Compact PDF schematic whose name points to a parametric equalizer combined with a subwoofer filter stage. Circuits like this are usually op-amp based and sit between the source and the power amplifier to shape frequency response and feed a low-pass signal to the sub. The file is small and names no maker or model, so read the drawing for component values and supply requirements. It is useful for building a preamp section or checking an existing board stage by stage.
The button opens the matching file on RM Schematics, or a source page if it is not catalogued there yet. The original attachment is from @pdf_servicemanual. Enginerys does not host this file.
How to open a .pdf file
The standard format for schematics, block diagrams, power-sequence charts and service manuals.