This is a headphone output to line input step-up transformer kit from Akizuki Denshi. I purchased and built this kit, intending to use four circuits with four transformers, but it became extra after I changed the design, so I'm offering it for sale. Operation has been confirmed, and it works well. Akizuki Denshi Kit Description: Have you ever had trouble with the volume being too low when connecting the headphone output of your smartphone or portable music player to the line input of your car stereo compared to other sources? Or have you found it troublesome to adjust the volume? Also, have you ever been surprised by a sudden loud sound when switching sources? This kit can help with these issues. These problems occur because the voltage of the headphone output is lower than the voltage required by the line input. This kit uses a transformer to step up the voltage, reducing the difference in volume compared to other sources. By eliminating the need for a power supply, you don't have to worry about battery degradation in the car, and wiring is easier. The input and output are isolated, which also helps to isolate the connected devices. It is also effective for cutting off GND loops (ground loop isolator). This is for those who have a good understanding of hardware. Due to the nature of the product, no returns or exchanges are accepted.