Build Bus’tr – a summing box to add two mixing boards together
We’ve all been in a fix from time to time when we needed to sum two audio signals into one. Much of the time, we need to convert a stereo signal into mono. The other situation is typically when you need to add the outputs of two mixing boards together, summing the left, right, effects and monitor outputs into common lines so that two boards act as one large mixer. For a church situation it could be that you need to add a “side-car” console beside your main mixing board so you can plug in another 8 mics for the youth Christmas show, or perhaps add a temporary console to handle wireless mic inputs for your Passion Play.
The quick but incorrect fix is to plug everything together with “Y” cords from Radio Shack, jamming two outputs into one with typically disastrous results. If the output drivers of the gear to be summed have build-out resistors, sometimes this will work. But most semi-pro and pro audio gear is designed around a 600 ohm output stage. If you attempt to combine most pieces of gear this way, bad things can happen, the simplest being a large signal imbalance between the two pieces of equipment. In some cases severe oscillations and signal distortion will result, with possible circuit failure. Yes, you can blow up the outputs of a mixing board this way, so don’t use simple Y-cables to combine them.
Mackie used to sell a very nice active bus mixer called Mixer Mixer, which did individual buffering and allowed for master volume controls to be added where desired. But that’s been a decade or more so don’t be looking on Craig’s List for Mixer Mixer. Time to build your own. My original Bus’tr I built back in 1990 isn’t quite so fancy: I designed it into fit in an aluminum project box with 1/4” phone jacks and 680 ohm build-out resistors. To use it, you just hook the outputs of anything you want to mix together into two of the jacks, and the signal comes out the third jack, simple as that. You can modify Bus’tr to sum as many separate channels as you like, from two to dozens of buses are possible. Don’t try to make it something its not. By modifying it to allow more than two signals to be summed into a common output, the losses become too great very quickly.
I usually build my own Bus’tr boxes with eight sets of inputs, allowing me to combine all of the functions of two mixing boards to feed my graphic equalizers and crossovers. There will be a 3 dB insertion loss the two boards because of the passive summing, but normally you’ve got lots of extra signal, so losing a little output won’t hurt you a bit.
Notice that all 3 jacks have a build-out resistor, allowing any two jacks to act as an input. Also, since this is a purely passive circuit, you can use this box to route a single channel into two outputs, while adding 680 ohms in line. This won’t affect the signal at all, except under some conditions of very long line runs.
You can substitute RCA jacks for the 1/4” jacks if desired, just be sure that the equipment you’re hooking together is rated at 600 ohm or less output impedance. Higher impedances can be made to work by substituting a large resistor value, but I’ve never encountered a modern piece of musical gear where this is necessary. Also, if you want to keep your signal balanced using TRS (Tip Ring Sleeve) or even XLR jacks, all you need to do is duplicate the same resistor trio on the “Ring” circuits for TRS or Cold-minus (pin 3) on XLR circuits. The actual value of the summing resistors isn’t too critical, with anything from 220 ohms to 1,000 ohms probably working just fine (just keep all three resistors in a stack equal) and even 1/8 watt resistors should work just fine. You can find resistors like these in 5-packs for a buck or two at Radio Shack or Allied Electronics, so even without raiding your junk box, Bus’tr only costs about $5 a channel in parts to build, so this is a cheap and easy project. A fancy Bus’tr in a nice aluminum case shouldn’t set you back more than a couple of Jacksons.
I always keep Bus’tr handy in my gig box for those times when I’ve got to hook up a second mixing board in an emergency, feed a press mult when I don’t have an extra output on the board, or simply make stereo into mono for a delay stack, and it has never let me down. There are no batteries to wear out, no possibility of overloading active circuits with excessive signal levels and no induced noise. Plus it’s cheap to build. What’s not to like?
Copyright Mike Sokol 2015 – All Rights Reserved