Abstract: An analog filter is connected at the output of a D/A converter. The analog filter eliminates higher frequency components than the original basic frequencies for a digital audio system and improves the phase characteristic. The analog filter comprises a plurality of band elimination filters that have respective cutoff frequencies of integral multiples of the sampling frequency fs. An audio amplifying circuit is connected to the analog filter. Each band elimination filter eliminates side band components close to integral multiples of the sampling frequency. By such construction, the analog filter has the necessary amplitude characteristic and lessens phase shift within the basic frequency.

Abstract: In a direct coupled type vacuum tube amplifying circuit, to stabilize the operation and simplify the feedback loop, a plate power source EB5 is connected to a plate of a vacuum tube V5 of a first stage through a constant current source 1. A grid of a vacuum tube V6 of a next stage is connected to the plate of the vacuum tube V5 through a first resistance R3. A second resistance R4 is connected between the grid and the cathode of the vacuum tube V6. A negative power source EC6 is connected to the resistance R4 through a constant current source 2. In this constitution, the grid bias voltage of the second vacuum tube V6 is defined by the voltage drop of the second resistance R4.