I believe this is a Bartolini preamp add-on unit for a bass. It's an old item, so my memory is hazy, but I'm pretty sure I bought it for a bass. I searched online, but it doesn't seem to be a current product. I would appreciate it if someone knowledgeable could judge from the images. Since the information is uncertain, I will treat it as junk. There are two units. They are not used simultaneously. Thank you.