Common class D audio power amplifiers are based on semiconductor devices
used as electronic power switches directly applying the power supply to the loudspeaker for very short pulses obtained by the Pulse Width Modulation of the incoming audio signal.
Power semiconductor devices can be replaced by vacuum tubes if the tubes can reach a very high speed switching rate and a low resistance conduction state.
High speed switching rate can be obtained by the means of signal integrity theory while the low resistance on state can be allowed polarizing the tubes in an unusual way described only at the beginning of their history and then completely forgot.
With these novel audio amplifiers a full digital audio path can be developed from the source to the loudspeakers but maintaining the special psycoacoustics perceptive feeling of the tubes.