One of my most popular blog posts has been The Stereo to Mono Summing Cable that No One Makes. I get contacted pretty much on a weekly basis from someone who wants to buy or build that cable to connect their iPod, phone, laptop, etc. to their PA system. I decided to take the concept a step further and build a balanced, line-level stereo to mono summing box. Just like the mono-summing cable, it appears that there aren’t any readily-available, off-the-shelf products that do exactly what this box does. There are some mic combiners (e.g., Galaxy Audio’s JIB/C) that are made for connecting two microphones to a single input, but those use much lower-value resistors, making them unsuitable for line-level signals (I even contacted Galaxy about the JIB/C, and that’s what they told me). (more…)