How to Build a DIY Passive Summing Box December 08, 2011 12:13 102 Comments
How It WorksMost 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.
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 TogetherBuilding 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.
Bill of Materials
|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|