Extensive compressor reviews and FAQ

 
What does "line level" mean?
 
The term line level doesn't always mean the same thing, so when you see a product claimed to have a "line level output", you still need to investigate further to find out what they mean exactly, in order to match it up with your other gear.
 
In simplified terms, line level is a very strong output signal, typically found coming from microphone preamps, mixers, effects-loop "sends", and the "line out" of an amp. Also this is the level needed to properly drive a power amp, and to get low-noise results when using some recording devices and processors. This is in contrast to "instrument level" which is the level that typically comes direct from a guitar, and "mic level" which is the typical output of a microphone, both of which are much lower than line level.
 
Generally speaking, if you send an instrument-level signal into a device that needs line-level input, you will get excess noise and/or weak sound. If you send a line-level signal into a device that's meant for instrument or mic-level input, you will get distortion. Since the effects loop on most amps is designed to both send and receive line-level signals, if you put a typical instrument-level effect pedal in the loop you will often get noise, weakness, and distortion. You may find a few exceptions though: either an amp loop that can operate at instrument level, or a pedal that can operate at line level.
 
The "loudness" or "strength" of an audio signal inside your rig is measured in AC voltage; the numbers you'll read in an amp's manual or on a website are not the maximum peak level of the output, they are an average of the sound between "silent" and its maximum peak. That average level is indicated by a how much it is above or below a reference level of 0.775 V. That reference voltage is called "0 dBv", and the term dB (decibel) is the term for the amount a signal level changes in relation to a reference point. So when you see amp specs that say "-10 dBu" or "+4 dBu", they are telling you how much lower or higher the average output is, relative to 0.775 V. The terms dBv, dBu, and dBm have slightly different meanings, but they all have that third letter that signifies a fixed reference value.
 
Some common levels you'll see:
+4 dBu is "professional" line level, common in modern pro recording gear, and it is about 1.25 V.
0 dBv is an average line level, typical output from rackmount guitar/bass preamps.
-10 dBu is "consumer" line level, common with older and cheaper recording gear.
-20 dBu is roughly in the neighborhood of a typical instrument's output.
-30 dBu is again in the neighborhood of a typical microphone's output.
However, instruments and microphones can have a very wide range of output levels in reality, so it is most practical to think of instrument-level and mic-level in/outputs as just "a lot lower than line level", rather than calculating specific dB amounts.
 
It may even be necessary sometimes to boost one "line level" output by using another gain stage, if the first output is around -10 dBu and the device you're trying to drive is designed to operate best with a +4 dBu input level. Another thing to note is that decibel numbers by themselves are just ratios in reference to a specific starting point, not a fixed value; in other words, 35 dB gain from one device can result in the same actual level as 50 db gain, or 10 dB, or even -20 dB from another device--it all depends on what values each separate engineer started with. 35 dB gain from a boost pedal is a lot, but it may not necessarily get you up to the +4 dBu level needed to drive most power amps, for example. So look for that third letter after the dB to know that you're dealing with a fixed reference point, and therefore a firm value for the highest average voltage output. +4 dBu is the same level all around the world.

 
 
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