Linear Variable Differential Transformer?

discussion of magnetic volume control design for both line and speaker levels.

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dirkwright
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Linear Variable Differential Transformer?

Post by dirkwright »

Hello David, have you ever considered the Linear Variable Differential Transformer as an audio attenuator? It seems to me that this is something that could be really great. No steps. You would need two for stereo obviously.
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dirkwright
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math?

Post by dirkwright »

I have no idea right now how to do the math needed to determine if this could be practical for audio. Obviously, the moveable core would have to be high perm metal like mu metal or nickel/iron. There would have to be a lot of turns of wire in each coil as well, but the good part is that it's easy to make the coils since they are just on a tube former. The tube has to be really thin though. I found some 20mm diameter pure nickel rod. I have not found other suitable metals yet.

The rod would have to be long enough so that the central coil is never completely empty, since that would dramatically reduce the input inductance and thus the impedance.
dirkwright
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Post by dirkwright »

Alibaba lists a few suppliers of mu metal rod:
https://www.alibaba.com/product-detail/ ... 9602Un1lLc
dave slagle
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Post by dave slagle »

interesting idea but I think the achilles heel is going to be that all of the coupling must be by magnetic core in order for this to work and that will be quite problematic at high frequencies.

I realize that the general excitation frequency of this can be in the 10khz range but I think the units need to be calibrated for whatever single frequency they work at since the coupling changes as the frequency does.

(to put this in transformer terms, there would be high leakage and this would behave about as well as a 1:1 wound on a split bobbin with the primary on 1 half and the secondary on the other)

dvae
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dirkwright
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Post by dirkwright »

OK thanks.
dirkwright
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Post by dirkwright »

You may be reading the schematic too literally. See the coil winding they use in this reference: http://nees.buffalo.edu/pdfs/lvdt.pdf

Also, since we only need attenuation, then we only need one secondary coil.
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dave slagle
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Post by dave slagle »

The theory of how it works dictates that the two secondary coils are unity coupled when the core is centered completely cancelling any output. When the core is such a position where one of the secondaries has zero coupling to the other two coils you would get full volume. The only way to get this behavior to happen is to design a device with high leakage inductance so 100% of the signal is coupled through the core which is the same way you make a really crappy audio transformer.

the one cool thing that this method would allow is for phase reversal of the signal depending on which way from null you moved the core.

dave
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dirkwright
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Post by dirkwright »

But yeah, you're right, these are probably too difficult to make with wide bandwidth. They have significant phase shift, for example.
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Lecture 7 - LVDT - Mechatronics.pdf
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