Passive Stereo to Mono Resistive Mixer

These days the most common audio format is two channel stereo, but there may be times when we need to convert this to one channel mono. Perhaps to feed the output of a portable audio player such as a mobile phone into the input of a mono guitar amplifier for example

The circuit is a standard combiner/splitter arrangement using 680 ohm resistors, giving two identical mono outputs from one stereo input. 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, in a Hammond 1590 LLB diecast box with four RCA chassis sockets mounted on the lid in a square. The resistors simply go from the centre pin of each socket to a common point forming a 'star'. This method of construction has the advantage of allowing any two sockets to be the input and any two to be the output. You can't go wrong!