Roman, imho look into coil feedback instead. A second coil is added (or
exists) to the existing one and used as instantaneous speed feedback in
the amplifier. This is analog and it's pretty good at suppressing speaker
self-resonance and other artifacts. Someone was making speakers like this
I think. You can also use a capacitive sensor between the metalized cone
center and an extra electrode you provide (coin, disc etc).

Physically sensing position (optical, etc) makes little sense since the
transmission medium (air) does not transmit 'dc' and the static sensor
will have a poorer linearity than the voice coil (which is 0.1% or better
usually). You are mostly interested in motions that were not oredered by
the voice coil, and getting rid of them, so the speed is more interesting
than the exact position of the cone.

The output from the speed sensor is added directly to the normal feedback
loop of the amplifier. It is VERY easy to set up an oscillator like this
(when having 180 degrees phase through mechanical resonances etc). Allot a
lot of time for tinkering and debugging.

Peter

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