Please read the profile before purchasing for a smooth transaction. This is the "Personal Studio ST7," the amplifier unit that is the heart of Yamaha's Silent Brass system for brass instruments. By connecting it to a pickup mute (sold separately), you can listen to your performance with reverb through headphones. It also has an AUX IN terminal, allowing you to connect a music player and practice along with backing tracks. 【Condition of the item】This item was found while cleaning the house and has been stored for a long time. When listing, I performed simple checks such as powering it on with batteries, but I have not been able to thoroughly check the sound output by connecting it to an actual instrument. Therefore, I am selling it as a product in its current condition, and I cannot guarantee that all functions will work properly. There are signs of use such as scratches, small marks, and fine dust accumulation due to aging. Please check the condition of the battery box on the back in the photos (everything in the photos is included). 【Recommended points】 This is recommended for those who "had their ST7 break and are looking for a replacement," or for those who want to keep it for parts or maintenance. This is an old model that is no longer in production, so I hope to sell it to someone who needs it.