Introduction: Passive 3 Input Stereo Mixer

This instructable will show you how to build a simple stereo mixer. Though this box has only 3 stereo inputs, you could easily upgrade it to as many as you need! I wanted to build this box to connect multiple audio inputs into one single output.

I am using RCA panel mount connectors because thats what I have, you could make this all with 3.5mm stereo jacks, 1/4" phono jacks, or even a mix of different types! I want to give credit to Curt @ scribd for his inspirational writeup, I'm essentially adding photos, visual design and diagrams.

Currently I have my mac tower, xbox 360, and laptop connected to the same set of speakers with no problems. I wanted to make a small, non-powered box that looked good sitting on the desk.

Build Time: 1 hour, not including research + documentation

Total Cost: $15
$8 -Diecast Project box
$2.50 -Panel mount connectors, (six RCA female, one 3.5mm stereo female)
$2 -18 or 20 gauge wire, solid strand (this should get you 10-20 ft. I only used 8 inches total)
$2 -4.7k-ohm 1/2 watt Resistors
$.50 - Small Ruber Feet

Tools Needed:
Drill, 1/4" bit
Soldering iron, Solder
Electrical tape or Heat Shrink tubes
Knife or actual wire stripper
*optional* Dremel to thin out the wall of my project box, yours might be fine without
*optional* Alligator clamps to test connections before soldering everything together

Step 1: Detailed Part List Photos

All parts for this project were purchased at the most honorable Debco in Cincinnati Ohio, except for the resistors from Radioshack.

The Diecast box actually worked out to save me about 20 minutes of soldering because it is conductive enough to ground all of these connections. Win.

Step 2: Drill + Screw + Tighten

Measure, mark and drill holes in your project box. I used the awl to make a small indent in the box so my drill bit wouldn't wander.

You'll need 6 holes for the RCA connections and 1 for the stereo jack. Screw the connectors down tight!

Step 3: Circuit Diagram

Heres the basic circuit diagram. I was having problems when I didn't use these resistors mentioned by Curt. When I connected more than two audio sources, one would be barely audible. This volume drop was being caused by signal interference. Adding the resistors solved the problem by raising the total resistance to interference to 9.4k ohms instead of just 4.7k ohms. I dunno entirely why it works, but just trust Curt on this one.

Basically, all input lines have a wire and then a 4.7k resistor attached. Each side (all 3 Right channels, and all 3 Left channels) are then soldered together after the resistors, and another wire runs from that connection to the appropriate pole on the 3.5mm stereo jack.

Instead of soldering everything at once, I used alligator clips to check as I progressed...but essentially, once things are soldered together, you just need to tape or heatshrink the connections.

Then you are almost DONE!

Step 4: Finish!

So, put on your optional rubber feet!

Then you are DONE! Enjoy!


sjoobbani made it!(author)2010-09-03

So what I'm doing is, I have two stereo inputs, and I have a stereo potentiometer set up on each, for volume. I wanna be able to play both at once ( ie, play cod on meh 360 and have music playing at the same time, but thru an mp3 player connected to the box, will it short out without resistors? or how does it work?

chefbrian made it!(author)2011-07-09

A potentiometer is essentially an adjustable resistor, you should have no problem, but it may be louder than expected, as the audio sources' volumes will stack (sort of)

Re-design made it!(author)2010-09-10

Great job!

iahaleem made it!(author)2010-07-20

I'm adapting this for my own setup (5.1 speakers) using an existing audio switch box. I've cleaned out the box and am going to put everything back in as a stereo mixer, but I have a few questions before I finish my project.

Is there a limit to how many signals I can mix? My box has room for 7 inputs (10 RCA L/R channels and 2 3.5mm channels). Will mixing these many introduce any noise into the mixer? I'm only going to use 4 of them, but because my box has 7 built-in inputs, I thought it'd leave room for my mixer to grow. Please advise!

I'm also planning on splitting the output into two streams, one for front speakers, one for rear. Do I need to do anything special to split the audio? Is it okay to use some 22 gauge wire to split the output to an OUT 1 and OUT 2 (essentially, a Y split)?

This is my first time building anything like this. I'm not a noob with electronics, I've just never constructed anything myself before. I'm very excited, though I want to make sure that the end result will work the way it's supposed to. Thanks for any advice you guys can offer!

I've linked an crude layout of my connections so you can get an idea of what it looks like, let me know if there is anything wrong with the wiring (for purposes of clarity, I left out the resistors in the sketch)

iahaleem made it!(author)2010-07-24

I tried making the passive mixer with different output leads. It didn't occur to me that it wouldn't work because if any wire makes contact with another wire at any point, all signals will be combined. So I couldn't split certain signals while letting others through. Not without a different and more complicated setup. Instead, I used 4 inputs to go to an output 1, and 4 more inputs to go to output 2. That was much less complicated, and it worked just fine! YAY!

spencerb8 made it!(author)2009-04-22

Would it be possible to change the 4.7K resistors to a panel mount pot? So I can turn the volumes up and down. If it is possible where do I put them?

Clevarr+Burton made it!(author)2010-03-08

 a pot is simply a variable resistor! please try it and update us!

caseyC088 made it!(author)2010-03-07

is this the same sort of thing as this ?? good instuctions btw :)

Clevarr+Burton made it!(author)2010-03-08

 caseyC088, the Radioshack item is only a switch. With a switch, you can only have one source playing at a time. The device I made is a mixer. All sources play at the same time. This works great when I need my computer and radio to come out of the same set of speakers.

diggidy_dylan made it!(author)2009-05-14

I suppose you could turn this whole thing around and use it as a six out/ one in sorta mixer too huh?

merkwitch_splat.superusr made it!(author)2008-10-17

Ditching the precompiled wireblazer for the nil-noise jamophone, huh? This is a perfect element for both utilitarian one-offs as well as a good bit of know-how for anyone interested in line level mixing in general. There are some really cool passive circuits that can be used as succatasch-powerless audio effects. Did you get a chance to send this to Curt? Nice Job on your first!

Clevarr+Burton made it!(author)2008-10-17

You're right, I should probably retitle this article to "nil-noise jamophone." Curt's article was originally posted by a sysadmin at Scribd. I sent a message to jamesyu, hopefully he can explain the information dis-integration between Curt, Scribd, and himself.

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