A feeding device for feeding a material in fibre form to a machine for preparing the material for subsequent spinning comprising a material feeding bin through which material is continuously fed to a substantially horizontal feeding table situated approximate a rotary preparing roll and defining an air gap therebetween, at least one feeder roll above said feeding table for conveying the material towards the air gap, said feeding table being supported to perform movements in a substantially radial direction relative to the rotary preparing roll, and means for positioning and supporting the feeding table which are actuated as a function of data detected by a device detecting the impurity level of the material installed upstream of said feeding table relative to a direction of fibre flow.