Introduction: Passive Audio Mixer W/ Channel Volume Control

This Audio Mixer mixes multiple audio inputs to a single audio output, with knobs to control the volume of each channel.

My particular build features 4 stereo RCA inputs mixed to 1 stereo RCA output. You could easily alter this design to any combination of jacks that you need. 

This mixer is passive, meaning it requires no external power. It also means that nothing is amplified. Consequently, the mixed output will have to be amplified by your stereo or another amplifier system. 

Step 1: Circuit Diagram

This circuit is actually really simple. All the complexity is due to repetition.

Basically, all circuits are as follows:
     input --> output

For an audio mixer, you need a resistor on each input. So, we have:
     input --> resistor --> output

But, it's stereo so you have to do everything twice. So, now you have:
     L input  --> resistor --> L output
     R input --> resistor --> R output

You could stop here, but if you want volume knobs, you have to go through that before the resistor: 
     L input  --> volume knob --> resistor --> L output
     R input --> volume knob --> resistor --> R output

Finally, we repeat this for each input. 

So, the circuit is really a very simple idea repeated 8 times, twice per channel for four channels. 

Step 2: Parts and Tools

This project requires few parts and tools. It is also relatively cheap, mine cost me about $20, but you can vary yours to fit your budget. 

-- Soldering Iron and Solder
-- Dremel or Drill
-- Wire Strippers

-- 1K ohm resistors x8 
-- 10K ohm logrithmic double-ganged potentiometer x4    (* explained below)
-- RCA panel mount input jacks x10
-- project box or other enclosure

*About the Potentiometers:
A potentiometer or 'pot' is the twisty knob that will act as a volume control. You want a logarithmic pot (aka audio pot) because volume is on a logarithmic scale. Double-ganged means that one knob controls two inputs at the same time, which is necessary if you have stereo audio. That way each channel can be kept separate, and one knob will control both channels. If you're building a mono mixer, you should just use single-ganged pots. 

The specific parts I got from Parts Express are as follows: 
-- 1.1K Ohm 1/2W Flameproof Resistors 10pcs.  Part #: 002-1.1K  Price: $0.75
-- 10K Audio Taper Stereo Potentiometer 1/4" Shaft.  Part #: 023-646  Price: $2.40 x 4
-- RCA Chassis Jack.  Part #: 090-280  Price: $0.73 x 10
-- Project Box 5-1/4" X 3-1/4"   Part #: 320-430  Price: $4.25

Total Cost:  ~$22

Step 3: Drill the Enclosure

Here we need to drill all of the holes in the enclosure. I put all of my audio jacks on the back, and the 4 pots on the front. 

1. Measure your components
2. Calculate where to drill the holes
3. Mark your enclosure with a pencil
4. Start drilling pilot holes, gradually increase the hole to full size
5. Test fit with your components 

I drilled all the input/output jacks first, and repeated the exact same process for the pots. 

Step 4: Begin Assembly

Start putting the components in their place and wiring them all together. 

1. Put output resistors on pots
2. Put jacks in enclosure
3. Put pots in enclosure

This is the majority of the assembly. After this, we have to do all the wiring. 
I recommend wrapping the resistor leads around the pot terminal, this makes a strong physical connection.

Step 5: Wire It Up

The wiring consists of a few simple steps that are repeated a lot. 
You end up with a lot of wires, so be organized!

Note that you can and should test along the way.
If you make solid physical connections, you don't even have to solder to test. 

1. Wire input jacks to pots. 
2. Wire the output resistors together.
3. Wire final resistor output to output jack 
4. Wire ground connections of pots and jacks together

Step 6: Final Product

Done!  We now have a functional 4 channel to 1 channel stereo mixer. Have fun twisting the knobs!