Card holder, also known as "cassette tape recording holder" or "cassette tape recording holder", is a stereo tape recording device. The card holder uses a standard cassette tape. The cassette tape is packed in a rectangular plastic box of 10cm×6cm, the tape width is 3.8 1mm, the tape surface has four tracks, which are divided into two playing surfaces: A/B, the tape thickness is between 9 microns and 18 microns, and the tape length is 30 minutes, 60 minutes and 90 minutes. Different from ordinary cassette recorders, high-end card holders usually adopt high-end technologies such as three-head, synchronous motor, closed-loop drive and Dolby noise reduction to obtain high-quality recording and playback. Usually, the card holder does not include power amplifier and speaker, so an external power amplifier and speaker system is needed.

In performing arts bars or leisure clubs, restaurants are usually two sofas facing each other with a small table in the middle.

Power amplifier, referred to as power amplifier, is an electronic device used to enhance signal power to drive speakers.

Power amplifier without auxiliary functions such as signal source selection and volume control is called post-stage.