Fixed Filters

1^st Order filters

These consist if a single Resistor and a single Capacitor which act as a voltage divider with the Effective resistance (reactance) of the capacitor calculated as XC =1/(2piFC). You will need to take into account the load impedance however the can be followed with an op amp buffer if required.

This filter will provide 6dB/Octave rolloff

1^st Order High-Pass

1^st Order Low-Pass

2^nd order Butterworth / Linkwitz Riley

This filter provides 12dB/octave rolloff but is somewhat more complex. The difference between a Butterworth and other second order filters is the ratio of R1:R2 for the highpass and C1:C2 for the lowpass however the Butterworth gives the steepest rolloff . (FC). The Linkwitz Riley which is extremely popular for active crossovers is essentially two 2^nd order Butterworth filters in series. This means that FC (Cutoff Frequency) for the linkwitz Riley is –6dB per filter instead of –3dB However the big advantage is that the output from the Highpass filter is in phase with the output from the lowpass filter at the crossover point.

2^nd Order High-Pass

2^nd Order Low-Pass

3^rd Order Butterworth

This filter provides 18dB/ Octave Rolloff. Although I have seen this filter often used in Active crossovers I do not recommend it as the Hi and Low signals will suffer from destructive interference at the crossover point as they will not be in phase.

3^rd Order High-Pass

3^rd Order Low-Pass

Phase Shift Circuits

These Circuit provide a phase shift either 90 degrees (lag) or 270 degrees (90 degrees Lead) the output level remains constant i.e they neither attenuate or boost the signal

90 degree lag

90 degree lead

Twin T Notch Filter

This filter can provide a very high Q notch especially if being fed by a very low impedance drive (such as the output of an Op-amp) and feeding a high impedance circuit (again such as an Op-amp)