https://docs.google.com/viewer?url=http ... edback.pdf
A motional feedback system is a combination of a power amplifier and a loudspeaker in which (a part of) the feedback is obtained from the movement of the cone of an electro dynamic loudspeaker. In the picture at the left the difference is shown. On top you will find a normal power amplifier and speaker system. Depending on the function β, which often is just an attenuator with a flat amplitude – frequency response, the amplification
of the amp is determined. Moreover a very important function of negative feed back is the reduction of nonlinear distortion!
In the picture below the so called motional feedback system has been displayed. The frequency response of the whole system is determined by β, the conversion of the input voltage to the piezo output and that of the correction. In all feedback (every time the word: ‘feedback’ is used, I
mean: ‘negative feedback’. In Dutch: ‘tegenkoppeling’) systems one has to deal with stability problems. The larger the bandwidth of a system the more feedback could be applied (e.g. with a unity gain stable op amp the gain could be reduced to 1). We all know the stability problems with a tube power amplifier because of the limited bandwidth of the output transformer. They are that serious that many people prefer such amps without any feedback….
But why should you construct a motional feedback system? Well, it flattens the acoustic frequency response and reduces the distortion even in a (very) small enclosure set up!