Guitar pickups produce a low signal with high impedance, known as instrument-level signal. While guitar and bass amps can use this directly, professional mixers need it converted to low-impedance, line-level signal for proper gain. Usually, a Direct (DI) Box is used, but ProFXv3 mixers have transformers on channels 1 and 2. Plug your guitar or bass into either of these combo inputs and activate the Hi-Z button for proper input!