What is the difference between Mic (microphone), Instrument, Line and Speaker signals?

In this post we’ll have a look at the 4 types of audio signals used in sound engineering and in the audio world. It’s a recommended reading for both apprentice-musicians and apprentice audio engineers.

The 4 types of audio signals

In sound egineering, there are 4 types of analog audio signals that we deal with on our mixers, amps and effects systems: Mic Level, Instrument Level, Line and Speaker Level.

Their difference is in their signal strength which is measured in dB (decibel), so for more info on decibel please click here and here.

– Mic level (or microphone) is the lowest (also known as the weakest), signal level of the four mentioned and requires a preamplifier to bring it up to Line level. It usually is -60 dBV (the V signifies voltage) at 0.001 volt to -40 dBV at 0.010 volt, with an average level of -30 dBV. Mic signal Impedance is also low, in the area of 250-600 ohms.

– Instrument level signal strenght is between mic and line level signals (and presents the most variation). We typically see this type of signal on electric guitar, bass and, more in general, from instruments with pickups (both type: single coils and humbuckers). Instrument level also requires a preamplifier to come up to line level. Instrument Level is usually around -10 dBV (0.300 volt) to -20 dBu.

– Line level signal is the highest level signal before amplification (in other words, before any form of power-amp). It is the type of signal that typically flows through our recording systems after a preamplifier stage and before the amplifier (power-amp) that powers our speakers. There are two types of Line Level Signal: Consumer and Professional.

Consumer line level is set around -10dBV (we find this type of signal on products like a CD players, mp3 players, ipods etc…). This also include some very old recording audio device and RCA (phono) connectors.

Professional line level is set around +4 dBu (or dBm) and is found in things like mixing desks and signal processing equipment. If you see that little switch on your gear that says +4 or -10, that’s what that’s for.

Nowadays, Line Level is generally considered around +4dB and its voltage is usually either 1.0 V, referred to as “0 dBv”, or 0.78 V, referred to as “0 dBu”. More info about the Line Level Signal here.

Please Note: be very careful not to send a line level signal to a preamplifier expecting a mic or instrument level signal.

– Speaker level signal is post amplification. After a line level signal enters an amplifier (or power-amp), speaker level signals are output to our speakers. This signal is much higher in voltage than all the other signals and require speaker cables for safe signal transfer.

You should never plug a speaker level signal into a source expecting anything less than a speaker level signal.

Converting Audio Signals

Now that we have learned about the available types and their nature, one may wonder if it’s possible (and how) to convert those signals when we need to. For example when we want to record a guitar sound completely clean to manipulate the sound in post production.

Converting an Instrument Signal into a Mic Signal

Attenuators (aka D.I. Box)

To convert a Instrument Signal into a Mic Signal, we can use a device called D.I. Box (or Direct Injection Box). These type of devices can offer a set of features and also a set of possible conversions. You always want to check for the offered features on the D.I. Box web page or to your local music shop. In General, the standard features for a professional D.I. Box are:

Converts an Instrument Signal into a Mic Signal (don’t get confused, this is exactly the opposite of a pre-amp! As we have learned on previous part of this post, Instrument Signal is usually stronger than Mic)
Converts unbalanced signals to balanced signals.
1. Unbalanced connections transmit a signal via a single connection, with a ground. These are susceptible to background noise and interference, particularly when run over long distances.
2. Balanced audio connections have a ground, live and return. This results in the cable rejecting interference as the signal is carried. Typical balanced connections are via 3-pin XLR cables (such as microphone connections) or balanced stereo jacks. So, by converting a signal from Unbalanced to a Balanced one the D.I. Box allow your signal to reach your mixer (or your Amp) with less interference, especially on long cables. More info on balanced audio here.
Offering Passive or Active circuitry.
1. Passive D.I. is the simplest form of D.I. Box available and it’s generally used to match line level or instrument level sources such like guitars and keyboards, and matches them to the mic inputs.
2. Active DI Boxes have active circuitry, which means that they require electrical power. This is usually supplied via phantom power or an internal battery. Whereas passive DIs use transformers to change signal impedance, the active DI uses powered amps.This negates the problems that occur with passive DIs at the limits of their dynamic range. However, when the signal is too hot, active DI boxes can distort in an unpleasant way. They don’t have the problem of disappearing low-end like passives do, though.This is all dependent on power levels- as a battery drains, performance will get worse. Plus, phantom power is not always 48V, which again can cause problems with performance.
Offering Ground Lift switch.
1. This feature can, sometimes, reduce unwanted hums and noise

When should I use a D.I. Box?

The most common use cases for the D.I. Box are:

When you want to connect your music Instrument directly to a P.A. System (“Public Address” System or also known as Pair of two-way speakers and the amplifiers used to power them, in other words those Speakers system you commonly see at gigs, clubs, music venues or conference centers used to amplify music or a speaker speech) when your PA System provides only XLR Mic input plugs.
Matching impedance when recording. Plugging something with instrument level output, like a guitar, straight into a line level input can result in a very ‘dead’ lifeless sound. Matching the impedance with a DI Box will ensure that the signal is recorded at its best.
When your audio card present only XLR Mic ports and you want to connect to it your guitar, bass, keyboard or synth audio output channels or your vDrums (or drum machine) output channels, as many of these equipment has actually an Instrument Signal Level output (check your device manual for more info and to be sure your device has an Instrument Signal Level output)

Converting a Mic Signal or an Instrument Signal into a Line Level Signal

Pre-Amp

Pre-Amp (or preamp) stays for preamplifier and is a sound engineering device that prepares an audio signal to be processed by other audio equipment. The word “preamp” is used in many different ways by different manufacturers, marketers, and users, so we’ll have to pay attention every time we’ll find this word attached to a device or on a post/article on sound engineering.

As we have seen, D.I. Box are used, in general, to convert an Instrument Signal Level into a Mic, so they are used to lower the strength of a signal. Although there are professional D.I. Box capable of doing more than that and, also, of to amplify the signal strength (in some cases), when we need to increase the strength of a signal (like when we want to connect a microphone to a Line Level mixer input) we generally use a pre-amp to convert a lower signal level into a Line Level.

The market is literally full of all possible kind of pre-amp, most of guitars and bass amplifier comes with a pre-amp circuit already onboard for example. There are pre-amp based on tube circuitry and pre-amp based on transistor circuitry. In general to do their job they need to have an active circuitry (i.e. they need electrical power to increase the input signal level).

Pre-amps can offer the following features:

Manipulate the level of Signal Gain. This means the amount your signal level is increased. The amount a specific preamp increases your gain is a critical thing to understand, because every device your signal will be sent to (power amp, mixer, instrument amp head, etc.) has an ideal range of signal level it expects in order to operate best. Not every preamp can effectively drive a power amp, for example. Some preamps have no gain at all. Others may be intended to increase the signal level just enough to overdrive the input of a tube amp. Some preamps have a gain control, while others (like onboard bass preamps) have a fixed amount of gain. Either way, they usually have a “volume” knob which just passively turns down the overall signal level at the very end of the preamp circuit.
Manipulate the level of Tone. This means that a pre-amp can increase (or decrease) the level of “warmth” or other subtle qualities, to outright distortion of your instrument signal (in general these are considered “EQ controls”). Some people want lots of tonal changing and EQ controls, other people want absolute transparency, and of course everything in between.
Manipulate the level of Impedance. This can be described as the efficiency of the signal transferring from one piece of gear to another.

There is a lot more to discuss on pre-amps, but hopefully this brief description has pointed out what’s really important to check in order to be able to correctly convert weaker signals into Line Level.

Don’t panic! Most mixers, for example, comes with a pre-amp at least on some of their channels, usually from channel 1 to 4 etc… (check your mixer or audio card manual for more specific info)

Converting a Speaker Signal back into a Line Signal

Speaker Simulation Units (CabClone and similar)…

The idea behind Speakers Simulation Units (SSU) is fundamentally a D.I. Box to lower the strength of a Speaker Signal back into a Line Signal, so, instead of connecting an SSU (or similar device) to your music instrument, you connect it to the Speakers output of your Amplifier and then, the output of the SSU, back to your mixer or recording device in the Line Level input.

Mesa Boogie CabClone is particularly versatile because you can decide which strength you want on the output signal (it supports Mic Signal, Instrument Signal and Line Signal). It also allow you to simulate which type of Speakers you want to use to manipulate your tone (open back, closed back or even vintage type of speaker).

Useful Links:

Conclusions

In this post we had a look at the 4 audio signals that are more common in audio engineering and how to convert each one into the other type and when this can be useful technique. I hope you enjoyed the reading and if you have any comment or suggestion, please feel free to comment below so I can improve the quality of this series of articles. Thanks for reading! 🙂