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cuibono 25th September 2008 12:24 AM

Beyond the Linkwitz ASP
I'm putting the finishing touches on a crossover for my current speakers, and looking to move it out of my computer - I currently use Thuneau's Allocator for all XO, EQ and time allignment, and am very happy with it (I just wish it weren't so buggy). The sonic results are great, but having to use a computer every time I want music is a pain, doesn't work well for turntables and other analog sources, and the multichannel volume control is a pain. So anyway, I want to build an analog version of my current digital XO...

I'm looking at various options, and have some experience with Linkwitz's designs, which I hold in high esteem. Rather than copying his work, which he makes readily available, I'd like to combine multiple filters around as few opamps as possible. I know its possible, I just don't know how, and I haven't figured out how to simulate them yet.

Any pointers?

cuibono 25th September 2008 02:15 AM

Allocator allows has a graphical interface which allows you to see the transfer function combined with your driver's measured response, which seems to work quite well for developing crossovers.

My current system is a two way:

The lows are crossed at 4th order at 125Hz, Q=.71 and highs crossed at 130Hz, same order and Q. There is no gain, both connected in phase, but the highs have a 1.1mSec delay. Last time I checked, the acoustic slopes were pretty close to 4th order.

In the highs, I use a shelving filter at 200Hz (+2dB, Q=1.0), and a notch filter at 600Hz (-4dB, Q=1.0). On the low end, I use the following notch filters - 80Hz, +3dB, Q=3.47 - 100Hz, -2dB, Q=13.13 - 110Hz, +4dB, Q=12.38.

These aren't final, but they have been stable for a couple of months, and are based both on measurement and listening. I will be changing the baffle, and have to remeasure then. If anyone is wondering, I'm using an Eminence Alpha 15 in an H frame on the bottom, and a Visaton B200 open baffle on top, modified with a phase plug and Enabled according to Planet10's recipe.

Next I'll probably post a couple opamp based filters, like Linkwitz uses - then I'm hoping someone can show me how to start combining them around one opamp. :D

cuibono 30th September 2008 10:44 PM

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Hmm, I hope someone is interested...

I got the sim working this weekend, and did some interesting stuff. Here are three 'equivalent' 4th order high pass circuits, the first is passive, the second is how Linkwitz does his active LR4 crossover, and the last is an simulated equivalent that I pieced together. It uses one opamp instead of two, the way SL does it.

cuibono 30th September 2008 10:48 PM

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Here is the simulation. There is something interesting going on - each filter has a slightly different 'knee', the passive circuit having the softest knee, and the single opamp circuit having the hardest knee. Y axis is in decibels..

(the green line is the passive circuit, the yellow is the double opamp, and the blue is the single opamp)

cuibono 30th September 2008 10:54 PM

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I've been successful at combining notch filters around a single opamp, and plan on doing that with the analog equivalent of the digital XO I'm currently using. Linkwitz uses 22 opamps per channel in his 3 way design, I'm hoping to use 3 or 4, depending on if I keep it a 2 way or make it a three way, which I'll probably do. Anyone have any objections?

In another thread ('correcting valleys', I think), someone asked about a peaking filter. I simmed one, and here are the results.

cuibono 30th September 2008 10:55 PM

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And here is the transfer function, Y axis also in dB. Its not optimized, but at least it works..

soongsc 1st October 2008 12:05 AM

for three way system, design the tweet and woof, and get the mid from subtraction.

cuibono 1st October 2008 03:01 AM

Hmmm, subtractive crossovers look ideal, but in the real world, I'm not so sure. Aren't they only 'transient perfect' on axis, at a specific distance (if this isn't true for subtractive crossovers, please correct me)? Subtractive XOs seem to have relatively shallow slopes, which isn't ideal: when you throw in the amplitude/phase variation of real drivers, and any additional equalization, it seems like there would be no chance of perfect summation, even on axis at a specific distance. Perhaps thats why I haven't been able to find any actual measurements of 'transient perfect' designs. Off smooth off-axis response seem like it might be more important than reducing phase distortion, and higher order XOs are better suited for that.

I'm interested in subtractive XOs, I'm just not sure they actually work, and even IF they are able to pass a square wave over a wide listening angle, I still haven't seen much evidence that lower phase distortion is more important than off axis power response irregularities.

infinia 1st October 2008 05:21 AM


Originally posted by cuibono
Subtractive XOs seem to have relatively shallow slopes, which isn't ideal: when you throw in the amplitude/phase variation of real drivers, and any additional equalization, it seems like there would be no chance of perfect summation, even on axis at a specific distance.

They (subtractive) work as xover's in situations, that would allow for 1st order designs. An example of their use could be from subs to a 2 way.

Here is a protype PCB that might intrerest you, it's not the most eff. use of opamps tho. see links
Group Buy thread

soongsc 1st October 2008 06:03 PM

It's hard to say if subtractive XOs will work unless one looks at all the data.

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