PASSIVE 3 CHANNEL AUDIO MIXER

This is the simplest of additive mixers. It consists of a resistor network and is purely passive. I use it to combine signals from various instruments so they can all use the same guitar amplifier. Without a mixer, signals from different sources will interfere with each other. Since all electric guitars and other inputs have their own volume and tone controls, this mixer has none. It adds up to 3 individual signals, giving out a composite signal that contains the frequency components of each of the source signals. 

You will need 4 female audio monaural sockets. I used 2 standard 1/4” and 2 standard 1/8” sockets although you could use all 1/4”. Also, 3 matching resistors, each should be 1K Ω -10K Ω. After experimentation, I found that 3K Ω worked best. Do not mix values as it will allow different volume levels for each input. A small project box, but alternatively a metal tin box could be used. 

A step-drill bit chucked into an electric drill is perfect for making holes in light plastic or sheet metal. You will need solder and a soldering iron.

After checking the size of your sockets, measure and drill appropriate size holes. Install the sockets.

Make your connections according to the schematic diagram. Remember, one socket is the output. In the photo, output is on the left. Wire all grounds (ring connections) together and solder. Attach a resistor to each input tip connector and solder. Connect the other end of each resistor to the output tip connector and solder. That’s it. Close the box and test.

NOTE: You can add as many inputs as you want within reason, just expand the circuit but decrease the value of the resistors or you will lose volume.

You’ll want to label the inputs and output to avoid mixing them up. I also attached double sided tape to the bottom so it can be attached to the top of the amp.