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Old 14th July 2006, 02:35 PM   #11
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Quote:
One advantage of the L/R is its power efficiency, i.e. phase matching.
The efficiency statement is indeed very true. But I wouldn't dare to say that the outputs are in phase. They are out of phase by 360 degrees actually. And how this influences efficiency on transients (the norm with music rather than sinusoids) has to be further investigated.

Quote:
Whatever the order of the filter one roll-off is always 6dB per octave, it also peaks, the higher the order the bigger the peak.

This has to be specified a little more: It is definitely possible to make symmetrical and asymmetrical subtractive crossovers of any desired order. In the "vicinity" of the crossover frequency they will always show a rolloff close to 1st order. And they definitely do have humps which are mathematically unavoidable for anything over 1st/1st order. These humps are higher for higer filter orders. They get smaller but broader for lower filter Qs involved.

The trick is to use the lowest order that is possible/necessary for a given driver combination and/or situation. So they are best used with sturdy drivers that are well behaved past the intended crossover frequency.

Regards

Charles
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Old 15th July 2006, 11:05 AM   #12
AndrewT is online now AndrewT  Scotland
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Hi,
I had a vague memory from many years back that subtractive filtering had some major failings, but my understanding then was much poorer.
However, Sreten & Phase accurate seem to have some (substantial) experience of this problem inherent in subtractive.
Sreten said
Quote:
they exhibit poor phase matching in the c/o region. Whatever the order of the filter one roll-off is always 6dB
per octave, it also peaks, the higher the order the bigger the peak.
and Phase accurate replied
Quote:
It is definitely possible to make symmetrical and asymmetrical subtractive crossovers of any desired order. In the "vicinity" of the crossover frequency they will always show a rolloff close to 1st order. And they definitely do have humps which are mathematically unavoidable for anything over 1st/1st order. These humps are higher for higer filter orders. They get smaller but broader for lower filter Qs involved.
Does the final statement
Quote:
use the lowest order that is possible/necessary for a given driver combination and/or situation.
mean that subtractive cannot be used for symmetrical multiple pole filters?
that 4pole filters are also impossible?
and how about 2pole L-R filters?

Some more extended clarification would be appreciated. It appears that if one already knows the answers then the message buried in the replies is clear, but for us poor amateurs, that level of understanding is lacking and I'm afraid I could be reading too much between the lines and drawing incorrect conclusions.
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Old 15th July 2006, 12:53 PM   #13
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So many threads to chase down. I'm going to pursue that now.
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Old 15th July 2006, 12:56 PM   #14
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I want the subractive approach because it seems elegant. It should be flat summing. Where as the HP and LP method really is not, and then there is component tollerance on top of that.

But this is why I posted here. Does it work? It would seem that you use HP to get the upper range driver signal. Then you subract that from the original to get the lower range driver signal.

If there is some component value error in the subraction, since it is the lower range driver, it does no harm.

But does it work. Is home or commercial equipment made this way?

All the books and online info I've ever seen and HP and LP.
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Old 15th July 2006, 01:12 PM   #15
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Andrew,

I don't know how you make symetrical slopes with subtractive crossovers, but Charles says it can be done. 4th-order for one side is possible, but the higher the order and/or the higher the Q the more pronounced the peak is on the opposite slope.

You cannot make a Linkwitz-Riley crossover subtractive because by definition a Linkwitz-Riley crossover is two 4th-order matched filters with a specific phase response. The 2nd-order variant hold the same as well.
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Old 15th July 2006, 04:36 PM   #16
AndrewT is online now AndrewT  Scotland
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Hi Zen,
I bought and tried a BKelectronics subtractive crossover.
It sounded terrible.

I then tried to measure what it was doing.
The slopes were unmatched but seemed to vary as well.
There was a lot of feedthrough on some of the channels, i.e. the passband was not flat and was much wider than it should be, but of variable response.

I then tried to reverse engineer the cicuit board and eventually convinced myself that the subtraction had been sent to the wrong place on the PCB. It appears the designer did not check his work or could not hear the errors. I dumped the PCB and kept the case & transformer.
That was about 15 years ago, my first silly attempt at going active.
BK, the last time I looked, still sold that subtractive but maybe by now they have sorted the errors.
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Old 17th July 2006, 07:34 AM   #17
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A subtractive crossover is just one method to derive a pair of lowpass and highpass that sum to unity. You can achieve the same thing also with seperate filters for high- and lowpass. But this would be much more complex than the subtractive variant since it would result in the summing of different filter outputs and it would be less tolerant to component variations than the subtractive method. This latter part is elegantly solved by the subtractive solution. With separate filters it would be easier to take driver response into consideration however. The third method for buliding these crossovers - the familiy name of which is constant voltage crossovers as John already mentioned - is the use of state-variable filters whose outputs are summed accordingly.

Basically you use a funtion that is always 1. I.e. its numerator and nominator polynoms are the same. The order of the polynom is the sum of all output orders minus 1. You then split the function into two halves having both the same nominator. And that is where the humps come from whoch grow larger with increasing filter order.

Though this is not very exotic from the mathematical point-of-view there are not that many sources showing this. I know only two personally. One is a recent paper by N. Thiele. The other one is this one, showing how it is done for a 2nd/2nd order symmetrical version implemented as state-variable filter (page 5):

http://www.national.com/an/AN/AN-346.pdf#page=6

How the same is achieved with the subtractive method is shown here:

http://www.diyaudio.com/forums/showt...789#post292789

Regards

Charles
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Old 17th July 2006, 12:07 PM   #18
sreten is offline sreten  United Kingdom
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Quote:
Originally posted by AndrewT
Hi,
I had a vague memory from many years back that subtractive filtering had some major failings, but my understanding then was much poorer.
However, Sreten & Phase accurate seem to have some (substantial) experience of this problem inherent in subtractive.
Sreten said
and Phase accurate replied
Does the final statement mean that subtractive cannot be used for symmetrical multiple pole filters?
that 4pole filters are also impossible?
and how about 2pole L-R filters?

Some more extended clarification would be appreciated. It appears that if one already knows the answers then the message buried in the replies is clear, but for us poor amateurs, that level of understanding is lacking and I'm afraid I could be reading too much between the lines and drawing incorrect conclusions.
Hi,

Without consulting my (couple of) references I'm going to stick
my neck out and say the following, from what I remember :

Whatever the order and shape of the main filter, the subtracted
output roll-off is 6dB/octave away from the c/o region. The
subtracted output tends to peak, the peak is bigger the higher
the order and Q of the "normal" filter.

So for 2nd order or > symmetrical filters are impossible, so no L/R.

4 pole main filters are certainly possible but the subtracted output
is nothing like you'd imagine, looks like a peaking 1st order roll-off.
(i.e. a 1st order roll-off + parametric peak at the c/o frequency.)

Say you use a 2nd order subtractive filter for a sub, 2nd order L/R
or Butterworth for the main filter @ 100Hz. The sattelite speakers
would need to work well and with minimal phase shift down to
50Hz at least to get correct all pass crossover summing of outputs.

I'd say the above is probably its only sensible use, when you
want to add a sub to a pair of main speakers with substantial
bass capability, and trying to get an all pass response.

(I say trying because the mains rolloff and consequently phase
response will probably scupper any real all pass capability, though
for lowish Q 2nd order main filter, transient response will be good.)

/sreten.
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Old 17th July 2006, 12:16 PM   #19
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Sreten

The part about "the higher the order the higher the humps" is right.
But it IS definitely possible to build these filters with whatever order one desires. But the higher the order the better the behaviour away from the crossover frequency at the cost of worse behaviour around the crossover frequency.

See:

http://www.diyaudio.com/forums/showt...629#post495629

Regards

Charles
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Old 17th July 2006, 03:48 PM   #20
forr is offline forr  France
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Phase-accurate
---A subtractive crossover is just one method to derive a pair of lowpass and highpass that sum to unity.---

I think the Phase-accurate's phrase above applies to the voltage domain at the output of the active filters. However, I apologise for criticising that it is not explicitly mentionned. Even not-beginners, me included, were or are very easily confused with that matter.

Filters for loudspeakers have three different outputs, at least : in the voltage domain, in the forward acoustic pressure domain and in the total acoustic power domain. This is complicated by the amount of mutual radiation of the drivers, ie. the distance between them.

Once all this is mastered, there is an easily forgotten fact : in the high pass section, there is an invisible additionnal filter which is the driver itself (medium or tweeter). It is a hi-pass device which has its own level and phase responses. That's why "ideal" crossovers (ie substractive) crossovers yield to results far away for ideal.

I know a guy who proudly says he is using a square wave perfect crossover. He mixes the voltages of the signals at the output of his active circuit and gets the same perfect square waves on his scope at the output of his active circuit that they were at the input. He never replied to my questions about the acoustic response of his whole system...


Zenmasterbrian
---I'm also interested in general issues for multi-amping, if you can make the power amp itself part of the active filter circuit.---

To avoid stability problems, there's a need for a high level input and just a unity gain power buffer. That's a good challenge for designers ! Pavel Macura designed one.
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