The aim is to create a small device that collects different audio signal sources, assigning them an independent volume, and mixing them into a single output.

Supplies

1 x Protoboard

5 x 100kΩ Resistor

1 x 1kΩ Resistor

2 x 100kΩ Potentiometer

1 x TL072

3 x Jack ¼ Connector

1 x Perforated board

2 x 9V Battery

Step 1: Understanding the Circuit.

First of all a voltage divider is added to control the audio signal amplitude Input. The potentiometer in the divider will be the volume knob.

After the first operational amplifier, we already have the inverted amplified mix of the two signals. Having the inverted signal shouldn’t be a problem in almost every case. However, problems can appear when working with systems that use oscillators, so another amplifier is added to invert the signal again, having the original phase.

Although this setup is only going to have 2 input channels, N channels can be added to this circuit as the third one represented in the drawing above.

Step 2: Circuit Simulation in OrCAD

In case of not having access to a circuit simulation software I have created an online representation of the circuit, where you will be able to play different types of signal and see how the circuit combines them. Click HERE.

Step 3: Setting the Circuit in a Protoboard

Before soldering the final circuit in a PCB or a perforated board it's very recommended building it in a protoboard and try that everything works fine. Be careful with the OP-AMP! (I burned two of them…)

In case you are wondering why it only appears 1 OP-AMP in the picture above, while in the schematics there are two of them represented. It is because the TL-072 works as 2 amplifiers in one. It has two positive inputs, negative inputs and outputs. Also one positive and negative Vcc.

The second picture shows how I checked the circuit was working, playing two tones (It is not funny to hear but it looks more professional). I also used the oscilloscope to look for any significant distortion in the signal.

Step 4: Soldering in a Perforated Board

The intention in this project is to fit the circuit in a box to make it portable. In this step we have to set all the pieces in a small perforated board and solder them with tin. I recommend soldering everything but the potentiometer and the connectors that should be placed in the box we choose first. This will help to avoid future welding defects.

The two remaining cables will be used for installing two batteries.

Step 5: Final Test

The purpose of this step is testing that everything works as it should. Plug and unplug things and rotate the knobs.

The video shows how the mixer works. The Input1 is connected to a PC playing the tariler of the TV show "The last of us" and the Input2 is connected to an iPad playing the soundtrack of the same TV show.