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May 9, 2020 / consort3

Inverse Chebyshev filter revisited



The original blog page was entitled 3rd order elliptical filter with one op-amp. The filter was in fact an inverse Chebyshev type. I had converted a simple Butterworth third order type by incorporating a bridged T circuit (C10 AND R14), however that had only one zero. I had not appreciated the significance of R10-13 in the schematic above. Basically they add a zero to the circuit giving a  greater notch depth, for the addition of only 2 resistors per filter. I also figured that it would be easier to tweak resistors rather than capacitors, so here is a new table. I used E96 series values for R1-3 to get the best flatness. An alternative is to use 2 resistors in series or parallel to make the value using a tool like Rescalc.

R1-3 R4 R7 R8 R14 Freq kHz
11.3k 9.1k 3.6k 56k 220k 3.1
12.4k 10k 3.9k 62k 240k 2.9
13.7k 11k 4.3k 68k 270k 2.6
15k 12k 4.7k 75k 300k 2.37
16.2k 13k 5.1k 82k 330k 2.15


Those who subscribe to the “some is good, more is better” school of engineering might be interested in this monster crossover. Put your unruly drivers in a strait-jacket of active filtering. It is a cascade of the above filters. It is actually a sixth order LR crossover with reversed tweeter.cascade


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