A music celeb. that I am helping get his catalog in order candidly asked me the above question (after not getting a straight answer after 40 years of recording.) I'll be happy to start it going but the real experts (Jamie, Barry, etc.) can continue on from there. I have stated in print many times that the analog Compressor/Limiter has to be the MOST MISUNDERSTOOD electronic device in recording/mixing/mastering. It's an essential device; most music could not be recorded without one, especially in the 20th Century. I won't bore you with what is inside the boxes (although that's pretty interesting). Here goes. Starting in 1925 with the development of the Western Electric System of electronic recording to wax disk (and later, around 1949, to magnetic tape), recordings needed to be "limited" so the instruments would stay in the same "space" with each other and also to keep the recording needle on the groove. There were (are) two ways to approach this, each valid, each used a heck of a lot in the history of recording. Probably 99.9% of all pop and classical recordings use some type of limiter/compressor in the circuit. Even the famous RCA-Victor "Living Stereo" recordings in the 1950's and the Mercury "Living Presence" recordings as well (Surprised?) So, there are two ways to do it. First: Compression. In the olden days, a compressor was called a leveling amplifier or optical compressor. It's purpose was to reduce or manage dynamic range. It would bring the dynamics of a performance down to a reasonable level for disk cutting (usually 20-30 db of dynamic range down from 100 or so depending on the music). This was needed to keep the music "above the noise and below the skipping point" of old phonograph records, both 78 and vinyl and the distortion point of analog tape machines and recording gear. Here is what I use and love, the Teletronix LA-2A Leveling Amplifier. See picture below. With the switch set to "compress", it is ALWAYS WORKING IN THE CIRCUIT and adds a distinctive flavor to the music as it works to smooth out the mix. You've heard this sound on many of your favorite records. Bill Porter's Roy Orbison and Elvis in Nashville recordings or anything from that studio in the Golden Age uses one as does almost every recording from the 1920's to the 1970's including everything from United/Western including all Beach Boys, Sinatra, Jan & Dean, etc.. It fools you because when used correctly, you are not even aware it's in the system at all. It's a truly magic machine. Second: LIMITING. This is a device that in the pure sense, does not interfere with the music until it hits a certain level in volume and then clamps down, hard. It "releases" the music at the rate you tell it to, sometimes leaving a drop in volume. It "limits" the sound from getting any louder and pushes down the music to keep it in place. You've heard this on albums like REVOLVER, etc. It's obvious when it's working and can be used (as the Beatles used it) as an effect as well as for the main purpose of keeping the needle in the groove. It was invented for broadcast use a loooong time ago to make sure the station did not overmodulate. Considered too "unmusical" for actual recording, over the years it's gained favor. When used correctly (to keep the occasional peak from wrecking a mix but otherwise preserving the original dynamic range of the recording) it works but we can always HEAR IT WORK. The famous Urei 1176: In the 1970's Bill Putnam's Universal Audio 1176LN solid state limiter/compressor blurred the line between a compressor and a limiter. You hear it used on almost all rock, pop, jazz and classical music from the 1970's and on into the 1990's. It's probably the most used compressor in recorded music. Many engineers still use it to record. I've always found it a bit unmusical but one can't have everything... So, any questions or comments, please. Do we understand the difference now? http://www.uaudio.com/products/analog/LA-2A/index.html The LA-2A and the 1176LN, below.