Hmm, is this an application of the other Lipshitz/Vanderkooy paper, not the one on how to make a subtractive LR4?
Looks like it can readily be applied to the Manger or any other driver with sufficient bandwidth, one just needs to take care of the geometrical offset by adding a constant delay to one output.
What bumps? The Manger bump of 250 Hz or so? Couldn't this be dealt with eiter before the XO or after the HP output?
Regards,
Eric
Looks like it can readily be applied to the Manger or any other driver with sufficient bandwidth, one just needs to take care of the geometrical offset by adding a constant delay to one output.
What bumps? The Manger bump of 250 Hz or so? Couldn't this be dealt with eiter before the XO or after the HP output?
Regards,
Eric
All transient-perfect crossovers with at least one branch having a filter-order of two or higher will have:
- some overlap in the crossover area
- a behaviour like a 1st order crossover in the crossover area
- have a hump in one branch or both
I was refering to the third point. If you have a look at the graph within the AN in question, you can clearly see those humps.
Regards
Charles
Something for the FR guys
There are always discussions about taking the driver's response into account when designing crossovers.
This can be done with subtractive crossovers as well of course but one has to rely on drivers that are suitable for 6dB filters.
One possible proposal you can see below. The task was to extend a FR driver's response at it's lower end.
The FR driver is high-passed with a first order filter. If it is mounted in a closed box this will amount to a total transfer function of third order (in an open baffle this might be exchanged, i.e. 1st order for the driver and 2nd for the crossover filter).
Another highpass following the actual FR crossover emulates the acoustical behaviour of the driver.
This signal is now subtracted from the input signal, leaving a 1st order lowpassed signal which can be fed to the woofer.
There is no doubt that the woofer has to be usable up to frequencies significantly higher than the crossover frequency (as already mentioned).
Another drawback is the polar response of the arrangement. This can be dealt with by the use of crossover frequencies placed at large wavelengths (compared to the driver sizes, which is quite often the case when FR drivers are used) or d'Appolito configurations (or both !).
It is still not an ideal crossover (which doesn't exist in practice anyway) but it is one possible solution for a transient optimised crossover that takes a driver's response into account.
Regards
Charles
P.S: I also did the maths for symmetrical subtractive crossover and will post it soon.
There are always discussions about taking the driver's response into account when designing crossovers.
This can be done with subtractive crossovers as well of course but one has to rely on drivers that are suitable for 6dB filters.
One possible proposal you can see below. The task was to extend a FR driver's response at it's lower end.
The FR driver is high-passed with a first order filter. If it is mounted in a closed box this will amount to a total transfer function of third order (in an open baffle this might be exchanged, i.e. 1st order for the driver and 2nd for the crossover filter).
Another highpass following the actual FR crossover emulates the acoustical behaviour of the driver.
This signal is now subtracted from the input signal, leaving a 1st order lowpassed signal which can be fed to the woofer.
There is no doubt that the woofer has to be usable up to frequencies significantly higher than the crossover frequency (as already mentioned).
Another drawback is the polar response of the arrangement. This can be dealt with by the use of crossover frequencies placed at large wavelengths (compared to the driver sizes, which is quite often the case when FR drivers are used) or d'Appolito configurations (or both !).
It is still not an ideal crossover (which doesn't exist in practice anyway) but it is one possible solution for a transient optimised crossover that takes a driver's response into account.
Regards
Charles
P.S: I also did the maths for symmetrical subtractive crossover and will post it soon.
Attachments
Hi Charles,
I've got a pair of Manger high in my shelf for a long time. And I'm having a similar crossover idea in my mind recently. Glad to see someone who had already done so much on the same topic.
My approach is a bit different. I'd like the high-pass for Manger to be passive, following the HP filter suggested in the Manger Zerobox 107.
The actual acoustic HP response of the Manger (with filter) will be measured by an near field MLS. And then, an active high pass filter, base on the measured HP response, will be built for the subtraction.
I cannot do the Math, but have done a little spice simulation.
Click here in case the pic didn't show up.
My simulation tells me, say the Manger with filter roll-off at 150Hz 3rd order, then the woofer would start to roll-off at frequency as high as about 500Hz 1st order. I'm not sure my SS8555 is capable for this.
Any comment?
Please let me know if you have any progess. Thanks in advance.
henry
I've got a pair of Manger high in my shelf for a long time. And I'm having a similar crossover idea in my mind recently. Glad to see someone who had already done so much on the same topic.
My approach is a bit different. I'd like the high-pass for Manger to be passive, following the HP filter suggested in the Manger Zerobox 107.
The actual acoustic HP response of the Manger (with filter) will be measured by an near field MLS. And then, an active high pass filter, base on the measured HP response, will be built for the subtraction.
I cannot do the Math, but have done a little spice simulation.
An externally hosted image should be here but it was not working when we last tested it.
An externally hosted image should be here but it was not working when we last tested it.
Click here in case the pic didn't show up.
My simulation tells me, say the Manger with filter roll-off at 150Hz 3rd order, then the woofer would start to roll-off at frequency as high as about 500Hz 1st order. I'm not sure my SS8555 is capable for this.
Any comment?
Please let me know if you have any progess. Thanks in advance.
henry
Hi Henry,
why would You want the high-pass to be passive and furthermore AFTER the power-amp?
IMO the same could be achieved on line-level with just an INPUT coupling cap before the power-amp. It`s not only cheaper (big value quality caps tend to be expensive) but easier and finally the better solution (IMO).
The idea, according Charles schema in his last post, to built a circuit simulating the acoustic high-pass response (as You want to do also) and to subtract it to derive the woofer-signal is very elegant.
But it get`s even simpler than this.
When You don`t connect the HP-output of this filter at all (power-amp. driven from a buffer or preamp circuit directly) but put a coupling cap of appropriate value in front of Your power-amp. as mentioned above You`d have no active crossover circuity in the high-pass branch nor passive crossover parts after the amp. - it just don`t get any simpler.
In my opinion the subtractive filter, used to be nice for a PA application, you can alter the frequency of a 2nd order symmetrical 2-way crossover, with one (easy available) stereo potentiometer.....
However:
You'll never get rid of the phase assymetry, the reason why you'll get a "hump" in the response of the subtracted output....
A compensating allpass network is a half-way solution: it doesnt change with frequency so it will only work around a certain point (very close to the crossoverpoint for instance)...
if you want to make a correct symmetrical filter dont make a subtractive one, just make both the highpass and the lowpass.
The only thing I can think of, to effectively use it is in an application where you want to be able change the crossfreq. a bit.
Then its easier to change the resistors instead of the caps, and then you make the opposite filter with a subtractornetwork...
However:
You'll never get rid of the phase assymetry, the reason why you'll get a "hump" in the response of the subtracted output....
A compensating allpass network is a half-way solution: it doesnt change with frequency so it will only work around a certain point (very close to the crossoverpoint for instance)...
if you want to make a correct symmetrical filter dont make a subtractive one, just make both the highpass and the lowpass.
The only thing I can think of, to effectively use it is in an application where you want to be able change the crossfreq. a bit.
Then its easier to change the resistors instead of the caps, and then you make the opposite filter with a subtractornetwork...
and what if we don`t want to achieve a "correct" symetrical filter (nor is easy adjustability the main issue here) but rather as much as possible transient-perfect crossover?
I do care less (usually - of course it depends of what is the goal) about interdriver phase assymetry as I do for overall transient system response and in this field a subtractive filter is potentially superior (at least for filters steeper than I.-order in one or more branches) .
Hi Christoph,
I want to have the high-pass filter after the poweramp because I'm currently using a passive preamp, without any buffer. Having the filter after the poweramp will also protect my precious Manger from any possible DC failure and my Goldmund SRA is a direct couple poweramp.
I agree doing the high-pass in low level siganl is a much easier and cheaper way. Maybe I'll make both and see which one I like, cause even an OPA627 buffer is cheap compare with the cost of Manger.
Henry
I want to have the high-pass filter after the poweramp because I'm currently using a passive preamp, without any buffer. Having the filter after the poweramp will also protect my precious Manger from any possible DC failure and my Goldmund SRA is a direct couple poweramp.
I agree doing the high-pass in low level siganl is a much easier and cheaper way. Maybe I'll make both and see which one I like, cause even an OPA627 buffer is cheap compare with the cost of Manger.
Henry
cocolino said:
and what if we don`t want to achieve a "correct" symetrical filter (nor is easy adjustability the main issue here) but rather as much as possible transient-perfect crossover?
I do care less (usually - of course it depends of what is the goal) about interdriver phase assymetry as I do for overall transient system response and in this field a subtractive filter is potentially superior (at least for filters steeper than I.-order in one or more branches) .
I understand that, but the transient response of the whole system (this means low and high acoustically summed after filtering) is at its best if the filters are phase-symmetrical, because they compensate the phases with each other. if the phase is not symmetrical , then you will just get a worse transient response.... and you'll get lobing either upwards or downwards due to the phase difference....
This doesn't mean that there is no situation where this could be an applicable solution for an already existing problem, or maybe you just like the sound of it or whatever other reason.....
But I just posted this, because of the contradiction I read about transient of filters and subtractive filters.... and because this is my experience with real life subtractive filters. I know that you can make the subtractive filter more complex, and compensate completely for the phaseproblem (thus transient response problem), but by the time you reached this point you'll end up using more components then with an ordinary HP/LP symmetrical setup.....
And I don't understand what you mean by the subtractive filter being steeper en having a better transient response..... im sorry but in my opninion it just isn't true.... if it is steeper it has more phaselag, and in the case of the assymetrical phase resaponse of the subtractive filter also a worse transient response....
but I can be wrong, i'm not really overseeing the phase of the summed output right now... it is possible that with 4th order (180 degrees -3dB) it comes to a point that it is phase correct due to the negative summing.... this means that in the 4th order case it might work, if used with an allpassnetwork.... hmmmm
It is not true that You get better transient response with symetrical filters leaving aside 6 db filters (HP and LP) .
Quite the opposite.
Where did You get the information from that symmetrical filters have better transient response - have You ever seen on a scope screen what two summed (HP+LP, equal slopes) signals for example from conventional second order (or any higher order) filters look like?
It is true that asymmetrical filters (be they subtractive or not) cause lobing but this must not always be of disadvantage.
For example operation of D`Appolito systems (filters + physical arrangement of drivers on the baffle, eg. Mid-Tweet-Mid or Woof-Tweet-Woof, with certain distance between drivers) are based on lobing effects due to 3.order filters causing phase differences between drivers, which in their total effect lead to a wider and more uniform radiation pattern though.
There is no contradiction of transient response and subtractive filters.
Unless (conventional) filler driver circuits and 6dB filters (and maybe unless digital filter implementation from which I have no clue), actually a subtractive filter is the only way to produce such filters.
Other than the 3 exceptions mentioned, I`ve yet to see another filter topology which can achieve transient perfection without subtraction?
Working subtractive transient perfect filters (even with driver-filter-effects taken into account) must not necessarily be more complex than ordinary HP/LP setups - see Charles`s (phase_accurate) example above.
I don`t say a subtractive filter is always a better solution. In fact due to the considerable frequency overlap and therefore particular at higher crossover frequencies, they have rather limited use IMO but there are cases in which they might perform better. A good example is Charles´s application above combining a FR-unit with a woofer (at rather low fc, ~80Hz-300Hz)
Maybe I have expressed myself not so good.
What I`ve meant is that there simply isn`t any (symmetric or not) conventional (not based on subtraction) transient perfect higher (>6dB) order filter (and again except filler driver filters which BTW are non symmetric by their nature).
In contrary to "ordinary" filters, filters based on subtraction steeper than 6dB slope (at least in one branch, although there are even symmetrical subtractive filters too) are possible and which are transient perfect (at least in theory and leaving aside for now how to achieve to maintain this behavior in the acoustical domain also).
No offense taken but I´d suggest further studies on this topic.
by more further examining (lipshitz & vanderkooij)
it seems to work, ... with proper allpass delays, for even order filters (2nd or 4th etc).
The phase difference between drivers is near zero, with the expense of the phaselag being translated to the whole system-sum..... Every step from one driver to the next is causing a phase change if you go upwards in frequency....
quite nice, but that means that transient response is really changed from input to output... hmmmmm
well i'm still in for symmetrical phase filters, have seen no one using subtractive filters somewhere, only in a very old kustom bass-amp from the 70's years ago, this one was without allpass correction..... (and because of that the hump)
(lipshitz & vanderkooij)it seems to work, ... with proper allpass delays, for even order filters (2nd or 4th etc).
The phase difference between drivers is near zero, with the expense of the phaselag being translated to the whole system-sum..... Every step from one driver to the next is causing a phase change if you go upwards in frequency....
quite nice, but that means that transient response is really changed from input to output... hmmmmm
well i'm still in for symmetrical phase filters, have seen no one using subtractive filters somewhere, only in a very old kustom bass-amp from the 70's years ago, this one was without allpass correction..... (and because of that the hump)
[cocolino said:
It is not true that You get better transient response with symetrical filters leaving aside 6 db filters (HP and LP) .
Quite the opposite.
Where did You get the information from that symmetrical filters have better transient response - have You ever seen on a scope screen what two summed (HP+LP, equal slopes) signals for example from conventional second order (or any higher order) filters look like?
yes, and like I said, the phase of symmetrical filters is compensating each other, thats why a -3 dB butterworth point is still giving a flat amplitude line, and not +3, because of the phase vector pulling it down. and sorry but wich scope does show transient response?
It is true that asymmetrical filters (be they subtractive or not) cause lobing but this must not always be of disadvantage.
For example operation of D`Appolito systems (filters + physical arrangement of drivers on the baffle, eg. Mid-Tweet-Mid or Woof-Tweet-Woof, with certain distance between drivers) are based on lobing effects due to 3.order filters causing phase differences between drivers, which in their total effect lead to a wider and more uniform radiation pattern though.
dont think so, two drivers placed vertical like d'appolito are having a small dispersion at higher frequencies (to be precise: 90 degrees/quarter wavelength if you measure center center of the bassdrivers, going smaller towards piston upwards, and with normal tweeters crossfrequencies and hifi-filterslopes this gets quite small, thats why 2.5 way is more effective to me)
There is no contradiction of transient response and subtractive filters.
Unless (conventional) filler driver circuits and 6dB filters (and maybe unless digital filter implementation from which I have no clue), actually a subtractive filter is the only way to produce such filters.
Other than the 3 exceptions mentioned, I`ve yet to see another filter topology which can achieve transient perfection without subtraction?
give me examples, i'm curious, who uses this with succes? I know that digital fir filters are used with succes, (also by me: the hugo/k&h pocessor actually a german design) and they are really effective and more versatile than any other filtertype I know, you can just overlap driverbands without any problem
Working subtractive transient perfect filters (even with driver-filter-effects taken into account) must not necessarily be more complex than ordinary HP/LP setups - see Charles`s (phase_accurate) example above.
hmm if they are transient perfect yes but they simply aren't. its impossible to turn back time so, with the first low pass it goes wrong, the low to mid step has to be compensated at the mids side (with an all pass network) for time due to the low pass of the bass unit...., this means a step in time backwards, and so the transient is disturbed, aside from if you really hear this, and i think you don't and that way its a nice filter to me
I don`t say a subtractive filter is always a better solution. In fact due to the considerable frequency overlap and therefore particular at higher crossover frequencies, they have rather limited use IMO but there are cases in which they might perform better. A good example is Charles´s application above combining a FR-unit with a woofer (at rather low fc, ~80Hz-300Hz)
what frequency overlap? seems impossible to me because you use the same filter for generating the both slopes... so the cross frequency is always the same... but yeah yeah charles combination is nice
Maybe I have expressed myself not so good.
What I`ve meant is that there simply isn`t any (symmetric or not) conventional (not based on subtraction) transient perfect higher (>6dB) order filter (and again except filler driver filters which BTW are non symmetric by their nature).
In contrary to "ordinary" filters, filters based on subtraction steeper than 6dB slope (at least in one branch, although there are even symmetrical subtractive filters too) are possible and which are transient perfect (at least in theory and leaving aside for now how to achieve to maintain this behavior in the acoustical domain also).
okeeokee im getting all these emails of responds on this topic so im gonna place this one and see what you guys have to say.... reelax maahn
No offense taken but I´d suggest further studies on this topic.
Apart from a FIR (which can be made subtractive as well quite conveniently if symmetrical coefficients are used) only the output signals of a subtractive crossover sum to the original input signal in time AND frequency domain.
It is simply so because one output signal is the difference between the input signal and the filtered signal (i.e. the one that is needed to complement the filtered signal to unity).
NO arrangement using seperate high and lowpass filters (apart from a 6 dB filter and the filler driver, which is mathematically related to a symmetrical subtractive filter) is able to do this. Period.
Regards
Charles
It is simply so because one output signal is the difference between the input signal and the filtered signal (i.e. the one that is needed to complement the filtered signal to unity).
NO arrangement using seperate high and lowpass filters (apart from a 6 dB filter and the filler driver, which is mathematically related to a symmetrical subtractive filter) is able to do this. Period.
Regards
Charles
As promised
Here is the symmetrical subtractive 2nd order crossover. I mathematically derived it from the one I called "advanced filler driver". I once read in a book by John Watkinson that symmetrical subtractive crossovers should be feasible but failed to find any decent info, so I did it by myself.
It's output looks the same as the one from the NS APP note mentioned within post #40.
There is one difference however: This one here takes less OP AMPs. If one uses an inverting 2nd order filter topology, it could be implemented with only two OPs (input buffer not counted).
It could also be implementded with a filtered lowpass and derived highpass by using a 2nd order lowpass and a 1st order highpass.
If the pole frequency of the 1st order filter and/or the gain of the intermediate stage is/are changed, the symmetry can be varied (din't do the maths for that).
There are also 3rd order symmetrical versions possible (slightly different topology) but it comes at the cost of larger humps and increased overlap. Even 2nd/3rd asymmetrical should be possible but I haven't done the math for that so far.
Regards
Charles
Here is the symmetrical subtractive 2nd order crossover. I mathematically derived it from the one I called "advanced filler driver". I once read in a book by John Watkinson that symmetrical subtractive crossovers should be feasible but failed to find any decent info, so I did it by myself.
It's output looks the same as the one from the NS APP note mentioned within post #40.
There is one difference however: This one here takes less OP AMPs. If one uses an inverting 2nd order filter topology, it could be implemented with only two OPs (input buffer not counted).
It could also be implementded with a filtered lowpass and derived highpass by using a 2nd order lowpass and a 1st order highpass.
If the pole frequency of the 1st order filter and/or the gain of the intermediate stage is/are changed, the symmetry can be varied (din't do the maths for that).
There are also 3rd order symmetrical versions possible (slightly different topology) but it comes at the cost of larger humps and increased overlap. Even 2nd/3rd asymmetrical should be possible but I haven't done the math for that so far.
Regards
Charles
Attachments
this is a very elegant filter phase ac., no question about that...
1st: I don't mean to be irritating anyone with my sayings here and i'm quite open to be convinced by you guys, but i'm still not....
If I look at the HP output, it has the normal 2nd order phasechange (compared with he input) around the crossoverfreq. and under that... If you sum the LP/HP outputs together and there is a flat line (flat summing amplitudewise) this means that the LP output also has the same phasechange , otherwise its impossible to get the flat line right?.
If the 2 outputs both have the same phasechange, there is phasechange in the acoustically summed signal, or if you like after summing the signals electrically this is visible/measurable.
But it is true that the phasechange isn't disturbing the amplituderesponse, what is your goal, but a good symmetrical filter isn't either, if you stay on axis.
Only off axis of the drivers, the lobing with your subtractive filter is broader and so vertical (if the drivers are placed so) it will behave much better because the outputs doesn't have to compensate. With a normal symmetrical filter changing vertical listening-angle means changing the compensating mechanism by varying the vectors... So the vertical lobe to the listener is smaller (only around the crossoverfreq), and in that way the subtractive filter seems more right to me.
However as long as the physical distance between drivers compared with the wavelength is small, this lobing (around the cross.) of standard symmetrical filters is much less, and not a problem to me.
But I must say its a very interesting topic, and it sure changed my feelings about the subtractive filter..... it is quite elegant, but the story about the transient respone seems untrue to me, because there is a common phasechange on both the outputs. But if you change for instance the low-end eq of your amp (wonder if any of you hififreaks have any ) you'll get the same effect. they say its inaudible, and i believe it.
cheers tonite guys, happy new filter
1st: I don't mean to be irritating anyone with my sayings here and i'm quite open to be convinced by you guys, but i'm still not....
If I look at the HP output, it has the normal 2nd order phasechange (compared with he input) around the crossoverfreq. and under that... If you sum the LP/HP outputs together and there is a flat line (flat summing amplitudewise) this means that the LP output also has the same phasechange , otherwise its impossible to get the flat line right?.
If the 2 outputs both have the same phasechange, there is phasechange in the acoustically summed signal, or if you like after summing the signals electrically this is visible/measurable.
But it is true that the phasechange isn't disturbing the amplituderesponse, what is your goal, but a good symmetrical filter isn't either, if you stay on axis.
Only off axis of the drivers, the lobing with your subtractive filter is broader and so vertical (if the drivers are placed so) it will behave much better because the outputs doesn't have to compensate. With a normal symmetrical filter changing vertical listening-angle means changing the compensating mechanism by varying the vectors... So the vertical lobe to the listener is smaller (only around the crossoverfreq), and in that way the subtractive filter seems more right to me.
However as long as the physical distance between drivers compared with the wavelength is small, this lobing (around the cross.) of standard symmetrical filters is much less, and not a problem to me.
But I must say its a very interesting topic, and it sure changed my feelings about the subtractive filter..... it is quite elegant, but the story about the transient respone seems untrue to me, because there is a common phasechange on both the outputs. But if you change for instance the low-end eq of your amp (wonder if any of you hififreaks have any
) you'll get the same effect. they say its inaudible, and i believe it.cheers tonite guys, happy new filter
Not at all !
The end result is the vector sum of the two signals i.e. for an LR4 where you have a phase difference of 360 degrees (i.e. NOT zero as some believe !!!) both signals add to unity at the crossover point because they are both 6 dB down.
With all the subtractive filter topologies there is a phase shift in the 100 degrees area (even for the symmetric 2nd order one) and significantly less decay than 6dB (most often even less than 3 dB) summing again to unity. The difference between the LR and the subtractive x-over is that the latter is phase_accurate (at the cost of less steep slopes and increased lobing).
Regards
Charles