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Analog Line Level Preamplifiers , Passive Pre-amps, Crossovers, etc.

8th-order subsonic filter
8th-order subsonic filter
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Old 29th October 2017, 03:55 PM   #1
alex5612 is offline alex5612  Austria
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Default 8th-order subsonic filter

I'm thinking about adding a subsonic filter to my system.

I've started with the filter from ESP, but since I'm going to make my own PCB anyway I've chosen to build an 8th order filter. This should help to keep any non-audible low-frequency rumble away without affecting frequencies that should pass through.

My filter consists of four cascaded sallen-key filters and the resistors will be made switchable using 4052 cmos multiplexers. I've figured that having the corner frequency switchable is a nice feature, because not all subwoofers are created equal and the smaller ones will usually struggle at reproducing low frequencies at full volume.

Am I missing something here? Are there any good reasons to use a lower order filter?

Ps.: The simulation is attached in the ZIP file.
Attached Images
File Type: png filter 3-1.png (50.0 KB, 128 views)
Attached Files
File Type: zip filter3-1.zip (2.3 KB, 13 views)

Last edited by alex5612; 29th October 2017 at 04:00 PM.
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Old 29th October 2017, 04:02 PM   #2
DF96 is offline DF96  England
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Have you considered phase response? As a general rule, the sharper the frequency cutoff the more wobbly the phase response.
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Old 29th October 2017, 04:39 PM   #3
alex5612 is offline alex5612  Austria
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I'm not sure if I've understood you correctly, but as far as I'm aware all filters will have some phase shift in the pass-band. However most subwoofer modules will have some sort of phase controller for correcting that anyway. Or let's rather say for making the best out of the situation.
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Old 29th October 2017, 05:49 PM   #4
Fast Eddie D is offline Fast Eddie D  United States
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Those phase controllers are primarily for adjusting phase in the passband. It is highly unlikely that any "controllers" will complement the crossover region phase shift of such a high order filter.
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Old 30th October 2017, 11:26 AM   #5
DF96 is offline DF96  England
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You get 90 degrees of phase shift per pole, so an 8-pole filter will have 720 degrees. Most of that will occur in the region of the frequency cutoff, but exactly how it is distributed depends on the detail of the filter design. Roughly, you can expect 360 degrees of phase shift in the passband, and the same in the stop band.
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Old 30th October 2017, 11:37 AM   #6
AndrewT is online now AndrewT  Scotland
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A single pole filter creates 90degrees of phase shift over the frequency range 0Hz (DC) to infinite Hz (lightwaves and beyond).

At the F-3dB frequency the phase shift of the simple RC or CR filter is 45 degrees.
When you see the phase plotted with the gain/attenuation curve you see the typical "S" shape.
This "S" shape is maintained as more poles are cascaded.
A 2pole S&K has 90degrees phase shift at the F-3dB frequency and a total phase shift of 180degrees.
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Old 31st October 2017, 01:17 PM   #7
alex5612 is offline alex5612  Austria
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I do get that, but will this phase shift cause any problems at all?
Phase shift is basically just group delay when it appears in multiples of 360. And to my understanding this shouldn't be a serious issue as long as it does not cause cancellation somewhere.
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Old 31st October 2017, 01:47 PM   #8
DF96 is offline DF96  England
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Any phase shift which varies with frequency causes group delay. If it varies nonlinearly with frequency then the group delay varies with frequency. That is why there are different filter shapes, so you can choose sharp cutoff or approximately constant group delay or something in between.
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Old 31st October 2017, 03:49 PM   #9
Hans Polak is offline Hans Polak  Netherlands
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In 1976, Tomlinson Holman came to the conclusion that 20msec groupdelay was the maximum allowable for an infrasonic filter to be unnoticed.

http://www.kallhovde.com/advent/phono-pre-research.pdf

With more complex elliptic filters, D Self managed to construct a very effective Rumble Filter, having only 0.3dB attenuation at 20Hz and >35dB at 10Hz and below, while still having only a 23msec groupdelay@20Hz.
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Old 31st October 2017, 06:21 PM   #10
Fast Eddie D is offline Fast Eddie D  United States
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Quote:
Originally Posted by alex5612 View Post
I do get that, but will this phase shift cause any problems at all?
Phase shift is basically just group delay when it appears in multiples of 360. And to my understanding this shouldn't be a serious issue as long as it does not cause cancellation somewhere.
Phase shift can cause audible anomalies. That's why we sweat the crossover so much - nonlinear phase shift between two drivers can cause some very audible effects; including cancellation at certain frequencies, and a (subjective) "garbling" of the reproduced signal.

Crossovers make or break a system. A good crossover can make two drivers play well together.
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