Passive Stereo to Mono Resistive Mixer
Why would anyone turn stereo into mono? It's true that most people want two or more separate channels, but sometimes there is still a need for mono. Maybe an audio mixing desk is running out of inputs and you want to free some up, or in my case, I wanted to feed speech podcasts to a speaker in another room, and by feeding a set of stereo PC speakers with this device, each speaker will output the same mono sound, so that one can be positioned locally, and the other elswhere
The circuit is a standard combiner/splitter arrangement using 680 ohm resistors, with two identical mono outputs that can be fed to the stereo input of a set of active speakers or a mixer/amplifier etc. That's it really, but be aware that as it uses fairly low value resistors, it cannot be connected across an existing stereo signal without reducing the stereo separation of that signal. It should therefore be used on it's own dedicated audio channel, or with an audio selector switch that isolates it from other sources. Also, being passive, it will attenuate the signal slightly, but it's hardly noticable in practice
It can be assembled any way you want according to your needs, but I built mine as shown above, on a quad 4 way phono socket panel, simply joining the earths together and connecting all the resistors to a single point in a 'star' formation. Although jack sockets may be preferable, this method has the advantage of allowing any two sockets to be the input and any two to be the output. You can't go wrong!