This simple circuit allows for control of the speaker cone. The power amplifier operates as a current amplifier. Without regulation caused the current amp a strong peak in the resonant frequency of the chassis. To avoid this, the voltage is measured at the voice coil and is compared with the input voltage. An opamp before the current amp corrects the frequency response. This applies not only to the resonant frequency but also for the whole band. As simple as it works you can't control the mid neither never mind the treble with an other technology.
The simulation works well so far.
The simulation works well so far.