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May 16, 2020 / consort3

4th order elliptical Linkwitz-Riley Xover

You can make a 4th order LR with 2 cascaded 2nd order Butterworth filters. I found you can cascade 2nd order elliptical filters and have the LR features i.e. 360 degree phase change and -6dB  at the crossover point. There is the notch with reversed tweeter that is useful for checking the phase. It makes better use of the op-amps than merely buffering an R-C filter on the third order types. I used the Microcap filter designer. Crossover frequency is 1.955 kHz. I set for the HPF Passband ripple 0.75dB Fc 2.4kHz stop atten. 20dB stop F 900Hz and for LPF Passband ripple 0.75dB Fc 1.6kHz stop atten. 20dB stop F 4kHz. I ignored the gain setting components and took the nearest E24 values. Impedance scale factor is 0.2.

LR24ellip1

LR24ellip

When I tried this with more realistic op-amp models, I got the tendency to oscillate at 3 or 4 mHz so put in the output resistors shown below. Not shown are the 0.1uF supply line decoupling capacitors required.

Lr24ellip2

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