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Old 23rd May 2007, 11:22 PM   #11
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I think Linkwitz did the Pheonix mostly in the second topology, however, the all pass filters used for the tweeter did show that he cared about the phase / delays. The mid and bass each has its own shelving pass filter, notch, etc., suggesting it is closer to the second topology.

In John K's NaO, it is similar. The dipole EQ, notch filter, tone control are at the root, suggesting it is using the first topology. However, the woofer EQ is at a branch, which would change the phase, similar to what is in the second topology.

Leonard Audio obviously uses the second topology.

From my memory, Rod Elliot suggests using the first topology.

So I am confused. I have not sufficient knowledge to be 100% sure about it. Of course, measurements will show if the results are correct or not. But I guess I should understand it first, design it then measure it.

My thought is that drivers are minimal phase devices, which means their phase is determined by their frequency response. If that is the case, if not considering the driver offset, provided that the active or passive XO network shapes the driver response to the target frequency response, such as LR4, LR2, BR3, etc, the network should sum up as predicted. In that case we don't need to worry about the phase but only the frequency response curves that determine the phase. If this assumption is correct, then I don't need to use the first topology, the second topology should do fine. This is very important because with the second topology, there are far less components (opamps, caps) in the signal path for each leg, beneficial especially for the higher frequencies.

I am currently designing my new WWMTMWW + Sub 4 way hybrid active / passive, hybrid dipole / cardiod / monopole speaker. For the 2.5k XO, I will use passive. I am planning to use the second topology. If I had to use the first topology, I would abandon the project because I don't believe the speaker can sound good with that many components in the signal path for a 4 way. I understand that driver offsets will introduce errors. But given the XO points are 300Hz, 65Hz the kind of phase errors would be at a minimum, and perhaps moving one leg of the XO higher or lower will adjust it perfectly, I hope.

Please correct me if I am wrong and save me from my troubles down the line.

Thanks in advance.
Bill
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Old 24th May 2007, 12:40 AM   #12
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Seems reasonable to me, as I think all that matters is that the phase is correct in the end.

However, I don't have any problem with my current 3 way using the first topology. My tweeters end up being driven through 7 op-amps, still extremely clean using OPA2134s. NE5532s were not quite as good. I cannot put my finger on exactly how they weren't as good, so it may just be that I spent a fair amount of money on my OPAs and want them to sound better.
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Old 24th May 2007, 12:52 AM   #13
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Quote:
Originally posted by HiFiNutNut
The normal 4 way passive XO would be something like this:

Click the image to open in full size.

Bill,

I think what you've got there is the normal active crossover layout. Parallel passive lay-out (this for a 3-way) is more like:
Click the image to open in full size.
If you ran a passive (4-way) the way indicated in your diagram, then you'd have the tweeter signal passing through 3 different high-pass filters before it got to the tweeter. You'd be hearing nothing at all, or at least, nothing good.
A parallel passive filter is completely independant of the filters for each of the other drivers in the system (in theory), which is why you can drive each of them with an amplifier.
I don't know what John Murphy was thinking when he came up with that block diagram that you posted...
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