You may recall my article about the differences between speaker cable and instrument cables, and how just because they look the same, it doesn't mean that they are the same. Well here is another bit of schoolin' to explain the differences between INSTRUMENT LEVEL, MIC LEVEL, LINE LEVEL, SPEAKER LEVEL, +4, -10, and what the heck (UN)BALANCED means.

This sort of explanation wouldn't be so necessary if there was any sort of obvious difference between the various cables and connectors for each of these signals, but here in the U.S. of A., more often than not, you'll see the same old 1/4" phone connection used for any one of these. That can and will get you into trouble.

First lets tackle INSTRUMENT LEVEL. If you consider the typical output jack of an electric guitar, you are dealing with a signal (which is really an AC voltage) of a very low level. The average electric guitar has it's signal generated by the strings inducing a voltage out of the guitar's pickups and this is what is sent to your amplifier. Heck, it's why you NEED an amplifier in the first place.... to amplify the level. Typically, a guitar's *passive pickups will give you a varying signal of 100 mV rms to about 1 V rms. If you were to plug your guitar directly into a speaker cabinet, guess what... you'd hear nothing. That's because the output level is way too low to even make the speaker flinch. You must amplify this level quite a bit to make that happen.

You typically would plug your guitar into a guitar amplifier, which at the most basic level, consists of two sections: the PREAMP and the POWER AMP. Thinking of a guitar amplifier in this way will help you visualize your way through all this mumbo jumbo. The job of the PREAMP is typically to raise an INSTRUMENT LEVEL signal up to LINE LEVEL, in order to drive a POWER AMPLIFIER which then drives a SPEAKER. Looking at it that way, your guitar's signal is amplified twice: once to LINE LEVEL with the PREAMP and then again to SPEAKER LEVEL with the POWER AMPLIFIER.

LINE LEVEL is further confusing because there are really 2 different levels that are considered LINE (in the US, at least). There is CONSUMER LINE LEVEL, which is sometimes referred to as "-10dBv" (pronounced "minus ten"). This is generally what you might find coming out of your CD Player that you then feed into your stereo receiver/amplifier that drives your speakers. This is also what you might find in the "LINE OUT" of a guitar amplifier, as well as the audio outputs of a keyboard/synthesizer. This level is often way too hot to plug into something that is expecting a low level (plugging your guitar amplifier's LINE OUT into your run-of-the-mill Fuzz Box-which is expecting to see a low INSTRUMENT LEVEL signal will certainly give that Fuzz Box a serious workout. You will distort the distortion box! You also may harm the input circuitry of the Fuzz Box and it would need to be repaired or donated to me so I can fix it).

Then there is PRO LINE LEVEL, which is often referred to as "+4dBv" (pronounced "plus four"). This is typically what you might encounter at the output of higher-end mixing boards, mic preamps, tape recorders, etc. It's called PRO as a means to separate the signals you will find in home audio stuff from what you will find in a professional recording studio. One tip-off between -10 and +4 is that you will often see those small RCA phono jacks on -10 gear as opposed to 3-pin XLR and 1/4" TRS ("tip-ring-sleeve", referring to the three connections on a jack.... which even more confusingly is the same as a stereo headphone cable!)

See why this can be so easily confused? The RCA phono jacks that you find in consumer gear like CD Players is the same kind of jack that you see at the SPEAKER output of a lot of inexpensive home audio gear! This is why you can't be too harsh on someone who can't automatically tell the difference. Without understanding the needs for different levels, you're left with going on your gut, which just might tell you to plug your guitar amp's SPEAKER OUTPUT into your fancy digital 8-track recorder. Is that smoke I smell? Sure is.

Okay, we're about halfway. We still need to explain microphone level and speaker level.

Your typical dynamic microphone, like a Shure SM57 will have a 3-prong XLR output. Like the guitar pickup, a dynamic mic gets it's signal from a teeny-tiny voltage that is induced by the microphone diapragm. It is usually even lower than INSTRUMENT level, and because of that, you need to amplify this signal even more than an instrument to get it to LINE level. That much amplification can be tricky because you run the risk of amplifying any stray noise, hum, RF etc. that happens to infect the signal as it goes along your mic cable to the preamp. What to do...

Here's where BALANCED audio comes in handy!!!! What an ingeneous idea someone had when they decided to create a balanced output on a microphone. You don't need to know exactly how it happens, but the teeny-tiny voltage created with a dynamic mic goes through a little transformer before it hits the output jack. This transformer splits the signal into two identical signals... identical except for one major difference. They are 180 degrees out of phase with each other. What that means is that when one signal is giving you 50mV with respect to ground (0v), the other signal is -50mV! Why is this significant? This is the cool part: You send these two signals down a mic cable, along with a 0v ground reference that serves to shield the cable. At the other end, you might find that you've added all this interference/noise to the signal because that sort of thing is all around us, all the time, and when you run a 20' cable from mic to preamp, you basically have a 20' antenna to pick up all sort of stray interference. Get ready for magic: you take those two signals that are equal, but opposite in polarity and if you add them together, they cancel each other out! 50mV plus -50mV=0v. Simple math. Well what happens if you invert the polarity of one of those signals so they are both, say 50mV is that you are cancelling any stray electrical interference/noise that the cable was subjected to because both the + and the - wires were both subjected to the same interference. I won't go too far into this concept because my head will hurt, but suffice to say, a properly BALANCED system will be much quieter than an UNBALANCED system.

Okay, so back to MICROPHONE LEVEL. It really is tiny. So we plug the MICROPHONE LEVEL signal into a PREAMP which typically brings it's tiny output up to +4dBv, which is a nice strong level to send to a mixer, a recorder, a power amplifier, etc. Again, the output level of most pro audio gear, like mic preamps, rack effects, mixers, recorders, etc is LINE LEVEL (sometimes +4, sometimes -10, and sometimes switchable).

So now everything is up to LINE LEVEL. Our guitar signal, our microphone signal, etc. Now we need to drive a speaker. There is a HUGE difference between LINE LEVEL and SPEAKER LEVEL, and this requires a different kind of amplifier. Whereas the types of circuits to convert INSTRUMENT or MIC levels to LINE level can usually be done with simple inexpensive and not-too-fussy transistor and op-amp circuits, the kind of power required to drive a speaker needs much more firepower.

Depending on the kind of system you're dealing with, either a guitar amplifier or a PA system, the task of the POWER AMPLIFIER is to efficiently get a LINE level signal up to a high-current, higher voltage signal capable of driving a speaker. I won't get too detailed here, but suffice to say that there is some brute force involved with making the cone of a speaker move, and it won't happen without a big hammer.

A guitar amplifier's POWER AMPLIFIER section will consist of circuitry that converts the signal amplified by the PREAMP section to a signal that will drive a speaker at something like 50 watts at 8Ω. Without explaining the details, you're increasing a mW signal all the way up to 50w. That requires some heavy circuitry that generates heat and requires a lot of power to do it's thing. Think of the preamp as someone dusting off your front porch with a broom and the power amp as a wrecking ball crashing into your house.

In the case of a PA system, you want to have an efficient way to convert your small LINE level signal into typically several hundred watts. Think of it like horsepower. It takes one Shetland Pony to pull a radio flyer wagon for ten feet. It takes a stable full of Clydesdales to pull a thousand-pound rock for a mile.

With that in mind, what do you think would happen if you plugged the speaker ouput of a 1400 watt power amp into the LINE input on your tape recorder? If you said "FIRE!" you are correct.

This is why it is important to at least have a basic knowledge of what is going on with regard to audio signals. Why does your guitar sound thin and tinny if you plug it directly into a power amplifier? You skipped the level conversion of the preamp. Why does your recorder smell like smoke when you plug the speaker output of your amp into the Track 1 line input? Because you're sending it a level possibly 500 times what it is meant to handle. Why does your delay pedal sound all distorted when you plug the line ouput of your amp into it? The delay is meant to receive instrument level, not line level.

I apologize for skimming over many pertinent topics in this article. Impedance, wattage ratings etc. are all important factors in all of this as well. For now, my goal has been to at least shed some light on the fact that there are different standard levels when dealing with electronic audio signals and many times, it's important not to interface the wrong things.

If there are any specific questions that I should have answered, but didn't, please comment and I will do my best to elaborate.

copyright 2008 Roger Lavallee