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Old 5th January 2007, 06:03 AM   #11
qguy is offline qguy  Canada
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one more question... what would be a better way of lowering the crossover point.. would be via the resistor or via the capacitor ?
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Old 5th January 2007, 06:21 AM   #12
F1 FAN is offline F1 FAN  Canada
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Either way is fine as the results will be the same.It is a matter of what parts are easier to obtain and change.
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Old 5th January 2007, 10:24 AM   #13
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Quote:
Originally posted by Drew Eckhardt
IC2D is a second order Sallen Key low-pass filter with unity gain.
Of course it is. Doh!

Note to self: don't post late at night when you should be in bed.

qguy, this might help:

http://www.daycounter.com/Filters/Sa...lculator.phtml
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Old 5th January 2007, 11:14 AM   #14
AndrewT is offline AndrewT  Scotland
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Hi Pink,
you're forgiven,
I too burnt the wick at too many ends over Christmas.

The last C could be increased from 0.47uF to 1uF and lower that final filter to about 72Hz & Q=0.71

The inverting input may be useful but be prepared to swap your sub input terminals to correct for the filter inversion.

If you want to preserve the Q of the middle filter then both 10k resistors need to be reduced by the switch (DPDT) or by adding extra parallel resistors. The range 5k to 10k is just fine.

If you want Butterworth for the middle filter then the top C =2 times lower C. Use a pair on top and single at lower location.
The final Q = Qmiddle * Qthird.
ie. Q= 0.69 * 0.71 = 0.9 or 0.73 * 0.71 = 0.52 (both close to Bessel or half of Linkwitz Reilly).
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Old 5th January 2007, 10:03 PM   #15
qguy is offline qguy  Canada
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Quote:
Originally posted by AndrewT
If you want to preserve the Q of the middle filter then both 10k resistors need to be reduced by the switch (DPDT) or by adding extra parallel resistors. The range 5k to 10k is just fine.

Hi Andrew,

When FI Fan said double the reisistance to lower the crossover point this means 10K to 5K and not 10K to 20k right


The two resistor that you mentioned are the one in red circle right ?

If you want Butterworth for the middle filter then the top C =2 times lower C. Use a pair on top and single at lower location.
The final Q = Qmiddle * Qthird.
ie. Q= 0.69 * 0.71 = 0.9 or 0.73 * 0.71 = 0.52 (both close to Bessel or half of Linkwitz Reilly).

What advantage would this changes make ?

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Old 6th January 2007, 09:07 AM   #16
AndrewT is offline AndrewT  Scotland
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Hi,
to set the frequency one needs to look at R*C. If one increases the combination one ends up with a lower frequency. So, you can increase R and/or increase C.

20k will increase the noise the circuit produces and at this level in a sub-base it will probably be tolerable, but it depends on the following gain and sensitivity.

I recommend you stay within the ranges below 12k. That's why I said 5k/10k before.

The red encircled resistors are the original (top) setting 50Hz and the extra (bottom) sets to the lower resistance and raises the frequency to 70Hz.
The other half of the pair is the next one to the left. You should change both of these to preserve the roll-off characteristic that you intended. The pair need to be adjusted to change frequency properly and the method being adopted is a cheap way of doing it.

I have now listened to Linkwitz Reilly and to me it sounds like there is a hole in the crossover where something is lacking (I had 150Hz 4pole). Many recommend L-R (Q=0.5) over Butterworth (Q=1/sqrt(2)=0.7071). This choice or anywhere in between will affect the sound. Once you have found the one that sounds right you want to keep that and fiddling with just one half of the pair of frequency setting resistors will make assessing/choosing more difficult. You should also be adjusting the passive filter to match but that is becoming even more complicated.

It just ocurred to me that Q=0.5 is not half of L-R, since L-R can only be achieved with an even number of poles i.e. 2pole or 4pole etc. 3pole(18db/octave) is just a Bessel characteristic.
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Old 8th January 2007, 09:42 PM   #17
qguy is offline qguy  Canada
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tried the 20K ohm resistor prior to reading your post, the sound was distorted, will try 5-9K ohm resistor and post the result - thanks
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