How to Build a DIY Passive Summing Box December 08, 2011 12:13 102 Comments

I've been curious for a while about the fuss surrounding analog summing. But with commercial boxes priced $600+ I wasn't that curious. Luckily, passive summing is a pretty simple concept and one that's very wallet- and DIY-friendly to achieve. Check out the video above for a quick run-through of how I built my 16-channel summing box.

Contents:

How It Works

Most analog summing designs take a number of mono inputs and assign them to the stereo bus via either pan pots or L-C-R switches. This route, while superior in functionality, is rather complex and expensive. The design below leaves out panning/assigning and makeup gain in order to avoid the need for power and forgo toggle switches (≈$7.50/channel). What we end up with is a passive bus mixer: 4 or 8 stereo pairs are summed via L and R buses, which are then sent to external microphone preamps for makeup gain. This basic design has been proposed by "New York Dave" and Fred Forsell, and can be found in the monitor section of old Neve consoles.
The greatest benefit of this approach, in my opinion, is the tonal flexibility. Every pair of preamps will impart their own sonic signature on your mix.
Passive Summing Schematic

Inputs are summed to the bus wires via resistors. All of these resistors must be the exact same value with as low a tolerance as possible. Recommended values are between 5-10k ohms depending on the input impedance and volume drop you're aiming for. The value of the shunt resistors depends on the number of inputs used and the value of your summing resistors. NYD gives us the formula for calculating it, where I is the input impedance, R the desired output impedance, and N the number of channels: (I/N)*R/((I/N)-R)

Putting It Together

Building a passive summing box always seemed like a bit more of a hassle than it should be for such a simple circuit until I found Rhythm in Mind's idea for mounting everything on a front panel. It's a great approach for this project, as all of the components weigh in at a grand total of around 3 lbs. Simply drill a panel as I've drawn below (or order one!) and mount components right to the panel as shown. I used nylon standoffs to suspend the bus wires with lock washers to hold them taut. diy summing box I've had no problems with noise or interference with this setup. But if you do find it to be noisy, you can always add a case.

Bill of Materials

Qty Value Note
8 1/4" TRS jack Panel mount, solder terminals
2 182Ω for 16ch 165Ω for 8ch Metal film, 1/4 watt, 1% tolerance resistor
16/32 6.8kΩ Metal film, 1/4 watt, 1% tolerance resistor
4' Bus wire At least 24 AWG thickness
6 1 3/8" Nylon standoff Female->male, 6-32 or 4-40 thread (must match other standoff)
6 3/8" Nylon standoff Female->male, 6-32 or 4-40 thread (must match other standoff)
6 Machine nut 6-32 or 4-40 thread (must match standoffs)
6 Lock washer #6 or #4 size (must match standoffs)
2 Male XLR jack Solder cup terminals
4 4-40 nuts and bolts For mounting XLR jacks

Using It

Since the inputs of our summing panel are hard wired to either the L or R bus, you can't simply send a mono signal (unless you want it panned R or L). Rather, assign your tracks to a number of stereo buses and then send those buses to the summing panel as shown below: passive summing diagram

Update: Adding Mono Inputs

Since posting this article, I've received a lot of questions about adding mono inputs for tracks like lead vocal, bass, kick, and snare that are often panned dead center. Luckily, this is really easy to implement with just a little more math. Of course we know that in the world of stereo, a signal that is "dead center" is actually two identical signals of the same volume in each channel. So instead of sending our mono inputs to either the L or R buss wires, we will send them to both. However, by doubling the signal, we've made our mono input 3dB louder than the others. We can rectify this by changing the input resistor values for the mono channel to 1.4x the value of the regular input resistors (1.4:1 being 3dB expressed as a ratio). For example, I used 6.8k Ohm input resistors for the "Endless Summer" kits. 6,800 x 1.4 = 9,520. The closest value stocked by Mouser is 9.53K (very close!), which are now included in the kits. Keep in mind that if you change the number of channels in your summing box, the ideal values for the output shunt resistors will change as well. Use NYD's forumula at the bottom of the "How it Works" section to calculate the correct value for your number of inputs. Any questions, ideas, or suggestions for this project? Let me know in the comments below.