Have you encountered a recording equipment like an audio interface or audio mixer with inputs or outputs labelled as “Line” or “Instrument” and even “Microphone”? For example see the two images below:
Microphone, Line and Instrument levels
Image credits: Focusrite Audio Engineering
In the first image, you see 2 inputs with label “Mic-Line-Inst”; this implies that it can accept microphone, line and instrument level inputs. Then in the second image, you see a label called “Line outputs”, this means that the output is at line level and should be connected to equipment requiring line level signals.
So what are Line, Instrument and Microphone levels? Beginners in home recording often ignore the importance in knowing the difference between line, instrument and microphone signals. But these things play a BIG role in achieving quality recording in your studio. Things associated with poor recording such as noise, hiss, weak signals, unintended distortion or clipping and tonal shifts are caused by the lack of understanding between line, instrument and microphone levels.
The first difference is the voltage level
The first difference between line, microphone and instrument are the voltage levels. In a microphone levels; the output voltage is not the same as the line level voltage or instrument level voltages.
In a microphone, there is a transducer that converts sound vibrations into electrical energy. The result is an induced voltage which is VERY low (within ~2millivolts). This low and very weak voltage cannot be used by other recording equipment or components in the signal chain. If you attempt to plug a microphone to an input requiring line level signals, then the resulting signal will be noisy or containing hiss. It is because the input assumes it is already pre-amplified which in this case it’s NOT since the output is still in microphone level voltages.
To make the microphone level usable by other components in the recording signal chain with reduced noise/hiss, it should be pre-amplified. This is where you should be using a “microphone pre-amplifier”. A microphone pre-amplifier applies a very high gain to amplify the very weak microphone signals (~2 millivolts) into a line level signal. Once amplified, this signal can now safely be plugged into any inputs requiring line level signals.
On the opposite if you attempt to plug a line level signal into an input requiring ONLY microphone levels, then you will get distortion. It is because the input signal is already highly amplified and plugging this input level again into a microphone input (which has a pre-amplifier on it) will further drive the signal level into intense levels causing distortion.
On the other hand, instrument level voltages (those that are coming out of the guitar pickups) have higher voltages compared to microphone levels but still less than line level voltages. It still needs to be pre-amplified.
Two types of Line Levels: PRO level and Consumer Level
OK so you now clearly understood microphone, line and instrument levels. So if you see input requiring line level signals, then you should feed that with a line level input right? Not so fast; this is where beginners in home recording can still commit some mistakes. There are two types of line levels that you should know which are very important.
First, there is a “consumer line level”. This is defined to have a line level equal to -10dBV. dBV is a voltage measurement relative to 1 volt. Converting this to equivalent voltage level that you can understand and relate:
dBV = 20log (Vlevel)
-10 = 20log (Vlevel), solving for Vlevel which is the consumer line level voltage:
Consumer line level voltage = 0.316 volts, this voltage is in RMS measurement.
Second, there is a “pro recording line level”. This is equivalent to a voltage level of +4dBU. dBU is a voltage measurement relative to 0.775 volt. Converting +4dBU to equivalent voltage level:
dBU= 20log(Vlevel/0.775)
+4=20log (Vlevel/0.775), solving for Vlevel which is the pro- line level voltage
Pro line level voltage = 1.23 volts
So you have seen that there is a big difference between a consumer level and pro line level voltages. Consumer line level voltages are commonly seen in non-professional audio recording equipment such as hi-fi equipment and CD players. Pro line level voltages are used by professional recording gears like recording pre-amps, etc.
Best practices when dealing with line, instrument & microphone levels
Below are some of the best practices you can implement in your home recording to get the best recording quality when working with any types of voltage levels.
1.) You will get a poor recording signal if you plug -10dBV equipment (a consumer line level) into an input requiring +4BU (pro recording equipment). To avoid this mistake, make sure you understanding the output and required input levels for signal compatibility. You can often get this information from the manual or ask the equipment manufacturer.
2.) Do not ever plug microphone directly to line inputs. A line input assumes that the signals are already amplified. As a result, you will get a noisy output from the microphone because it’s not pre-amplified.
3.) Use balanced XLR inputs in the mixer for plugging the microphone. This will ensure that the microphone signal will go to the pre-amplification stage. For example this is the screenshot of the Behringer Xenyx manual:
balance XLR input for microphone
Image credits: Behringer Xenyx Manual
4.) Understand what type of line input is required (is it consumer level or pro-level inputs). Again refer to the equipment manual for the specifications.
5.) Guitars with active pickups can be plugged directly into line level inputs provided they have an output voltage of 1.75volts (which is greater than the pro-line level required voltage of 1.23 volts). When in doubt, contact the guitar pickup manufacturer and verify.
6.) Guitars and bass with passive guitar pickups cannot be plugged directly into a pro-line level input (such as in the mixer) because they have output voltages less than the required level. You need to plug the guitar input into the pre-amp input of your mixer or audio recording interface.
7.) A good way to assess whether you are connecting the right input-output combination of your gears is to look at the level meter of audio interface, in DAW or in the hardware mixer. For example, not using XLR input for connecting dynamic microphone to the mixer and plugging it directly to the line input will get a very low recording signal level (because it is not being passed into pre-amplification stage) with much hiss and noise (as you increase the trim or main output volume). In this case, the level meters will barely move and the signal is not good.
Content last updated on August 7, 2012