# Naively mixing two (or perhaps more) audio signals

this is my first question and I am very much a beginner in electronics so I apologize if this is a too stupid question.

I have Atari punk console and another small homemade audiomaking circuit. These two have mono audio signals as outputs and I would like to combine these as one in the simplest possible way. I know next to nothing about audio processing but I believe that the strength of the two signals is roughly the same.

What would a neat solution be? It would be really nice if I could extend the solution later on to more than just two signals but thats not really important.

Any hints, references, guides, links or ideas would be highly appreciated.

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The simplest way to combine two signals is with two resistors as shown in the circuit diagram below.

simulate this circuit – Schematic created using CircuitLab

The ground connecton is shared between both inputs and output. The quality can be improved by adding an output buffer (opamp), but that complicates things in requiring an IC, power supply, (de)coupling caps, ...

simulate this circuit

The advantage of the lower circuit is that the inputs see a constant input impedance, one that doesn't change with the other signal's level. This improves output quality. Practically speaking the inverting output (-) of the amplifier is virtual ground, which means it is set at a static voltage (4.5V in this case). The series capacitors are required to decouple the DC bias of both in- and outputs, they allow the signal to pass, but block the DC voltage.

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Circuit 1 could be improved by connecting the 2 sources to the ends of a linear 10k pot, with the output taken from the wiper. "Similar" audio levels often aren't as close as you'd like and this would allow balancing. Using a dual-gang pot would upgrade to stereo easily. –  Chris H Mar 19 '14 at 14:45
In the second circuit, do you mean the non-inverting input is at 4.5V? Or am I misunderstanding? –  Peeja Mar 19 '14 at 15:23
@Peeja: the non-inverting input is at 4.5V. Feedback then forces the inverting input to be there, too. –  Ross Millikan Mar 19 '14 at 15:28
@jippie Thank you so much for your great answer. I will shortly attempt to build this =) Regarding the capacitors, does it matter if I use ceramic or polyester ones? –  Jonas Mar 25 '14 at 12:08
@Chris H Shouldn't the pots be log not lin? –  peterG Mar 26 '14 at 2:31

In your situation you would probably want to use an op amp in a summing amplifier configuration, shown below:

This circuit provides an output signal voltage according to the following equation: $$V_{out} = -R_4(\frac{V_1}{R_1} + \frac{V_2}{R_2} + \frac{V_3}{R_3}+ ...) = -(V_1 + V_2 + V_3)$$ The equation simplifies nicely in this case since all of the resistor values are equal, but you can easily change the values to set fixed gains for the channels. Additionally you can add quite a few more resistors for extra channels as needed (although its best to disconnect unused resistors/channels as they contribute noise to the circuit).

You can also take this configuration one step further and add individual gain control for each of your channels, simply by adding a potentiometer before the resistor on the given channel, as shown below. Since you are only dealing with mono channels, you would only need one potentiometer per channel (and only one op amp).

As far as actual components go, the TL071 is a single op amp available in an 8-pin DIP package and can be run off of +/- 3.5V. DIP packages are usually much easier to work with than SMD parts if you're just starting out in electronics. Since your devices are pretty low fidelity, you won't have to worry about using a high-performance chip at all and could get away with a less expensive chip.

For extra information on audio mixing, you can check out this link, and more information on specific audio op amps can be found here

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