This is the "HM-400" from MACKIE. It's a 4-channel headphone amplifier that can connect up to 12 headphones. Up to 3 headphones can be connected to a single channel. Each channel features low and high EQ, individual ear muting, stereo/mono switching, and AUX input. Please check the manufacturer's website for detailed specifications. I purchased this about 3 years ago for recording choral performances. Actual usage time is about 3-4 hours. After that, I stored it in a rack, hoping to use it again, but I haven't had any opportunities to use it in my usual work, so I'm selling it to organize my equipment. I used it on a site where about 10 people were monitoring simultaneously and repeated dubbing for about 10 groups, and it worked perfectly. I checked the operation again before listing it, and it's working without any problems. Although it was stored in a rack, I don't think there are any noticeable scratches. I think it can be used in various ways, such as recording and live monitoring, rehearsals with in-ear monitors, and simultaneous listening for multiple people. The sound is also clear and solid, typical of MACKIE. The condition is quite good, but it's a used item, so please check the condition in the photos. Thank you for your consideration.