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Old 8th May 2011, 11:45 AM   #1
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Default 24db LR crossover polarity

Hey I'm bussy makeing a 24db/octave active Linkwitz Riley crossover for biamping. I have a question about phase reversal... With the 12db crossover there is phase reversal and either the high or low channel needs to run inverted.

Do I still need this for 24db? I still need that last opamp in the stage for level adjustment anyway, I'm just unsure of the polarity.

Attached is what I made so far until I started doubting that question part.
It is a set for two channels, with left and right input, and two pairs of Low and High pass outputs.
The two sides are isolated from each other as my amplifier is in monoblocks in one enclosure format.
Things are cramped as I had to fit it in one eurocard thanks to eagle trial.
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File Type: jpg 24dbLRstereo.jpg (94.6 KB, 185 views)
File Type: jpg 24dbLRstereoback.jpg (76.2 KB, 178 views)

Last edited by digits; 8th May 2011 at 11:52 AM.
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Old 8th May 2011, 11:49 AM   #2
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No. The outputs are in phase.
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Old 8th May 2011, 12:02 PM   #3
mbar is offline mbar  Poland
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Yeah, 4th order LR are in phase, like 360 deg. "in phase". No polarity switch necessary fortunately. But remember, there is still some phase difference between woofer and tweeter at the crossover frequency.

Originally Posted by digits View Post
thanks to eagle trial.
Use Kicad.
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Old 8th May 2011, 12:16 PM   #4
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I'm so used to eagle, I'd probably die trying something else.
I would like designer though, it has neat features above and beyond the board.

Thank you guys for the awesome news... well almost, I was hopeing for an easy way to make it switchable between 12 and 24db for experimentation.

It is the standard Rod Elliot implimentation with the little bump to flatten the dip formed by 24db crossover point back to 0db point.

I wasn't quite sure what values to start with in the calculator, so I used 1nf, and it basically threw out 1nf, 2nf, 50.02k and 100.04k resistors, which is why you see pairs of resistors everywhere to parallel two to make the value up within a few .000s

Is there realy a NEED for the inverting stage with 12db crossovers? wouldn't just reversing the speaker cable on one driver be more elegant?

Last edited by digits; 8th May 2011 at 12:24 PM.
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Old 8th May 2011, 01:34 PM   #5
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"Is there realy a NEED for the inverting stage with 12db crossovers? wouldn't just reversing the speaker cable on one driver be more elegant?"

dunno if it is elegant or not, but probbly i would go with that solution.
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Old 9th May 2011, 02:55 AM   #6
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Hi Arty, that is what I intend to do myself. I'm going to be using a B1 Buffer as output rather than an opamp. an opamp can easily be configured for inverting or non-inverting, which a simple buffer I do not believe can be...

The main advantage I see with putting an inverting buffer in after a 12db crossover is that you don't have to remember to reverse the speaker wires on one of the amps. I can imagine myself cringing just a little every time I hook up the cables in reverse on one of the amps

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Old 9th May 2011, 12:18 PM   #7
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All I can say is, it is not a cheap exercise, Just the capacitors and opamps come to $60, fancy resistors would add another $20 at least. Its a big circuit, containing 45 capacitors and 45 resistors. i'd say its the hardest PCB I designed yet.

I could drop 8 caps, and use 2nf polyprops, they'd take up about the same size, but costs about 5 to 6 times what the wima 1nf polyprops go for, so it would be even more expensive, although you do gain the move up to 1% tolerance caps.

I reccon I will make my own dedicated listeng room, so I don't have to compete with mommy, got a whole sittingroom standing empty, sad thing being 2 people in a big old house.
I Think I will start with 4 sets of LM1875 (don't knock em until you hear a solid implementation). I do have some nice solidstate gear too, but stereo sets only, I want 4 exact same amps to drive the 4 outputs, one each side. And I think when all is accounted for the 75's are about as much as my budget (over several months) will take.

If money was no problem, I'd fire two sets of Precision I's at em, but I don't think I could even afford the electricity. Its the one amp Carlos got right, DX, too much bass, HRII, too refined, PI, leaves tears in your eyes and goosepimples on your arms, with you looking around to see where sounds came from. It takes absolute control of bass frequencies unlike anything I ever heard, but you need to power it with a small substation and mine two continents for all the heatsinking required. Well I guess not if you actualy like earned lots of money and stuff.

Last edited by digits; 9th May 2011 at 12:32 PM.
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