medium frequency speakers 20gds-3. soviet vintage acoustics, s of a music column

audio equipment midrange speaker suspension diffuser repair 20gds3 20gds 15gd11 basket cap coil magnet ferrite vintage frequency revision upgrade soviet professional tweeter studio sound technology monitor driver loudspeaker high quality powerful listen box metal equipments classic membrane preservation back dj electronics russia sixties device nostalgia radio engineering recording lowfrequency grids