Balanced Line-Level Stereo to Mono Summing Box

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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).

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Here are a few examples where such a mono-summing box would come in handy:
• You only have one channel input available on your mixer and need to connect a balanced, line-level stereo source to it.
• You need to sum the left and right stereo outputs of a mixer to connect to a single amplifier input for a mono PA system.
• You need to sum a stereo signal to mono for connecting it to one or more subwoofers (low frequencies are omnidirectional, so it’s generally desirable to send a summed mono signal to your subwoofer/s).
• You need to connect the stereo outputs of a studio monitor volume controller to a single active monitor (“grot box”) such as an Auratone, Avantone Mix Cube, Behritone, etc., to test your mix for mono compatibility. This is the primary way in which I intend to use the box in my own recording studio.

This build is slightly more complicated than the regular mono-summing cable. First off, it’s for balanced connections, so you have to solder a resistor (470 Ohm, 1%) into the path of each signal conductor (left +, left -, right +, right -), and there’s also a shunt resistor (20k Ohm 1%) that gets installed across pins 2 and 3 at the output connector. It also requires more parts (two female and one male panel-mount XLR jacks and a box to mount them in), making it slightly more expensive to build. But it’s still a fairly simple project.

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If you want to build your own, check out the Why Not Wye article from the Rane website for more details. Or, if you’d prefer to buy one, check out my ebay listings. If I don’t have one up for sale there, you can contact me directly.

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