# TT crossover

I recently discovered the Inverse Chebyshev filter, which has the ripple in the stop band. But there is a dearth of information on how to design these. I found that the free Microcap simulator had an active filter designer which could design the type. So I downloaded the application. It turned out that the technique used to get the notch was the twin-T filter, hence the title.

I aimed for the re-entrance to be 30dB down and to use a single stage. If you make the requirements too onerous, the designer adds stages. I noticed that the high pass and low pass had different configurations of the twin-T circuit. I believe the circuit tweaks are due to Kerwin who confusingly also has a bi-quad filter named after him. I managed to get a reasonable high pass filter but could not easily get a symmetrical low pass type. So I used my trick of changing resistances to reactances and vice versa to get the low pass type. I then tweaked the frequency offsets to get a smooth phase curve for the combined response.

I also removed a stage which was not doing much and buffered the final RC filter. The crossover is third order with the tweeter reversed. Thanks to the erstwhile owner of Microcap whose making it free was the inspiration for this.

Note that the table uses E24 capacitor values which are unobtainium but you can use capacitors in parallel to get the values.

Freq | 1.7k | 1.9k | 2.1k | 2.3k | 2.5k | 2.9k | 3.1k |

Hp null | 742 | 818 | 897 | 984 | 1090 | 1226 | 1345 |

Lp null | 4027 | 4415 | 4852 | 5377 | 5871 | 6517 | 7173 |

C1 | 30N | 30N | 27N | 27N | 24N | 24N | 22N |

C2 | 13N | 13N | 12N | 12N | 11N | 11N | 10N |

C3 | 7.5N | 7.5N | 6.8N | 6.8N | 6.2N | 6.2N | 5.6N |

C4 | 1.6N | 1.6N | 1.5N | 1.5N | 1.3N | 1.3N | 1.2N |

C5 | 5.6N | 5.6N | 5.1N | 5.1N | 4.7N | 4.7N | 4.3N |

C6 | 5.1N | 5.1N | 4.7N | 4.7N | 4.3N | 4.3N | 3.9N |

C7 | 2.7N | 2.7N | 2.4N | 2.4N | 2.2N | 2.2N | 2N |

C8 | 5.1N | 5.1N | 4.7N | 4.7N | 4.3N | 4.3N | 3.9N |

C9 | 9.1N | 9.1N | 8.2N | 8.2N | 7.5N | 7.5N | 6.8N |

C10 | 39N | 39N | 36N | 36N | 33N | 33N | 30N |

R1 | 2.2K | 2K | 2K | 1.8K | 1.8K | 1.6K | 1.6K |

R2 | 36K | 33K | 33K | 30K | 30K | 27K | 27K |

R3 | 18K | 16K | 16K | 15K | 15K | 13K | 13K |

R8 | 10K | 9.1K | 9.1K | 8.2K | 8.2K | 7.5K | 7.5K |

R9 | 18K | 16K | 16K | 15K | 15K | 13K | 13K |

R10 | 16K | 15K | 15K | 13K | 13K | 12K | 12K |

R11 | 6.8K | 6.2K | 6.2K | 5.6K | 5.6K | 5.1K | 5.1K |

R12 | 3.3K | 3K | 3K | 2.7K | 2.7K | 2.4K | 2.4K |

R17 | 56K | 51K | 51K | 47KR1 | 47K | 43K | 43K |

R18 | 12K | 11K | 11K | 10K | 10K | 9.1K | 9.1K |

You can get deeper notches if you fine tune R1 and R18 to make the zeros equal using the tool here:

http://sim.okawa-denshi.jp/en/TwinTCRkeisan.htm

Extended table with 0.6 to 6k crossover frequencies

More on Kerwins involvement with the Twin -T

https://repository.arizona.edu/handle/10150/557787

The notches could be used to take out resonances or break-up peaks on for example, the RS52 metal dome tweeter. So the band pass filter looks like this with -3dB points at 850 and 5.2kHz.

More on this circuit configuration in Williams and Taylor

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