Given many audio inputs from consumer devices, it is sometimes necessary to mix the sound such that all sources play through a single input. For example, given household electronics and a PC sound card, it may be desirable to feed all sound sources to the sound card but, unfortunately, sound cards are not equipped with sufficient inputs.

Most consumer devices with audio output already contain an amplifier such that investing in an amplifier or mixer with many lines is not really necessary. A mixer or amplifier may contain dials for each mixed line, in order to adjust the line levels individually, frequency analysers and much more that simply is not necessary and would be a costly solution.

A simple schematic for a line mixer using RCA plugs for left (L) and right (R) could be the following:

Inputs (RCA)             Outputs (RCA)

L                             L
+---+-------------------------+
+---|---------/\/\------------+
    |         4.7k
R   |                         R
+---+-------------------------+
+---|---------/\/\------------+
    |         4.7k
+---+          .
+---|          .
    |          .
+---+          .
+---|
    .
    .
    .
    |
   --- (GND)
    -

The schematic could be extended for as many inputs as necessary and will manage to mix all sources in one. The outputs can then be attached to a sound card for further processing.

The line mixer is a work in progress and gets extended every now and then.

Using a few resistors, some RCA jacks and some wires, the line mixer can finally be pieced together.

The amplifier can also be fitted in a small junction box.

The third version adds two 1:1 transformers and selects the upper RCA jacks as the output. In doing so, better isolation is received such that any ESI would not seep into the circuit to which the output of the line mixer is connected to.

Additionally, the wires are replaced by copper threads salvaged from other transformers to reduce the clutter inside the junction box.

The resistor could perhaps be replaced with a variable resistor to control the volume of each input but, as mentioned previously, for this usage case, controlling each line volume is not necessary.