This could get a bit long, I apologize in advance. I'm working toward a crossover circuit for audio. My plan is for a 4-way crossover, but to begin with I was experimenting with just a 2-way.
I should mention that my understanding of filter design is a shabby hodgpodge of (mostly useless) info from low-level EET textbooks, various internet findings I never bother to bookmark, The Art of Electronics and an older version of The Electronic Filter Design Handbook, which pretty much goes over my head. I see transfer functions and my eyes glaze over; I have no idea how to relate them to anything practical.
So in various readings I got the impression that Butterworth filters are the general type used for audio, but I wanted to give Bessel a shot as well. Linear phase sounds like a good idea in audio; I know there are commercially made "linear phase equalizers", so at least someone else must have thought it was a good idea too.
I started with the Bessel, making and testing a simple 4-pole low pass filter. Works as expected. Then I made a simple 4-pole high pass. Works as expected, including this note's mention of Bessel high pass filters not exhibiting phase linearity. Also notice about halfway down that note, there's a graph showing a combined low- and high-pass output, in which there's a slight bump around the design frequency.
When I summed my otherwise normal Bessel high- and low-pass filters, I end up with a drop of about 12dB around the design frequency. This makes no sense to me since taken individually, the low- and high-pass sections are (of course) 3dB down at the design frequency. This is where I need help. What could be causing this? Is it a phase issue?
One difference in the filters I designed from the Bessel designs most references give is that I followed The Art of Electronic's method of equal component design. Equal capacitors and equal resistors in each section, with gain greater than unity. The math behind this escapes me (those transfer functions again) but I followed the tables given and got the expected results when testing the filters individually. Is there something about this method that could be causing the dip where I should see a rise according to the Rane note?
Also let me preempt a couple of responses I expect:
Why not do this digitally? - Because I don't want to, and because that would be even more beyond me. The ultimate goal is an analog processor.
Anything about SPICE/simulation - I don't have any simulation software. I don't know of any for Mac OS.
I should mention that my understanding of filter design is a shabby hodgpodge of (mostly useless) info from low-level EET textbooks, various internet findings I never bother to bookmark, The Art of Electronics and an older version of The Electronic Filter Design Handbook, which pretty much goes over my head. I see transfer functions and my eyes glaze over; I have no idea how to relate them to anything practical.
So in various readings I got the impression that Butterworth filters are the general type used for audio, but I wanted to give Bessel a shot as well. Linear phase sounds like a good idea in audio; I know there are commercially made "linear phase equalizers", so at least someone else must have thought it was a good idea too.
I started with the Bessel, making and testing a simple 4-pole low pass filter. Works as expected. Then I made a simple 4-pole high pass. Works as expected, including this note's mention of Bessel high pass filters not exhibiting phase linearity. Also notice about halfway down that note, there's a graph showing a combined low- and high-pass output, in which there's a slight bump around the design frequency.
When I summed my otherwise normal Bessel high- and low-pass filters, I end up with a drop of about 12dB around the design frequency. This makes no sense to me since taken individually, the low- and high-pass sections are (of course) 3dB down at the design frequency. This is where I need help. What could be causing this? Is it a phase issue?
One difference in the filters I designed from the Bessel designs most references give is that I followed The Art of Electronic's method of equal component design. Equal capacitors and equal resistors in each section, with gain greater than unity. The math behind this escapes me (those transfer functions again) but I followed the tables given and got the expected results when testing the filters individually. Is there something about this method that could be causing the dip where I should see a rise according to the Rane note?
Also let me preempt a couple of responses I expect:
Why not do this digitally? - Because I don't want to, and because that would be even more beyond me. The ultimate goal is an analog processor.
Anything about SPICE/simulation - I don't have any simulation software. I don't know of any for Mac OS.