In the audio world, input and output levels come in two basic flavors: Mic Level and Line Level.
Line Level is technically the correct voltage level for our audio signals to travel along wires from device to device, and is based on the voltages used by early telephone systems. Without getting too technical, it represents approximately one volt (“0 V.U.” is actually a little less than one volt, since it represents an average voice level and not peak loudness). But this article is not about telephone wiring, but modern audio devices.
Audio signals that have been mixed or recorded, and output from an amplified device such as an audio deck or mixing board, are at line level. Raw signals, such as from a microphone, are much lower in volume/strength, and at some point in their processing will require being amplified up to line level. Hence, the term “microphone pre-amp” which refers to electronic circuitry that functions to boost mic level signals to line level, so that they may be properly mixed and recorded.
Microphone levels are typically 50dB weaker than Line levels. Since Line levels are considered to be zero, mic levels would be around minus 50dB on a metering scale. If you (like all of us) are wondering why audio metering ranges from negative numbers (measuring weak volume) and increases up to zero — again, this dates back to the old telephone wiring systems. If it were up to me, numbers would increment from a low of zero up to say, an ideal line level of 100%. But it is not my call to make, so learn to live with it!
In addition to sheer gain or volume, there is also the impedance factor of the input and output circuits, and that is measured in ohms. Without getting into advanced electronic theory, suffice it to say that line level signals are generally considered to be 600 ohms, and mic level is around 250 ohms.
Here is what we have so far. Line Level is a much louder and stronger signal, metered as 0 dB, and rated as 600 ohm. Mic Level is much quieter and weaker, approximately 50dB less than line level, and rated as 250 ohm. Mic Level, when brought into a mixing or recording device, is processed by a Mic Pre-Amp to boost it up to Line Level, so that it can be properly dealt with.
Another way to understand the difference between mic versus line level is to think in terms of voltage and light bulbs. Consider that we have two light bulbs: one is rated 12v and the other is rated 100v. Along with these bulbs, we have two power sources: 12v and 100v.
Think of what each combination would result in. If we sent 12v of power into a 12v bulb, everything would be fine. Ditto, 100v of power would mate perfectly into the 100v bulb. However, the 100v bulb would ultimately be brighter/stronger and more visible even in a somewhat well-lit room!
Similar to audio. Mic level signals going in to a device set to receive these weaker signals will sound fine; and Line level signals going in to a Line input would also sound fine. But if there was a fixed level of interference near the wires (think of the stray light in the room in the bulb example), it would have much more impact on the weaker signal than the stronger signal.
Now imagine the results if we mix up power sources and our two light bulbs. If we send out a paltry 12v into the 100v bulb, the filament will just barely glow. However, 100v of electricty into the 12v bulb would overpower the bulb, causing it to go super-bright and eventually burn out.
So, back to audio. Sending a mic level signal into a line level input will result in a low and feint signal, just barely audible in the headphones even with the volume know turned way up. Not good.
Sending a line level signal into a mic level input will overload the circuit causing the sound to be way too loud and badly distorted. (Don’t worry, your audio device will not physically burn up or self-destruct like our light bulb example; it will just sound really bad in the headphones.)
Learn to recognize the symptoms of a mis-matched output/input combination. When you have the menu options to do so, it is always preferable to run line level signals to line level inputs in order to minimize the risk of interference. But when you are connecting a raw microphone to a recorder or mixer, you are dealing with mic level output and that can only mate with a mic level input. However, the processed (mixed) output from a powered mixing panel to a recorder is normally a line level output, so make sure that the recorder input is set for line level (or else it will be too loud and distort).
One more thing to note. In the real world, there are actually two different levels of Line Level. Professional Line level is referred to as being +4dB, while consumer Line level is known as minus 10dB. Line level outputs from sophisticated (professional grade) audio devices with XLR output connectors are usually +4dB, whereas consumer line outputs using RCA and stereo mini connectors tend to be the slghtly lower -10dB level. The difference between these two levels is not a problem when you connect to the input of a professional recorder or mixing board, and can be adjusted for via a switch, gain control knob, or main volume control/fader. Sending a +4 signal into the RCA or mini input of a consumer device may or may not cause a minor distortion issue, depending on the quality of the consumer device. For what it is worth, I have found this issue to be more theoretical than actual; in over 40 years it has never created an issue for me in real practice.
Review
- Mic out to Mic in. Sounds OK, but weaker signal more prone to interference.
- Line out to Line in. Sounds OK, a stronger signal less prone to interference. Best settings to use if you can.
- Mic out to Line in. Very low and feint volume. Think of sending 12v into a 100v light bulb.
- Line out to Mic in. Too powerful a signal for that input. Audio will be loud and distorted. Think of 100v going into a 12v light bulb.
- Line level is normal level for most devices. Mic level requires pre-amp to raise level up to Line level for mixing or recording.
