Summing Mixer


Well-known member
Here is my first attempt at a passive summing mixer. It has 18 inputs total, 4 mono and 14 stereo. I figured that I would use the extra stereo input as a link from a sub mixer. The outputs are on the front since I will be racking this unit next to my mic/mic pre patchbays. I also installed a ground lift incase I needed it (you never know).

I painted the back of the rack to indicate the mono (blue) from the left (white) and from the right (red). Its got that flag look going for it. I was going to paint some design on the front, but the case (Middle Atlantic) has a vinal coating and paint did not stick well. I manage to get the paint to somewhat stay on the back, but it needs some touch up. That is why I did not show the backside as it does look like A$$, but once I clean it up I will add the photo. The case top has a serious lip on the front side, so I need to notch it for the XLR outputs before I can attach it.

The resitors I used are the standard 10K for the stereo, 14.3K for the mono, and 220 for the shunt. Each mono channel uses four resistors, two for the left and two for the right. I tested it last night and everything seems to be in order.

I changed my design midway through the build. I was going to use a PCB I had and install multiple resistors (10k and 4.3k) to branch and sum the MONO channels. I had also drilled the case for that PCB. The PCB became a mess and I realized I could do the same thing with 14.3k resistors. I used one of the PCB's drill holes for my mount and it was not perfectly in line with the mount on the other side (you can see the old holes in the bottom of the case). I also tried to solder each resistor sliding shring wrap between each resistor which made the buss wire pull even more out of line. Next time around I will get the mounts perfectly lined up and string the buss wire across before I do any solding.


That looks pretty good. Three questions:
1. What 1/4 TRS jack did you use?
2. Any tips on working/drilling with the Middle Atlantic case?
3. and was it worth it?

I'm getting ready to start mine in a week or less. Wish me luck.
  1. Switchcraft 112BX
  2. a. The Middle Atlantic case has a lip on the front that I had to cut because it interfered with the XLR connector.
    b. The coating on the case is a brittle plastic coating. You will need to be careful when cutting that you don't chip it off it tears; a normal drill hole should be ok though. This coating will prevent some paints from sticking. I thought it was metal so I got some paint for metal and it did not stick well to it.
    c. If I had known these two issues above, I would have spent a little more for a Par-Metal case, but if all your connectors are on the back and you want to leave it black, it is a good case. I used a drill press so making the holes was not a problem.

    I used some 4-Position Dual-Row Barrier Strips from Radio Shack to secure the wires; they workd perfect.

  3. It was worth it. I have wanted to build my own passive mixer for quite some time. the following links were VERY helpful if you have not seen them yet:

    Link 1
    Link 2
Sorry if this isn't a very helpful reply, but what purpose does a summing mixer serve? I thought the usefulness of a mixer was to mix signals with the option to give varying volumes to each. Please point me to where I should read more about this.

Sorry if this isn't a very helpful reply, but what purpose does a summing mixer serve? I thought the usefulness of a mixer was to mix signals with the option to give varying volumes to each. Please point me to where I should read more about this.


There are several reasons why I like using a summing mixer over straight ITB or OTB mixing. My DAW is SONAR, I have use the Cakewalk software for well over a decade now, so I no desire to switch to another DAW. That being said, there are several shortcomings in the software.

The first is that I never liked the way my mixes sounded mixed ITB. To me my mixes never sounded as full as the those done OTB. The other thing I have not been able to do in past versions was incorporate external gear (comps, EQ, FX, etc.) I am on SONAR 7PE but 8 is supposed to be MUCH better with their external insert technology.

There are many things I can do INB that I cannot do OTB. Some of those tasks would include: total recall of my mixes, automation of just about everything, use of plug-ins.

Using a hybrid method of mixing with use of a summing mixer I can use all of the ITB features that the DAW provides plus with the ability to use outboard gear and mixing to an analog mixer.

There is nothing wrong with mixing ITB, OTB, or hybrid. With the technology out today, you can get great mixes with any of the three methods. The difference is which one works best for you and your work flow. If I did not already have all the outboard gear that I have now, I could easily be happy with the tools in my PC (Duende, UAD, Breverb, etc.) is great place to learn about summing mixers and a whole lot about other recording gear. There are people at all levels of recording there, but like with any forum, you will need to wade through the posts to find out who know their craft and those who are full of it. Search will always be your best friend. :D
Pretty cool.

I built a few passive mixers back in the 80's... one I still have and use. They can be handy for certain applications. Mine were not so elaborate as to build in a rack. I used Radio Shack aluminum cases for the most part..

The one I still have is a simple 4 into 2 stereo box that I used to combine the outputs of an Ensoniq ESQ-1 and ESQ-M.

Anyway, nice work. :)
I built mixer once whith pan controls too and it worked well except for earthing noise. Im just wondering if those resistors are the right values? I suppose Op amps would add a bit of noise. But you can't beat the quality of a cheap behringer?
I built a passive mixer a few years ago that I couldn't live without.

I put a master on/off switch on the front that kills everything, plus another switch that kills just the sub send.

I put a huge old volume pot on it (an expensive Noble with detents) that allows me to really keep track of what volume I'm running my studio at, and that was always a problem before.