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January 16, 2020 / consort3

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:

Extended table with 0.6 to 6k crossover frequencies


More on Kerwins involvement with the Twin -T


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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