DAC (digital-to-analog converter, also called decoder) refers to a digital-to-analog (D/A) converter that converts on/off pulse signals into analog sound signals. DAC has been installed in CD player, but external DAC can improve the sound quality of CD player or other digital players. ■ D/A converter: a device that converts digital audio signals into analog audio signals in digital audio products (such as CDs and DVDs). D/A converter can be made into an independent machine to cooperate with CD turntable, which is often called decoder at this time. I usually say the decoder simply: the decoder is actually a conversion tool device. The main purpose of decoders used in the market is to convert digital signals (such as optical fiber and coaxial) into analog signals (such as channel output on sound cards). Pure decoder needs to cooperate with digital input to work. Of course, some decoders have analog outputs of sound cards such as USB, which is another matter. An operational amplifier is an operational amplifier. Operational amplifier is a widely used linear integrated circuit. Moreover, there are many kinds. In application, it can not only amplify weak signals, but also act as an inverter and voltage follower, and can add and subtract electrical signals, so it is called operational amplifier. It is not only widely used in other places, but also in acoustics. For example, preamplifiers and buffers, except for some discrete components and electron tubes, most headphone amplifiers use integrated operational amplifiers. Sometimes it is used in voltage stabilizing circuit to make high-precision voltage stabilizing filter circuit. Various operational amplifiers have different timbre characteristics because of their different internal structures and distortion components. Originally, what we are pursuing is high fidelity, and the operational amplifier should be the best with the lowest distortion, which can truly restore music and has no personality. However, due to the need to cooperate with other audio components, such as digital sound source, post-amplifier tube, etc., if it is dry and cold, it can be matched with an operational amplifier with a delicate and warm tone, and if it is too soft, it can be matched with an operational amplifier with a bright tone of Leng Yan, so as to achieve the best effect of the whole machine's cooperation and complement each other. Therefore, it is not that the more expensive the operational amplifier, the better the effect. Matching is very important, and the best hearing is the best way to achieve the goal. If it is used in low-voltage analog filter circuit, it is necessary to choose the operational amplifier with good low-voltage performance.