While one can make this connection physically, it is not advisable. The AudioBox USB and AudioBox 22VSL devices accept microphone or instrument level signals only. A microphone, passive electric guitar, passive electric bass, etc., create signals that are very weak - this is because these signals are physically generated by the instrument or mic itself, not through electronic amplification.
The AudioBox USB and AudioBox 22VSL devices are designed to preamplify these weak signals to line level so that they can be better treated by the device. However, these devices are not intended to accept an already preamplified, or line level, signal. A line level source is too powerful for the preamps of these devices and could potentially damage the unit, requiring a repair.
Common devices that output a line level signal include:
- Mixing consoles
- CD/MP3 players
- Keyboard workstations
- Guitars/basses with "active" pickup technology
- The outputs of other audio interfaces
If you wish to connect one of these devices to the AudioBox USB or 22VSL, you would want to first connect your source to adirect box, or DI. These are small, inexpensive devices that can attenuate the line level signal down to instrument level, allowing safe acceptance by the AudioBox preamp. Direct boxes can be purchased online or at almost any pro audio or musical equipment store and have a wide variety of uses, making them useful to have on hand.
The AudioBox 44VSL contains line level inputs on the 1/4" stage of inputs 3 and 4; the AudioBox 1818VSL has line level inputs available on the 1/4" stage of inputs 3 through 8. These devices require no direct box when connecting your line level source to the proper input.