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 37 reviews
Isabelle Olivier
Warmth and Depth

That was my first ever DIY project. I'm super happy with the result. It gives warmth and depth to my mixes. Would give 10 stars out of 5 if it was possible.

Addy Tsoi
SB2 summing box

Read a lot of reviews before the order.
Yes, the box with my ez1073 added depth and dimensions. Me and my clients Loooooovvvvveeee it.

Theo MIserlis
Amazing Little Box!

I've been wanting to build the SB2 for awhile now but it's been out of stock until recently. So, when I saw they were in stock again I excitedly ordered one. Delivery was quick and while waiting for it to arrive I familiarized myself with the build instructions. It is a simple kit to build, but if you have zero soldering experience study the technique necessary, be sure to use a fine tip on your gun and take your time.

As a passive unit there's always a little concern until you get audio passing through it. I calibrated it in Logic with the Test Tone oscillator routed to four stereo outputs inputing the SB2, then outputting to my mic pre for the makeup gain, a Phoenix Audio, at unity gain going back into Logic and everything was working as expected. Delight!

My first mix through the SB2 was a song I recently released that I was pleased with the mix (ITB) but instantly noticed how much more distinct the high and low frequency ranges were. The digital sum vs analog sum contains the same information, but I know on my ITB mixes I'm constantly automating things to get them to sit properly in the mix and those things were simply more present in the analog summed mix which made it more interesting as a listener. Low end was definitely less muddy and high end was smoother, less smeared. I think I will continue to mix ITB initially, routing everything to four stereo busses, and then when it's time for the final mix route the busses to the different stereo outputs feeding the SB2. Really looking forward to future mixes run through this amazing little box!

Mitch Park

I was curious about analog summing, but didn't want to shell out a couple thousand dollars to find out. My thoughts:

It's more about the d/a converters than it being analog. Cramming all those signals through a mediocre or poorly implemented converter. The analog summing technique spreads the signals over several d/a resulting in less distortion. I notice it on the "ends" - the low and high ends of the mix. Much clearer and defined. I also separate the tracks based on that - for example I don't send the kick and bass through the same converters. Usually opposites will share converters like a bass and flute.

This specific box is better suited for me. No active circuitry means no noise or added distortion. Otherwise I could just use a cheap compact mixer.

I also have a 500 Chassis full of expensive preamps which now get double duty. I can choose between API 512c or Heritage 73jr for different flavors. I also have a pair of EH ATY Tube Pre, but haven't tried them yet.

All in all, I'm really happy and amazed the cheapest solution turned out to be the best for my situation. Highly recommended!

Pete G
Easy build and awesome sound

Been using an 8 channel summing mixer built into my monitor controller for the past few years ($2k unit) and just built an SB2 to try out since it’s so inexpensive. Sending the SB2 into the mic+ input on my Louderthanliftoff Silver Bullet was an eye opening experience. The soundstage expanded and everything separated. Mixes come together faster and sound bigger. Panning is defined and everything is really focused compared to the ITB mixes and old summing mixer mixes I shot out (both I fed into the mix inputs of my LTL SB btw). Took 30 minutes to build and is an absolute no-brainer.

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