Passive Summing

What It Does

Summing is used to combine multiple input signals into a single output signal.

Remark

Passive summing is the simplified (cheaper) version of active summing, but it won't provide perfect summing.

Basic summing.

To "sum up" voltages use resistors. Although this seems a very basic trick you have to understand some "quirks".

Notice in the left circuit Vout (7.5V) is the average between the two incomming voltages (5V and 10V).
Notice in the middle circuit Vout is pretty close to the 5V input (5.455V). This is because here I gave the 5V input a much smaller input resistor (10K), compared to the 10V (100k).
Notice in the right circuit the 10V has no impact at all on Vout!
Do you also recognize the voltage dividers?
Notice summing two 10V's won't giving 20V but 10V.

TIP

Although it is called "Summing" passive mixing is actually mathematically averaging.
Use for each voltage you want to add a resistor. Mind the ratio between them.

You can offset an alternating signal with a fixed DC voltage in a passive summing mixer, but take note that it won't be a perfect addition.

Notice that the output not only has an offset but also an attenuation.
Try changing the resistor values, and the value of potentiometer (edit component), and notice that all these things affect each other.
The main issue is something what I call "network effects", which are more difficult to design for.
Nevertheless sometimes this simple setup is just good enough, and the non-lineair effects are neglectable.

TIP

For more improved summing please see Active Summing.