# 3rd order low pass filter table

Just to finish the 3rd order theme of the last post here is a Butterworth low pass filter table with 3 equal resistors.

C1 | C2 | C3 | 20K | 22K | 24K | 27K |

33N | 82N | 4.7N | 340.2 | 308.9 | 283.3 | 252.4 |

47N | 120N | 6.8N | 236.1 | 214.4 | 196.6 | 175.2 |

68N | 180N | 10N | 160.8 | 146 | 133.9 | 119.3 |

100N | 270N | 15N | 107.9 | 98 | 89.8 | 80 |

150N | 390N | 22N | 73.3 | 66.5 | 61 | 54.3 |

220N | 560N | 33N | 49.5 | 45 | 41.2 | 36.7 |

So if you want a filter with 50Hz cutoff look down the 20K column and you see 49.5Hz, the nearest value, so the capacitors are 220n, 560n and 33n and the associated equal resistors are 20k. The table covers a decade of values. If you want 500Hz as opposed to 50Hz you scale the capacitors by a factor of 10, so the capacitors would be 22n 56n and 3.3n. Alternatively if you wanted 16Hz the capacitors would be 680n, 1.8u and 100n

Here is a Bessel filter table, the frequencies are the -3dB points.

C1 | C2 | C3 | 20k | 22k | 24k | 27k |

22n | 33n | 5.6n | 366 | 330 | 303 | 269 |

33n | 47n | 8.2n | 242 | 223 | 204 | 179 |

47n | 68n | 12n | 168 | 150 | 142 | 125 |

68n | 100n | 18n | 115 | 104 | 95 | 85 |

100n | 150n | 27n | 77 | 70 | 65 | 57 |

150n | 220n | 39n | 52 | 47 | 44 | 39 |

PCB’s to produce 3rd order crossover filters available here:

http://www.kk-pcb.com/cross.html

Never mind the old TL074 try the MC33079 quad op-amp for this

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