The Zoom H2 is a popular portable audio recorder. This recorder can record four tracks simultaneously, but unfortunately, this only applies to the signals from the four built-in microphones.
The modification described here also lets you record four signals at line level. For this, we add four phono sockets to the recorder, where the signals are attenuated by 40 dB via a resistor network. The capacitor blocks the supply voltage for the electret microphones, which would otherwise appear at the line inputs, which is obviously not desirable.
Two switches are used to select either the line input or microphone as the source. A short YouTube movie  shows all the steps required to modify the H2.