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.
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.
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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.
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.
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.
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.
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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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.
I'm still wondering how ANY audio system, and especially a playback system (getting mastered recordings) could ever need 8th-order bass filtering.
2nd and 3rd order cover the case of too-small sealed and vented woofers getting bass-heavy sources. Another 2nd order might be needed on badly isolated phonograph. There are mis-designed phono preamps with subsonic bumps which *might* want another slope. So worst-case I figure 5th or 6th order. And that's a troubled system. 4th order sound more than enough for any non-sick system; 3rd for too-small woofers.
2nd and 3rd order cover the case of too-small sealed and vented woofers getting bass-heavy sources. Another 2nd order might be needed on badly isolated phonograph. There are mis-designed phono preamps with subsonic bumps which *might* want another slope. So worst-case I figure 5th or 6th order. And that's a troubled system. 4th order sound more than enough for any non-sick system; 3rd for too-small woofers.
You do have a point there. The filter was supposed to go into a subwoofer controller/preamplifier. It does work just fine on its own, but when one tries to simulate the whole system the response is plain awful. Including poor response at the crossover region and many, many notches all over the audio band.
I'll guess this will be addressed in a different way. Maybe I'll post something later on when the project has progressed.
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