This is an adapter to connect a source such as a smartphone stereo headphone output to the input of an MP3 recorder. The view and diagram is looking at the back of the adapter where the plugs are inserted into the jacks. The switch handles are on the far side.
The switch on the right determines which of the two outputs my earphones connect to. The switch on the left changes from stereo straight through to mono where the right channel is connected to the left channel. It will be left in stereo position as long as the source is stereo.
There are four jacks as shown in the wiring diagram. There are two switches labeled in the diagram. The common sleeves of the jacks and the metal bodies are all connected together and are a common ground.
There are two 10k adjustable trimpots to reduce the source output to the level of the microphone input of the recorder. They should be set once and then the source volume control should be adjusted to suit the level of the source’s audio.
