So… What is the Difference Between Mic and Line Level?

How many times have you plugged in your PA, only to find that you just barely have to crack open the volume knob on your mixer, powered speaker, or “other”, and you’re either getting crunchy distortion from the iPod you have running, or blasting feedback from an attached microphone?  Or maybe you have the opposite problem. You find you need to turn the input and/or output volume all the way up to get an acceptable audio level, (or maybe it’s still not loud enough, and… “Where in the world is that unusually loud hiss coming from!?!”)

Many times my mind starts running down the laundry list of gear that could be broken

–      Bad cable?

–      Mixer? 

–      Power amp? 

–      Microphone? 

Sooner or later, after all components are verified as working and the issue remains, there is one other cause that has literally been staring you in the face (if you’re looking at the mixer).  Welcome to the topic of this blog entry: Proper Gain Staging in audio systems.  “WHAT is THAT!?” you say?  I am so glad you asked. ;)  Here we go:

Most people that work in the A/V industry know that you can’t just plug a microphone into a rack of power amplifiers and expect the attached speakers to make noise loud enough to be heard by anybody.  Why is that?  It’s usually an XLR cable connection on both ends, so why doesn’t it work?  And likewise, most people with even a little experience in this kind of work also know that you can’t take an output from a power amplifier, and connect it to a mixer input without frying the mixer… or power amp… or both.  Thank goodness (nowadays) that most manufacturers have made the respective cable/connector types on those two pieces of equipment so incompatible, that it would be hard to make that particular scenario happen.  If you don’t take anything else away from this little essay, here is the point I want you to remember:

There are three “operating levels” when it comes to electronic audio signal transmission.  Listed here, in order of “quietest to loudest”: 

1) “Mic” level  (usually measured in millivolts),

2) “Line” level (usually measured around one volt),

3) “Speaker” level (usually measured in multiple volts).

 

Speaker level voltage can vary greatly, all the way from a few volts for a small speaker/amplifier combination, to almost 100 volts for an extremely large speaker/amp combination.  Quite possibly you have heard these terms thrown around, or seen them silkscreened next to an input or output jack on a piece of gear.  Steps 1 through 3 above are the basic steps that an electronic audio signal needs to go through in order to achieve the necessary voltage amplification to move a speaker cone.  And here’s how it goes through those steps:

           

When a microphone picks up an acoustic sound wave, it generates a very small voltage waveform.  This is the “raw” voltage level created by the microphone.  And, as noted above, is usually best measured in microvolts.  Over the years, this type of audio voltage level has been given the nickname “Mic Level”.    Mic levels are very, very small.  That is why in nearly every case I have ever seen or heard of, that signal needs to be amplified to a larger level. 

Once it is amplified to a workable voltage level, it can be routed, mixed, and distributed.  Enter the “Preamplifier”.  This piece of electronics is a critical component in almost any audio system in existence.  It can be a no-frills circuit with a set voltage gain that is built into a piece of equipment that you never even notice, or a several thousand dollar stand-alone rack mountable piece of premium gear, 

Hey man nice preamp!

(with hundreds of options in between).  High-end preamps are usually used in recording studios, or possibly on the lead vocal mic in larger scale live concert rigs.  Most often though, this is the rotary knob that you see at the top of a channel strip on just about any mixing console surface, small or large.  Even the digital consoles will have this component located somewhere on or in the control surface.  With a digital console it may either be a software knob, or hardware knob, but whatever form it takes, it will still be there.  The microphone preamplifier has been given several different nicknames over the years, and different manufacturers have labeled it with their own favorite terminologies.  For instance, Yamaha prefers to call them “head amps”, as seen in the silk-screening on the console surface or console software, which is typically abbreviated “HA”,  (if you have ever wondered what the heck “HA” has to do with it).  You will also hear people refer to them as a “mic pre”, “pre”, or “mic trim” (trim is sometimes a different knob from the preamp control knob on fancier mixing consoles, however it is also what other manufacturers, like Mackie

have termed their preamp control).  I will say more about the nuts and bolts of dialing in mic pre’s in paragraphs to come, but the overall design objective of the preamp is to get the voltage signal generated by the microphone to a “loud” enough level that it can be manipulated without any further voltage degradation, often the phrase used here is “signal loss”.

XLR

Let me take a paragraph now to discuss the physical connections on the back of an analog mixing console, and also introduce another important electronic circuit.  Quite often you will have two physical input choices for every channel on a mixer.  One is an XLR jack, and the other is a ¼” (quarter inch) jack.  Typically, the silk screening next to each input jack will say “mic input” for the XLR, and “line input” for ¼”.  Aside from the physical difference in connectors, there is an important electronic difference as well.  The XLR jack on the back of a mixer is generally equipped to accept signals from microphones, because they need quite a bit of preamplification to work properly.  The ¼” jack is designed to accept signal sources like CD players, ipods, and computer audio outputs.  The console is expecting that the voltage levels from these devices will to be larger than that of a raw microphone, and because of that, they do not need nearly as much pre-amplification.

1/4"

May I now introduce the “pad” circuit?

This is an attenuator circuit that is sometimes built inline to the ¼” connector’s signal path before it even gets to the preamp circuit.  Midas, for example, builds in a 10dB pad into the ¼” inputs on their Venice consoles.   This is why a CD player with a hot output signal may overload an XLR input, but when plugged into the ¼”, will then be at a workable level.  On a more full featured console, this may also be an additional circuit that can be inserted into the signal path with the push of a button. This button is almost universally physically located next to the preamp knob on an analog console.  Different manufacturers will design different degrees of attenuation into these insertable pad circuits; anywhere from 10dB to 30dB, but it seems that 20dB is the most common.  The pad circuit is extremely useful for knocking down the input level on a very “hot” signal, which would otherwise overload the preamplifier and cause noticeable (and unacceptable) distortion.

This is where we finally get to the heart of gain staging.  Between these two electronic components, the preamplifier and the attenuator pad, we now have the flexibility to accommodate any level of voltage signal presented at a mixing console.  The overall goal of these two components is to get all of your input signals, as discussed above, to a workable voltage level so they can be routed, mixed, and distributed as needed within the mixing console, and beyond. 

Ideally, we want to get each and every one of our input signals to be as strong as possible without distorting.  This is because there is a certain amount of inherent noise in every electronic component inside a mixing console, and the entire audio system as a whole.  It is important to keep the signal voltage well above this inherent “noise floor” throughout the entire audio signal path so the resulting “signal to noise ratio” at the very end of the signal path is as high as possible.  Why is this a big deal?  I will give you an example from personal experience:

In the fall of 1997, I was getting my audio rig for “Anything Goes” ready to hit the road for a national tour.  We loaded into the first theater, and I started to rough in volume levels for the actors 25 wireless systems, plus all the mics from the orchestra pit.  As our workday ended and the theater quieted down, I couldn’t help but notice a hiss in the idle system while the pit orchestra mic faders were live.  Being mostly familiar with the equipment, I knew that this hiss noise was not something that should be this audible.  After a short bit of troubleshooting, I realized that the issue was the way I dialed in my preamps for all the pit orchestra mics.  I neglected to add enough gain to my preamps on each of my channel inputs for those mics, so the console was adding system noise as the signal made its way through each of the various mixing stages.  I went back the next day and added more gain at the mic preamps.  I was then able to dial the outputs of my main EQ back a bit, (thereby keeping the same apparent loudness to the audience) and was able to maintain a much more acceptable noise floor.

This kind of issue is exponentially increased with each signal you add to a mixing console.  It is cumulative.  Perhaps one mic might not be such a big deal, but when you start adding the noise of each channel together into a single, summed output, it can be very noticeable even to the casual listener.  The opposite problem can also be an issue as well, when many very hot inputs are summed together, it can actually overload a mix or output buss on an audio desk, and cause it to distort.  This is most evident in concert audio situations, where input levels can get hotter as the concert goes on, because each of the performers start pushing harder throughout their performance.  A good engineer will plan ahead for this, and leave themselves a little bit of headroom in the system inputs, (and outputs) so they will not have to back everything down (or “hit a ceiling”) later in the performance after distortion issues start to appear.

           

Let’s return to a more practical application for the average A/V tech.  Let me give you examples of common gain staging issues I see in small audio setups on a day-to-day basis. 

-Quite often I will see systems that use ceiling speakers permanently installed in a facility (hotel meeting or conference room, etc). 

-Someone has plugged a small Mackie mixer plugged into an XLR input jack on the wall of the room, and I hear a noticeable hiss coming out of the speakers. 

-Even though the master output level on the Mackie is turned way down, the audio sounds a bit “crunchy” when I speak into an attached microphone. 

-Almost every time I run into this situation, it is because the mixer output has been patched, (currently outputting a line level signal), into an XLR jack on the wall that is expecting a mic level input, and the signal is overloading the mic preamp attached to the XLR wallplate.  It just so happens that most small Mackie mixers (and other manufacturers as well) have designed an attenuation pad switch into their output stage to accommodate this scenario.  It is a small button usually located next to the output jack on the mixer.  Sometimes the silkscreening will say “Line/Mic” or “+4dB/-50dB”.  If you cannot find a line level XLR input to the house ceiling speakers, the next best thing is to take advantage of this switch and turn the mixer output into the mic level signal that the XLR wallplate jack is expecting to see.

I see very similar situations with many smaller events using powered speakers mounted on tripod stands.  I will see a mixing console output run to a pair of powered speakers, and the XLR cable from the mixer is patched into the mic input on that speaker, instead of the line input, (or the switch on the powered speaker is set to “mic” input).  This creates the exact same hiss and distortion situation as in the paragraph above, except there is a different, and better solution.  In this case, it is best to take the time to re-patch the cable into the line input XLR jack on that speaker (or flip the switch, like on a JBL Eon).  This is the ideal solution because you are now able to keep the voltage level on the XLR cable as high as possible, and any noise that might get picked up in between the mixer and the powered speaker will be “drowned out” by the larger signal voltage available to us in a line level signal.  This way the powered speaker will not have to re-amplify the mic level signal coming from a mixer that is set to output a mic level. 

Most audio mixing consoles are designed to output a line level voltage.  This is because once the signal leaves the mixer, it may have to travel a very long distance before it interfaces with any other equipment, be it an equalizer or a power amplifier, etc.  The longer the run of cable, the greater the chance that it is going to pick up some type of interference along the way (usually Electro Magnetic Interference or Radio Frequency Interference, but that is the start of another topic en.wikipedia.org/wiki/Electromagnetic_interference); so it is best that the actual signal we are interested in is as powerful as we can make it in order to “drown out” any interference that might get picked up along the way.  To reiterate what has already been stated, some mixing consoles have the flexibility to pad the output signal back down to mic level, although you should avoid doing this to keep as much unwanted noise out of the system as possible, unless it is necessary to interface properly with the downstream gear (e.g. house patch).

So far we have discussed mic level signals and line level signals, and the appropriate uses for each, but there is a third operating level that I mentioned briefly at the beginning of this article, and that is “Speaker Level”.  This voltage level is the largest of the three operating levels, and for a couple of reasons will probably require the least explanation.  “Speaker level” is the voltage you get after line level voltage has been run through an audio power amplifier.  As the audio world is evolving, we are seeing fewer stand-alone power amps, and consumers are starting to take for granted that this component will be built into the back of any modern speaker cabinet – both small and large.  One of the nice things resulting from this trend is this: The possibility of connecting a speaker level output to a piece of equipment that is expecting to see a mic or line level input is becoming harder and harder to do, and that is a good thing, because as stated at the beginning of this article you will destroy something by doing this.

Cheesy but effective audio level diagram

I have not really even begun to discuss the actual voltage standards and references associated with each operating level in an audio system, but it is worth also noting at this point, that there are actually two standards of line level audio.  One is the “professional” standard (also referred to as “+4dBu”) and the other is the “consumer” standard (-10dBV).  I will not get into reasons for the specific nomenclature associated with each standard in this article, but the important thing to remember is that a “+4dBu” signal is hotter than a “-10dBV”, even though they are both commonly referred to as line level.  Most CD players, ipods, and laptop computers use –10dBV as their operating level, and most professional audio equipment uses (and outputs) +4dBu as an operating level.  This is mostly important if you may be trying to connect a piece of professional audio gear to a semi-pro or consumer level piece of equipment.  You may end up with a situation similar to that of the line level mixer feeding a mic level input, and end up overloading the destination.  If that is the case, there are several gadgets on the market that you can insert in the signal path to change it to the correct standard of line level.

As you can see, this issue of proper gain staging can become complex in a large audio rig, but a working knowledge of the three operating levels in the world of audio signals is fundamentally important to ensure a good end product; even if all you're doing is connecting one microphone attached to a single speaker.

Bob Conley

Lead Audio Engineer

J&S Audio Visual Houston Branch

Bob is a Berklee College of Music graduate (Boston, Massachusetts), with a degree in Music Production and Engineering.  The majority of his professional experience comes from mixing Broadway tours world wide for about 15 years, with occasional forays into summer music festivals, club venues, and audio for houses of worship of all sizes (which is his passion), during theater down time. Bob started freelancing for J&S in 2006, worked on staff in the Dallas show services division for a short while in 2008, and is the currently working as the lead audio engineer for the J&S Houston division."