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Old 28th December 2007, 02:51 PM   #1
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Default 48db/oct Subsonic filter

Has anybody built a filter like this?
Is for subsonic filtering, maybe 15-17Hz 48db/oct highpass.
It be worth?
Thanks
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Old 28th December 2007, 03:08 PM   #2
AndrewT is offline AndrewT  Scotland
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Hi,
each filter has a Q of 0.7
When you cascade four of these, the Q of the whole circuit is 0.7^4=0.25
This is far too low.
You should alter the Q of each section to deliver a final Q between 0.5 and 0.7
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Old 28th December 2007, 03:25 PM   #3
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Hi AndrewT:

Thank you for your answear
Maybe modify this, is too much to my knowledge.
Certainly its easyer build something proven, like this?
http://sound.westhost.com/project99.htm
Thanks
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Old 28th December 2007, 03:37 PM   #4
AndrewT is offline AndrewT  Scotland
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Hi,
ESP has altered the Q of each section to give a fairly fast turn over.
That's why he warns you not to experiment with changes.

This type of circuit is much easier to design and build if you adopt equal value Sallen & Key filters instead of unity gain filters.

The disadvantage of evsk is that each stage's gain is greater than 1 and the total gain is usually too high. This can easily be solved by using MFB filters for the two highest gain (highest Q) sections. MFB are inverting stages and using two cancels out the inversion.
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Old 28th December 2007, 04:57 PM   #5
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Hi Andrew:

I found the "Texas Instrumentes Filter Pro" design software, maybe this help me to understand more the complexity of filter design.
Thanks a lot
Iván
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