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March 9, 2019 / consort3

Hardman crossover filter

Readers of this blog will be aware of my efforts with the notch type of crossover filter. Bill Hardman in Electronics World  Aug 99 came up with a crossover using a notch. Bill pointed out that the group delay of his filter was better than high order LR crossovers, while having a similar amplitude cut-off to 6th and 8th order LR types.  I guess a similar argument applies to my notch crossover types bearing in mind the order. The phase tracks for the circuit.

Bill used  Bainter filters to provide the notches. These have 3 op-amps, which offends my sense of economy but give a sharp notch and  use only 6 resistors and 2 caps. The depth of the notch is not dependent on components values or tolerances but on the gain of the op-amps at  the notch frequency. Thus it may be worth investing in more op-amps!  The component count is roughly similar to the Thiele notch crossover.

I used a 10.9 dB gain amp on the tweeter to get the equalized levels shown. R3 is made up of a 2.7k and a 220R in series and R9 is made up of a 47k and a 130k in parallel. The Bainter notch filter can be configured as low pass, notch or high pass .Bill uses the LP and HP with an underdamped response compensated by the overdamped equal component Sallen & Key output filters. If R3 is less than R4  you get a LP and if R9 is greater than R10 you get a HP.

The easiest way to scale the design from the original 1.5kHz is to change all capacitors to 3.9n for a 1.8kHz crossover frequency. 3.3nF caps give you a 2.15 kHz crossover and 2.7nF caps give you a 2.7kHz crossover.

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Bill also included a tweeter delay circuit to compensate for the acoustic centre misalignment problem The delay is 22.5uS

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