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RE: Can I wind an optimized x-former for my needs?

Hey,

The short answer is Yes... However there is a 600 pound gorilla in the corner is begging for some attention. Here is a basic simulation showing that the behavior with an autoformer can be made to behave the same as an L-pad.





While being rather small the thing that everyone (except fostex) overlooks is the 8.5 ohm R2 in the autoformer example. This resistor does two important things and gives some unexpected results if left out. First it sets the crossover impedance and second it damps the CL resonance that all crossovers have. Here is the same siumulation run with a value of 8.5 ohms and 1 meg for that shunt resistor. (1 meg essentially takes it out of circuit)





Well the first thought is YIKES... but then you realize that the autoformer is reflecting back a higher impedance than the 8 ohms the driver presents so of course the crossover needs to change. So Lets change the crossover values for the 100ish ohm reflected impedance and see what happens.





OK now we are back in business except for one small problem and that is the non-linear impedance curve of a driver. Lets say that out nominal 8 ohm drive starts to increase in impedance as frequency goes down and is actually 12 ohms in the crossover region. The L-pad handles this nicely but the autoformer "magnifies" the impedance change and that nasty LC resonance starts to show up.





This was just a simple "first cut" at some of the things autoformers do that typically get ignored and the moral of this short story is to remember to use a damping resistor across the primary to avoid the gremlinsh that are sure to pop up.

(The people at Fostex knew all of this and if you look inside the T100 you will se a couple of 8 ohm resistors across the primary)

dave






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