sound.westhost.com
He's helpful.
iUSERTLO72p
Hey guys,
As i mentioned earlier im basically torn between two peerless speakers - the 830843 (10") and the 830845 (12"). I want a sub that will work well as a stand alone sealed enclosure. I also want to leave room in the design to flatten and extend the response down a few Hz.
Currently i have the 10" in a 55L enclosure (I came to this value using spread sheet maths but winISD suggests 59 for Qtc = 0.707 - which should i go by?) and the 12" in a 62L enclosure - as suggested by the data sheet (110L for Qtc = 0.707 is too big for me!)
Im happy with these sizes and am not looking to miniaturise the enclosures but rather to improve the response. I figure i can flatten each frequency response curve down to ~30Hz in each of these enclosures.
My DC gains seem quite high. Have any of you guys designed a Linkwitz transform by using winISD?? What should i look out for?
Cheers
As i mentioned earlier im basically torn between two peerless speakers - the 830843 (10") and the 830845 (12"). I want a sub that will work well as a stand alone sealed enclosure. I also want to leave room in the design to flatten and extend the response down a few Hz.
Currently i have the 10" in a 55L enclosure (I came to this value using spread sheet maths but winISD suggests 59 for Qtc = 0.707 - which should i go by?) and the 12" in a 62L enclosure - as suggested by the data sheet (110L for Qtc = 0.707 is too big for me!)
Im happy with these sizes and am not looking to miniaturise the enclosures but rather to improve the response. I figure i can flatten each frequency response curve down to ~30Hz in each of these enclosures.
My DC gains seem quite high. Have any of you guys designed a Linkwitz transform by using winISD?? What should i look out for?
Cheers
Guy,
I've used WinISD for this before - it's quite excellent IMO.
The only thing to remember with it is there's a bug regarding the LT and efficiency - WinISD will lie to you about the power required to get a certain SPL. If you want to check real efficiency levels then simple switch of the LT and see how much power it takes to reach a certain SPL at a certain frequency - this figure isn't going to change when you add the LT in in the real world.
The thing to watch out for with the LT in general is just whether you're ending up with silly amounts of LF gain. I don't think there's an established maximum or anything but if you end up with more than, say, about 20dB of gain you might have trouble with amplifying DC offsets in the system and, also, input noise. I myself am using something near this amount of gain though and am experiencing no problems - so maybe it's safe to go higher. However, I'm doing mine digitally, from a digital source, and so there's no DC offset to amplify - this isn't going to be the case in an analogue system.
Just exercise restraint, I think. Perhaps integrating a very-low-Fc highpass filter in to block DC might be a good idea? Someone else might have an opinion.
As for the two sizes (55L and 59L) - those are so close to being the same I don't think it's worth worrying. You might as well err on the side of larger just to account for the volume the driver will take up when it's installed in the box.
I've used WinISD for this before - it's quite excellent IMO.
The only thing to remember with it is there's a bug regarding the LT and efficiency - WinISD will lie to you about the power required to get a certain SPL. If you want to check real efficiency levels then simple switch of the LT and see how much power it takes to reach a certain SPL at a certain frequency - this figure isn't going to change when you add the LT in in the real world.
The thing to watch out for with the LT in general is just whether you're ending up with silly amounts of LF gain. I don't think there's an established maximum or anything but if you end up with more than, say, about 20dB of gain you might have trouble with amplifying DC offsets in the system and, also, input noise. I myself am using something near this amount of gain though and am experiencing no problems - so maybe it's safe to go higher. However, I'm doing mine digitally, from a digital source, and so there's no DC offset to amplify - this isn't going to be the case in an analogue system.
Just exercise restraint, I think. Perhaps integrating a very-low-Fc highpass filter in to block DC might be a good idea? Someone else might have an opinion.
As for the two sizes (55L and 59L) - those are so close to being the same I don't think it's worth worrying. You might as well err on the side of larger just to account for the volume the driver will take up when it's installed in the box.
Wingfeather,
As always, thank you for your support. Sorry about the delay. The holiday period always gets pretty hectic.
Your right about the volumes, i think i will err on the higher side of things.
Im yet to select a good amp available in Aus. I would prefer not order from overseas so i might appeal to some of my aussie brethren and see what they know?
Cheers,
Guy
As always, thank you for your support. Sorry about the delay. The holiday period always gets pretty hectic.
Your right about the volumes, i think i will err on the higher side of things.
Im yet to select a good amp available in Aus. I would prefer not order from overseas so i might appeal to some of my aussie brethren and see what they know?
Cheers,
Guy
Guy Incognito said:
I think I posted this before, but with an LT cct it is wise in the extreme to size your box such that whatever maximum power you intend to apply (LT or not) is incapable of bottoming or melting you driver.
1) choose amp/power below thermal max of driver
2) downsize box such that driver will not bottom... you can actually ignore the LT at this point. If the box is small enough (or power low enough) such that driver cannot be bottomed at max power, then the LT will not be able to bottom it either... amp might clip, but this is better than driver bottoming.
3) after 1 and 2, fiddle with LT to get system Q/extension back to where you want them.
GB
This thread is a little old so I don't know if this will help you at this point, but the LRT crossover if designed correctly will drive your speaker to the max excursion it is capable of at lower frequencies than the sealed box (usually smaller than desired) would enable. (if designed correctly)
This has been stated before and is basic to the intent, but elaborated means that no matter what box you put the speaker in it will not produce those frequencies at a higher SPL without distortion or possible bottoming. (Contrary to what has been stated earlier in this thread) Your trade off? You cannot drive the speaker to higher frequencies. A 40Hz 12db rolloff is good.
Why? The power requirement become outrageous. Without a LP filter you would need 1000's of watts to drive higher than 80Hz at the same SPL. I.E. it would be power limited and rolloff anyway...
Bottom line: If you want 120dB at 15Hz you need more than one speaker or ridiculous Xmax and power (you still won't get 120dB unassisted). The Xmax compromise is in efficiency and the power required is usually enough to pull down the mains voltage. So keep it in mind when designing.
This has been stated before and is basic to the intent, but elaborated means that no matter what box you put the speaker in it will not produce those frequencies at a higher SPL without distortion or possible bottoming. (Contrary to what has been stated earlier in this thread) Your trade off? You cannot drive the speaker to higher frequencies. A 40Hz 12db rolloff is good.
Why? The power requirement become outrageous. Without a LP filter you would need 1000's of watts to drive higher than 80Hz at the same SPL. I.E. it would be power limited and rolloff anyway...
Bottom line: If you want 120dB at 15Hz you need more than one speaker or ridiculous Xmax and power (you still won't get 120dB unassisted). The Xmax compromise is in efficiency and the power required is usually enough to pull down the mains voltage. So keep it in mind when designing.
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