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In Reply to: RE: Amandarae's beautiful thread posted by Randal P on January 01, 2017 at 17:41:48
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.
Follow Ups:
I have a pair of 3 ohm caps and 8 ohm 802D's as well as a pair of Werner Jagusch autoformers. I can't find it at the moment, but he shows a variable resistor in parallel with the primary of the autoformer. Presumably the autoformer will change impedance based on attenuation.
Bob Crites' 3636 autoformers are decent, have 1dB steps and currently sell for $35 each. One could use a copper alligator clip on a jumper to go around the taps. Sometimes its nice to use the high Z approach as smaller capacitors are generally less expensive than large. A swamping resistor across the driver can be used. Klipsch touted the high reflected Z reduced "Transient Intermodulation Distortion" (Otala).
3636 AUTOFORMER
Karlson Evangelist
What would the swamping resistor do in this case?
a swamping resistor would reduce the variation in impedance and possibly, response.
Karlson Evangelist
I have a pair of 3 ohm caps and 8 ohm 802D's with a pair of Werner Jagusch autoformers. In one of his diagrams he shows just such a variable resistor.
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