SB2 Passive Summing Mixer Kit


16 channels of passive, analog summing in one tiny box.

The SB2 is an 8/16x2 passive summing mixer in a ultra-compact format. Using minimal, completely passive circuitry, it sums 8 or 16 balanced line inputs down to a single set of stereo mic-level outputs. Pair the SB2 with your favorite pair of mic preamps to add glue and color to your mixes.


  • Ultra-low cost and small footprint
  • Accepts 8 or 16 balanced inputs
  • DB-25 (D-SUB) input connectors
  • Balanced, microphone-level outputs
  • XLR output jacks
  • Neoprene foam pad for desktop use

What the Pros are Saying

"As a mixer that works almost exclusively in the box, there are some tracks that need the warmth, glue, and spacing that only analog can offer. When I run my mix through the SB2 paired with CP5s loaded with Rogue-tec Airs, I get that great low-mid presence that I'm looking for along with airy highs and a great stereo image. Other analog summing always made me feel like something was changed in the balance of my mix, but the SB2 gives me the analog enhancements I'm looking for without sacrificing anything. I currently have this setup replacing summing units in my home room at Lounge Studios NYC worth over $10k combined."

-Mikaelin "Blue" BlueSpruce
Solange, Mariah Carey, Blood Orange

Input Impedance 10k balanced 
Output Impedance 150R balanced
Crosstalk (1kHz) <-80dB

8 or 16

Outputs 2
Attenuation 45dB
Recommended Load Impedance 1.5k (standard mic preamp input)
Recommended Source Impedance 100R (standard line output)

How is the SB2 so inexpensive? Most summing mixers cost at least 10x more.

The SB2's radical affordability is due to its radical simplicity. By stripping the passive summing concept down to its essentials and "outsourcing" the pan controls (see below), makeup gain (to mic preamps), and assembly (to you!), we're able to make the SB2 low-cost and pass the savings on to you.

Why are there no switches/knobs? How do I pan the inputs?

You may have noticed that typical summing boxes are rather expensive, despite containing limited circuitry. This is mostly because they feature panning knobs or switches. In addition to being expensive themselves, these controls necessitate a larger front panel, case, and circuit board.

The SB2 bypasses all of these costs by "outsourcing" the panning to your DAW. Each of the SB2's inputs is hard-wired to one of the output channels: odd-numbered inputs to the left, and even-numbered to the right. You simply send pre-panned stereo buses to your interface's hardware outputs and connect them to the SB2. For example:

1/2: Drums
3/4: Vocals
5/6: Guitars
7/8: Keys/Effects

For a single track you want panned center, simply send it to a pair of outputs.

Do I need to provide makeup gain after the SB2?

Yes, being completely passive, the SB2's outputs require 45dB of makeup gain to be returned to line level. The best way to do this is with a pair of mic preamps.

Does the SB2 have a "sound"?

Nope, a passive summing mixer has essentially no sound of its own. The sound of your summing setup will be determined completely by the mic preamps you use for makeup gain. Indeed, one of the unique benefits of passive summing is that you can tailor your makeup-gain preamp choice to each mix.

What is the circuit design based on?

The SB2's circuit is a classic passive summing network--the same as found in countless mixers and consoles. The only difference is that instead of dedicated makeup amplifiers, the SB2 has mic-level outputs which must be connected to mic preamps. The circuit is the same one shared by NYDave on GroupDIY and featured in How to Build a Passive Summing Mixer.

Do you sell an 8-channel version?

You can use the SB2 as an 8 or 16-channel mixer. Just use inputs 1-8 for 8-channel operation.

Can I daisy chain two SB2s for 24 or 32-channel summing?

No, there is no way to daisy chain two units. We made this design decision because the performance of this kind of passive summing circuit degrades after about 20 channels.

Does it come with D-sub breakout cables?

Nope, cabling is up to you. Want to feel like a real DIY ninja? Build your own D-sub cables!

Can I use the SB2 with unbalanced gear?

No, the SB2 only works with balanced inputs and outputs.

Customer Reviews

Based on 50 reviews
Chad W
Simple and AWESOME

I built this a while ago but tried it for the first time when my mixes through a Midas Venice F32 weren’t sounding that great. This little box, run into CP5 loaded with CTX, then into my EQP5 (with vintage transformer) is simpler than using a big console, easier to recall, and sounds way better. I’m using 8 channels from the interface into this unit. Would prefer to use the whole 16 but I don’t have the proper snake. Even using 8 inputs (4 stereo) sound better than my big console.

Great summing mixer as part of a system with DIYRE preamps and EQs

Rene Dalderop
SB2 Passive Summing Mixer Kit

I purchased the SB2 as an addition to my Dangerous Music 2Bus. Just needed more channels and I wanted to try an affordable solution before taking the plunge and get a second 2Bus. Putting the SB2 together was easy enough as it came with clear instructions. I’m using two CAPI VP26’s to amp the signal and have just completed the first mixes with the new setup. In a nutshell; it is a fantastic addition and works beautifully. In all fairness, it may not be as clean as the 2Bus, possibly because of the preamps used but it definitely stands its ground. It’s really quiet and translates the character of the outboard feeding it very well. I’m now considering some relatively simple additions like mono switches or perhaps stepped switches on the inputs, this little box is here to stay!

Travis Atkinson
Custom ACA summing using SB-2’s

I’ve used SB-2’s with mic pres in the past with great success, but always had a multi stage grouped summing idea in mind, not using mic pres- more of a console style system.
I built a CAPI ACA system with four stereo pairs of groups (8 channels each) that sum into the master CAPI ACA boards. I’m able to keep the master section clearer sonically and mix and match on the sub group pre summing pairs with my choice of discrete opamps. The master ACA has JLM 990a and Scott Leibers SL-1731 opamps in it, and a mixture of CAPI 0252, AM-10, JLM 99v, and Gar 2520 opamps in the group summing stages.
The SB-2’s are used as 8 channel pre summing boxes that go into the ACA group summing amps, then to the master ACA, loaded with transformers.
I removed the shunt resistors and they worked phenomenally as an affordable elegant solution for this setup.
Thanks so much to Peterson for the great kits, and the help when I needed to reach out about nodding the boxes!
If anyone has any questions or would like to do a similar thing, don’t hesitate to reach out.

Daryl Stacks
Very solid kit!

Super easy to build, well built and very compact. Minimal sound testing done but sounds clean as far as any unwanted noise issues, very promising. Nice work!

Ole Jensen
Nice and easy

Easy to assemble . Sounds neutral and fine

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