Capacitive sensor servo subwoofer

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Since nobody has come up to post an answer.... Are you sure it was capacitative? Seems unlikely given the distances involved in the motion of the cone assembly. Of course, there have been numerous types of sensor suggested over the years.

Could you be thinking of a driver which had a special winding added to the voice coil former that provided a motional feedback signal? There's a Sony driver that fits that description.

In another active thread is a discussion of the latest-and-greatest manufacturer to try MFB and they are using an extra winding to provide the negative feedback. Large pro-sound speakers. Discussed towards end (at present moment in time) of:

Servo controlled subwoofers - why arent they used more often

B.
 
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Here we go, 40 years ago,
 

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Yes. Interesting. The capacitor plates are sliding up and down relative to one another, not getting closer and further away.

Right off the top of my head, I can think of a bunch or reasons it seems unfeasible. All kinds of non-linear physics involved. Hard to control the relative motions of the plates in a moving cone assembly. You need a signal (RF?) which is influenced by the change in capacitance. Humidity in the air.

By comparison, you can take a little piece of sheetmetal and create a photocell mask that changes percent of masking with the cone motion. Which mechanical assembly makes more sense? Hint: neither makes too much sense. And that's why these mechanical approaches are cheap, but not nearly as good as a modern accelerometer.

B.
 
All kinds of non-linear physics involved. Hard to control the relative motions of the plates in a moving cone assembly. You need a signal (RF?) which is influenced by the change in capacitance. Humidity in the air.
B.
They are not plates, they are cylinders. The outer cylinder is put on the inside of voice coil, a metal foil can be pasted on the inside of coil or even conductive ink. The inside cylinder has to be made, it should not be difficult to make a cylinder place concentrically with the voice coil. I did not understand the non linearity in the technique.

Now, it does need a high voltage source, now that may be a problem.
 
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