How to Build a DIY Passive Summing Box December 8, 2011 12:13 105 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.
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.
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.
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:
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).
DIY Kits
We now stock the SB2 Passive Summing Mixer kits in the DIYRE store. The SB2 accepts 8 or 16 balanced inputs via D-sub jacks and puts out two balanced, microphone level outputs. The kit contains everything you need to build a passive summing mixer. SB2 Passive Summing Mixer Kit $49
Comments
Jason Harris on November 12, 2014 17:59
I was thinking about making a 24 channel mixer not sure if i’m doing the math right. if I wanted it to be 200ohm’s then I could use 10k’s for the input and a 227.27k shunt resistor? or would you recommend the 6.8k’s?
Kan Kaban on November 12, 2014 17:59
Thanks for the great will of maintaining this info online. I´m after a 8ch (4st) balanced network. So…. understood everything. Just one little question. First please check this specs:
Output impedance RME FF800 = 75 Ohms
Input impedance VP28 = Mic 1K2 / Mic+Pad 1K5 / Line 10K
Input impedance RME = 10K
1. Do you think I must use the VP28 in line (10k) or mic (1K2) mode?. If you ask me, I prefer how they sound at 10k line mode. Since the network will go from RME to VP28.
2. Very numb question: do you need the 48v (phantom) running trough the network for it to be balanced?.
Thanks again for your time…. KK.
Peterson Goodwyn on November 12, 2014 17:59
Hi Kan,
1. Either mode will work, since the output impedance of the passive mixer is set by the shunt resistors. So it will be around 200 Ohms. That’s plenty low to go into the 1k2 mode.
2. No you do not need phantom power. It’s passive!
Pablo on November 12, 2014 17:59
Im halfway building this but the bus wire I’m using (kanthal a1 resistance wire) seems impossible to solder. Ive tried different types of flux (rosin and no clean liquid), a temp controlled iron and ipa to clean the wire. I tried different tips (finer pint and chisel) with all different temps, but the joints just don’t hold well. Can I use stranded wire? I find that much easier to solder.
Peterson Goodwyn on November 12, 2014 17:59
Yes, you can absolutely use stranded wire!
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