Be sure to add me to your favorites list Original Yamaha Pro Audio Sound Information Series Tech Sheets, from 1992. These are 4 original Yamaha information sheets, part of the Sound Information Series of technical information sheets for users of Yamaha Professional Audio Products. The are: Grounding Affects Noise Safety Optimizing Gain Structure Understand Those Specs! Multi-Way Loudspeakers? Three of these original Yamaha sheets are in very good condition, and one (Multi-Way Loudspeakers?) is in good condition as it has some light stains on one section of it. These 1992 Yamaha sheets are both very helpful, and nice to have for the Yamaha collector! I've found that I have another set of the same 4 sheets as well, all in very good to very good + condition. This is a manual or document, this is NOT a piece of equipment or instrument. If you have any questions, I'm always happy to answer them. Thanks for looking, Check out my Store!