Other Audio Inputs

Mic vs. Line-Level Audio

Microphones put out a very weak signal we call “mic level.” For condensers, the signal generated is so weak that it takes a transformer in the mic’s housing or in an outboard unit to get it to the low level that a dynamic puts out. Mixers and interfaces have preamplifiers to take this signal to a usable level.

The outputs on a mixer or interface, your phone, computer, or instrument put out a “line-level” signal. This is much stronger than a microphone and has been standardized so that most devices are immediately compatible with one another (provided the impedances are compatible, but that’s a more advanced topic). It’s important to know whether something is mic or line-level. Plugging in a line-level source into a mic preamp could overload and damage it. Plugging a mic into a line-level input will produce such a low level that it’ll be completely unusable. This is why an amplifier is required to raise a mic’s output high enough to drive a speaker.

Often, a line-level input will have a switch to go from -10dBV to +4dBU. These should match on both sides of the cable so that the sending device is sending the same level the receiving device is expecting.

Guitars and Resonant Body Instruments

The most common input besides a microphone is a guitar. Most recording interfaces have 1/4″ jacks on them for guitars, either as part of a combo XLR/TS jack or as a two separate inputs/jacks.For live work, there are plenty of options.

For live work, there are plenty of options. On a small stage, a guitar can go into an amplifier and perhaps be loud enough to bypass the house system. The most common approach is to put one or more microphones in front of the cabinet; receiving the amp’s sound is essential for a lot of players, since the amp is often the most important part of the desired sound. Another option is to use an output on the amp to go into the mixer. Alternatively, a DI (direct injection) box is used.

A mic picking up the sound from a guitar amp.

Direct Injection Box

These are nifty pieces of hardware that usually have a high-impedance, unbalanced 1/4″ input and a low-impedance, balanced XLR output. This allows instruments like guitars and electric pianos to run long distances back to the mixer. They may also have additional features, like dual ins/outs for stereo sources, pads, ground lifts (to break ground loops), provide phantom power, and/or multiple outputs for splitting the signal.

Direct injection box by art.

Direct Injection box input side, with two pads and a filter.

Direct injection box by ART.

Direct injection box output side, with 9V battery compartment (for +48V power), ground lift, and phase switch.

An active DI is one that uses phantom power. A passive DI doesn’t require an external or internal power source. In some cases, a DI will be lauded for its high-quality transformer (such as a Jensen transformer, found in more expensive DIs).

Keyboards and MIDI Instruments

An electronic keyboard will usually go into two DI boxes (or a stereo DI) and up to the console. Again, if the stage is small, you can sometimes get away with an onstage amplifier and call it good.

Often a keyboard player will have a small mixer to deal with audio coming from several keyboards and synthesizers, then send a stereo signal to the main mixer. As software instruments are gaining ground, it’s becoming more common for keyboard players to have a laptop on stage with them, then output a stereo, line-level audio signal.

MIDI instruments in a studio output either via a common computer interface or a MIDI cable, then the sound is generated by the computer. On stage, MIDI instruments may go into a laptop that’s run by the player or use onboard sounds; when this is the case, they output similar to a regular keyboard or synth.

Resources for this Section

This book goes into detail on how musicians and engineers should use all sorts of stage inputs.