1. Field of the Invention
The present invention relates to a music reproducing device that outputs an audio signal to a headphone.
2. Description of the Related Art
In a headphone output of a music reproducing device that outputs an audio signal to a headphone, there are systems that are called unbalanced output and balanced output (for example, see JP 2013-005291 A). In an unbalanced system, a three-pole terminal with a diameter of 3.5 mm is used, and the audio signal is transmitted by two kinds of “hot” and “cold”. Meanwhile, in a balanced system, a four-pole terminal with a diameter of 2.5 mm is used, and a signal is transmitted by three kinds of “ground”, “hot” and “cold”. “Cold” is opposite phase of “hot”. In case that external noise occurs, noise of the same phase is superposed on both “cold” and “hot”. By inverting phase of “cold” and mixing inverted “cold” signal with “hot” signal, external noise is cancelled and amplitude of audio signal becomes twice. For this reason, the balanced system is strong against noise and sound quality is good in the balanced system.
FIG. 11 is a diagram illustrating amplifiers that amplify the audio signal in the balanced system. A positive-phase signal is input to one amplifier. The one amplifier amplifies the positive-phase signal. A negative-phase signal is input to the other amplifier. The other amplifier amplifies the negative-phase signal. The amplified positive-phase signal and negative phase signal are mixed. Hereinafter, the balanced system is called the balanced mode.
FIG. 12 is a diagram illustrating the other system that amplifies the audio signal using two amplifiers. An audio signal is input into one amplifier. The one amplifier amplifies the audio signal. The other amplifier is connected to ground. For this reason, output of the other amplifier is maintained to 0V (ground). In this system, ground is stabilized by the other amplifier. and noise can be reduced without burden on the amplifier. Hereinafter, the other system illustrated in FIG. 12 is called active control ground (hereinafter, referred as to “ACG”) mode.
In the music reproducing device, a user can select either mode of preference by enabling to switch the balanced mode and the ACG mode. However, amplitude of the audio signal that is output in the balanced mode becomes twice compared with amplitude of the audio signal that is output in the ACG mode. For this reason, volume of the audio signal that is output in the balanced mode also becomes twice compared with volume of the audio signal that is output in the ACG mode. In both mode, volume is different each other and it is difficult for the user to judge difference of sound quality because sound quality depends on volume.