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Old 16th January 2013, 03:54 PM   #1
Join Date: Dec 2011
Default need help to wind Autotransformer (-d)

Hello world,

some speaker XOvers (like Klipschorn) use Autotransformers to do the -d function to attenuate the voltage steering to any of the drivers. some other simply use L-pad resistors. I want to wind Autotransformer which is able to do -3, -6, -9 and -12db of attenuation. I want information about the the turns number and everything I need to wind it. I'll use 57EI lamination which is common to wind this kind of Autotransformers.

all help is much appreciated

Last edited by oldies; 16th January 2013 at 04:01 PM.
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Old 31st January 2013, 09:00 AM   #2
Join Date: Dec 2011
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Old 31st January 2013, 10:26 AM   #3
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speaker dave's Avatar
Join Date: Nov 2009
Location: The Mountain, Framingham
Tapped Autotransformers

If you can log into Lansing Heritage there are some threads on it over there, as JBL used them from time to time.

Not too hard to design yourself if you do some experimenting.

Some hints: inductance is proportional to turns squared. Double the turns and quadruple the inductance. Pick a suitable gauge wire and start winding and measuring and you will easily figure out the constant for your core material. L = n sqrd constant.

Taps are just voltage drops in proportion to total winding length. That is, if you need 3 dB step down, that is 70% of voltage or, say, 30 turns from the end on a 100 turn coil. Calculate what you need or use a voltmeter and AC (say, 1 kHz across total coil).

David S
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