Sound card mixer

This circuit receives inputs from both external connectors and the sound card's digital-to-analog converters.

It selects or mutes, amplifies (with variable gain) these signals, adds them together, and finally routes the result to both external output connectors and the sound card's analog-to-digital converters.

[1] Sound card mixer controls are provided through the GUI interface in the computer's operating system.

On most desktop environments, the mixer can be accessed via a Volume icon in the system tray.

Additional hardware or software may add other effects such as low-pass filters, distortion and boost controls.