The invention concerns an online low frequency amplifying interface operating in automatic cutting feed, particularly designed for multimedia computers. It concerns a impedance adapting device produced around an electronic tube. It can be arranged between a source transmitting low frequency signals and a receiver. The invention is characterised in that it comprises an low frequency amplifier sub-part having high impedance at the source whereto it is connected, and having a low impedance at the receiver whereto it is connected next. The high voltage power supply of the anodes of the low frequency device is produced by a working high frequency automatic cutting feed regulator feeding a voltage amplifying device. The automatic cutting feed regulator operates in direct current low voltage. The inventive device is particularly designed for sound boards and for sources of hi-fi installations, whether incorporated or not.