This is a set of four BEHRINGER monitor headphone amplifiers. They were very useful for using a click track in live performances. They offer a surprising degree of connection flexibility. For example, you can connect a microphone and monitor it through headphones, and also send the signal through to a mixer. You can daisy-chain them to send a synchronized click track to up to four people. They currently sell new for 4,980 yen at Soundhouse. They have been stored in a smoke-free, pet-free environment, and have only been used in live performances about four times. The set includes four adapters. One of the four units has a chipped area (see photo 5). Thank you.