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Old 22nd August 2007, 02:08 PM   #1
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Default Calculate PLLXO

I purchased an XM46SB passive line-level crossover from Marchand; 80 Hz BW2 high pass. The circuit is shown in the manual:

http://www.marchandelec.com/ftp/xm46man.pdf

The input impedance of my power amp is 22K. When I told that to Phil, he said it doesn't matter. I don't get it. Wouldn't the amp's impedance affect the XO frequency? The supplied component values are:

R1 = 2.55k
L1 = 5.6 H
C1 = 0.47 uf
C2 = 0.22 uf
(C1 + C2 = 0.7uf actual, according to the written notes in my manual).

I'd like to adjust the component values to make it possibly more accurate. At the same time, I'd like to use a pair of .33uf Auricaps I have lying around. The inductor value can't change, but I could easily change R1 and the caps. Can someone please help tweak this circuit?
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Old 22nd August 2007, 03:31 PM   #2
sreten is offline sreten  United Kingdom
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Hi,

You have not specified R2. R1 and R2 make the filter fairly
tolerant of load impedance and normal source impedances.


Absolute accuracy of 80Hz high pass 24dB/oct (should be L/R)
is not necessary if you have an adjustable 24dB/oct low pass
on the subwoofer.

Careful speaker + sub placement + sub adjustment will be needed.

Theoretically you need a 22K resistor on the output (no level adj.)

/sreten.
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Old 22nd August 2007, 03:55 PM   #3
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Quote:
Originally posted by sreten
Hi,

You have not specified R2. R1 and R2 make the filter fairly
tolerant of load impedance and normal source impedances.

Absolute accuracy of 80Hz high pass 24dB/oct (should be L/R)
is not necessary if you have an adjustable 24dB/oct low pass
on the subwoofer.

Careful speaker + sub placement + sub adjustment will be needed.

/sreten.
Hi sreten.

There's only one resistor, R1. Note that this is a 2nd order Butterworth, high pass only. The high pass filter combines with the natural roll-off of my sealed BESL MTM's to provide 24 dB slope.

This is for a stereo system, where the dual subs are placed directly below the main monitors. I know absolute accuracy is not strictly needed, but I want to upgrade the caps, and if I'm going to do that, I might as well try to improve the crossover point as well.


Quote:
Originally posted by sreten
Theoretically you need a 22K resistor on the output (no level adj.)

/sreten.
I'm not sure what you mean. There is no pot in my crossover. R1 is 2.55K, and my amp impedance is 22K. Which means I have a total impedance of 2.285K. If I plug the numbers into the formula, I get a -3dB point of 65 Hz due to the inductance, and a -3dB point of 100 Hz for the caps.

Shouldn't I change R1 and the caps so that I get -3dB at 80 Hz for both legs?
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Old 22nd August 2007, 04:45 PM   #4
sreten is offline sreten  United Kingdom
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Hi,

So your using half of the board twice ? C1 = C1a, C2 = C1b.

....... BW2 = butterworth 2nd order

7uF + 5.6H with zero source impedance and 2Kohm load = 80Hz BW2.

You cannot treat each component seperately. Reduce R1.

Find an on-line BW2 calculator to play with values.

/sreten.
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Old 22nd August 2007, 05:16 PM   #5
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C1 = C1a, C2 = C1b

Oops, yes, sorry for confusing the issue.
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Old 22nd August 2007, 05:34 PM   #6
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This looks like a good calculator: http://www.the12volt.com/caraudio/crosscalc.asp#cc
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