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RE: Amandarae's beautiful thread

Posted by D Mike on January 2, 2017 at 05:39:24:
The crossover I use just has a series cap, a variable shunting resistor across the autoformer inputs, and the autoformer itself.

I had a nice pair of matched oil caps tested at 2.07 uF. To get the desired crossover point around 7400 Hz, I needed a reflected impedance seen by the cap of about 10.4 ohms. The reflected impedance is the combined parallel resistance of the shunting resistor and the impedance seen from the autoformer. I'm using 9 dB of attenuation, which has a turns ratio of 2.833. The autoformer impedance is the 8 ohm driver x the square of the turns ratio, which equals 64 ohms. To get to my desired 10.4 ohm combined impedance, the shunting resistor needs to be 12.4 ohms.

By using a variable wire wound shunting resistor, I can tune the crossover frequency up and down using the same cap value. There are other ways to use an autoformer to get the desired results. This way seems to work well for me.