• What is a compressor and how do you use it?
• Pedals versus rackmount, and how to use the effects loop of your amp
• Where do you put a compressor in your chain of effects?
• "I tried a compressor and it totally killed my tone and dynamics."
• What is an expander or gate?
• How do you get a punchy sound, and what does "punchy" mean anyway?
• Why are compressors so noisy? Aren't they supposed to help with noise?
What is compression, and how do I use a compressor?
• From GuitarNoise.com
A compressor is just an automatic volume control. At a basic level, it reacts to volume spikes from your music, and turns down the volume a bit as your levels go up, to keep things more even sounding. People talk about compressors fattening their tone, increasing sustain, or other effects; while those effects are possible, they are really just side-effects that may be achieved depending on your settings and the particular unit being used.
There are a few core elements to compression; once you understand them, you'll have an easier time setting up any compressor with your rig:
• Input level is critical because it determines the range of signal that the compressor "hears" and reacts to. Some comps don't have an input level control, so you'd have to use the output volume controls on your instrument, preamp, or another pedal to regulate the level going into the compressor.
• Threshold is how loud your signal has to get before the compression kicks in. This is essential because you may not want your entire signal to be compressed. Some units have no threshold knob- for them, the threshold is "fixed" and can only be adjusted by changing the input level of your signal.
• Ratio is how much your signal gets compressed once it passes the threshold. Generally, ratios of 2:1 or 4:1 are considered moderate compression, and ratios of 10:1 or 20:1 are considered heavy compression or "limiting". Check out the charts in the articles linked above for more information about ratios, and also about the terms "hard knee" and "soft knee".
• Attack controls how quickly the compression reacts to your signal, and Release controls how long it takes to "let up" and stop compressing after it's triggered. These controls are interactive, and the right settings for them will vary depending on the music and your playing style. You'll have to experiment, but a decent rule of thumb for bass is to start with slow attack and release times, and adjust from there.
• Output Gain controls how much the volume of your signal is increased coming out of the comp, and this is necessary because compression lowers the overall peak levels of your signal. So almost all compressors have a booster at the end, which provides "makeup gain" to bring your signal back up to the level you want. This is what accounts for both the increase in audible sustain and harmonics, and also the increase in the noise floor.
For compressors with only two knobs, "Compress" and "Level", the Compress knob usually controls the ratio and/or threshold, and the Level knob controls the output gain.
Pedals VS. Rackmount, and when to use an fx loop:
Generally speaking, pedals are meant to go between your instrument and the main input of your amp, while rackmount units are meant to go in an effects loop, or between a preamp and a power amp.
Pedals are usually designed to run at "instrument level", which means they will operate best with the input signal from your bass or other pedals, and the output needs to go into a preamp (or amp head/combo). The output of most pedals is not the right level or impedance to work well going directly into a power amp or the effects loop of most amps.
Rack units are usually designed to run at "line level", which means they operate best taking the output of a preamp. An amp's fx loop typically runs at line level, and is a direct connection between the preamp and power section of that amp. The signal from most basses is not the right level or impedance to go directly into most rack processors. Technically you can plug your bass straight in, but it will not work nearly as well as if you put a preamp between them, and it may not work at all depending on the particular gear.
There are exceptions: Some rack units have a "Hi Z" input, which is designed for you to be able to plug your instrument straight in; and some rack effects (especially older guitar-oriented units) are designed to operate at instrument level. There are also some effects loops which may be able to operate fine with pedals in the loop- you'll need to check the manual for your specific amp, or just experiment. One common example would be a loop that operates at "-10dB" line level, paired with an fx pedal designed to work well with active basses -that combination can often work out fine.
When experimenting, listen carefully and ask yourself these questions: Do you hear any distortion? Does the signal seem weak? Does the compressor seem to react too strongly, or not enough? How bad is the hiss, and does the hiss change in level or "quality" when you switch positions? Does the comp have meters to indicate signal levels or compression amount, and do those meters seem to read the same or differently in each position? Another thing to consider is that connecting a comp to the instrument input of your amp head will require an unbalanced (AKA mono, TS, instrument) cable; whereas connecting it in an fx loop, or between a preamp and power amp, may require a balanced (AKA stereo, TRS) cable for optimal signal quality. It won't hurt either your amp or your effects to experiment with connecting them different ways. The only thing you cannot do is place your compressor or other fx after the output of a power amp. That would kill your gear.
Effects loops can be "series" or "parallel". Series means one device leads straight into the next, with no signal splitting. Generally speaking, a serial connection is best for a compressor (although there are esoteric exceptions). Parallel means the signal from your preamp is split into a "clean" channel and an "effected" channel, and then those two are blended back together. Parallel loops typically have a "blend" knob (wet/dry). If your fx loop is parallel, you'll need to set the blend knob to 100% wet in order to avoid problems with phase cancellation.
Rackmount gear is not necessarily better quality than pedals. There are good and bad units in either format. That said, rackmount gear sometimes offers more controls over the compression than you'll find on most pedals, and good-quality rack gear may offer a better signal-to-noise ratio.
Where to put a compressor in the fx chain
• An envelope filter, or any other effect that relies on dynamic control from your instrument, should usually go before the compressor since a compressed signal will not trigger the envelope follower properly. Also, many filters put out volume spikes that can't be restrained just using hand technique.
"It killed my tone and my dynamics."
First, it really is an unfortunate reality that many compressors roll off some of your high frequencies, and our ears/minds hear that as a deadening of the tone. With units like that, the only easy solution for brighter, livelier sound is to get a different compressor. The more challenging solution would be to modify the unit you have with a better op-amp or wider-range input or output filtering, if you are experienced in that sort of work. Don't bother adding an EQ or an enhancer/maximizer after the compressor, as those devices cannot actually "put back" any frequencies that have been cut off.
Second, some compressors can alter your tone when you set them to heavier compression settings. If you don't like the tone you hear from your comp, try using a lower ratio and a higher threshold. How much that helps will vary widely depending on the specific unit you're using. Also, try lowering the gain of the signal going into the comp- that can help because some units use the input gain to determine the threshold.
As far as "killing the dynamics"... that is 100% a result of using too much compression. Compression is all about modifying your dynamics, that's the point. But if it "kills them", for your purposes or tastes, then you've got it set too strongly. Raise the threshold and lower the ratio. Or if your pedal just has two knobs, dial back the one that controls the amount of compression. In the case of something like the Boss CS-3, you'd dial back the "Sustain" knob, and also reduce the level of the signal going into the pedal in order to raise the threshold. You may have to turn down the output volume on your bass to make that happen.
Part of the problem is that people expect to hear the compression working, or hear some magic improvement to their tone (extra "fatness" for example), so they turn it up until they can hear a big change. But most of the time the correct setting of a compressor is found when you can't hear it working. Extreme compression can be useful and cool-sounding in some specialized circumstances; but the vast majority of the time you'd actually benefit most from a transparent "invisible hand" keeping your levels under control without messing things up. And that requires lighter settings. If there will be any tonal improvements from the compressor, expect them to be very subtle.
What is an expander or a gate? And what is "ducking"?
Expanders and gates are processors that respond to incoming signal levels just like a compressor does, but they affect the levels in a different manner. An expander increases the dynamic range of your signal, at a controllable ratio, rather than decreasing it. A gate is a type of expander; the difference is that when your signal level reaches the threshold, a gate goes from "closed" (no signal passes through) to "open" (all of your signal passes through). Some gates offer more advanced control such as ratio or attack/decay, but basic ones are just open or closed. This is a common way of controlling noise and hum from high-gain distortions, single-coil pickups, and other noisy circumstances; when you pluck your note, the gate opens, and when the note dies down the gate closes, keeping quiet until the next note.
"Ducking" is a system where you have two audio channels, and the output levels of one are controlled by the input levels of the other. A common example of this is the voice-over: music plays during a show or commercial, then an announcer or actor's voice comes on, and the volume of the music is lowered to make it easier to hear the talking voice. Ducking uses compression circuitry to raise and lower the levels automatically whenever the controlling voice/audio comes on and passes the threshold.
PUNCH!
People use the word "punchy" to mean different things, so often there is confusion when somebody says a particular pedal or sound is "punchy" and the next person doesn't hear it that way. For some people, it is a sharp clear attack at the leading edge of each note; for others it is the "hump" of amplitude peak that comes a few milliseconds after the leading edge. So for example, comparing the EBS MultiComp with the Demeter Compulator, each of them will sound "more punchy" or "less punchy" compared to the other, depending on the expectations of the person listening. Either way though, compressors in general improve punchiness by increasing the amount of time the signal stays near its peak amplitude. That way there is a greater perception of "loudness" at or near the beginning of each note, without raising the overall level of the signal, which helps your sound punch through the mix without overwhelming the other instruments.
You can achieve a more punchy sound with your comp by carefully listening to the way your signal is being compressed, and using whatever controls and indicators are available to adjust the response so that the first moment of each note, maybe around 200 to 400 mS, is being compressed and held at a steady level for that short time. Ideally you would use a high ratio (maybe 8:1 or so) and a hard knee, and a slow attack may also help depending on your playing style. I find that it helps to visualize the wave shape of the signal: try to picture a line showing the envelope of each note rising and falling, and adjust that line in your mind as you adjust the controls of the compressor, trying to match what you see with what you hear. Ideally you will achieve a signal that has a high plateau at the beginning, then drops off sharply, and then trails away with a smooth slope.
Is it true that compressors are noisy? Can the noise be avoided?
All compressors, even the really expensive ones, have the potential to add noise. Most of them will add a bit of noise in normal applications. The reason is that compression reduces the peaks of your signal, which also brings down the average signal level. Our brains focus on the average levels, not the peaks, so reducing the peaks makes the whole signal seem quieter. To make up for that, nearly every compressor has a boost stage at the end, called "make up gain" (sometimes just labeled "level"). This gain stage boosts the level of the entire signal, including any noise that may have been in the signal path already from your pickups, preamp, pedals, room wiring, or a variety of other sources. You may not have noticed the noise before, but it was there- and the gain stage on the compressor just turned up the volume on it.
Additionally, any active signal processor may create a bit of its own noise, depending on how well it was designed and the quality of the parts used. So with compressors you get a combination of those two noise sources: noise that was already there in the signal, now amplified; and noise created by the actual circuit design and components of the compressor. That second noise source is where you can see improvement between a crummy cheap compressor and one of better quality. That's the factor I am referring to in my reviews when I say a compressor has "low noise" or "more noise than I'd like". Good comps and bad ones will both amplify existing noise, but good ones are well-designed and made using high-quality components, and so will not add much noise of their own.
Now, to complicate matters a bit, there are many different types of compression circuit, and some types may be inherently less prone to amplifying noise. But it's hard to predict the results when shopping for a specific type of processor, as again it will vary depending on the way that particular model was built.
Another issue that I've run into many times is that some designs are very prone to picking up ambient electro-magnetic noise in the room. Tube compressors in particular give me endless problems this way. You may find that you have a tube compressor that I say is "too noisy", and you don't hear any noise at all. It's partly because you're in a different room, with different electro-magnetic fields.
However another factor is the system you use for listening to your sound. Some amps and speakers are not designed to reproduce the very high frequency range that noise occupies. Some bassists use cabs with no tweeters, or with the tweeter turned down. Some headphones have a limited frequency range. And some people tend to roll off the highs with an EQ. All of these options can contribute to not hearing any noise. Conversely, if you have a very high-fidelity listening system, or if you turn up the highs for a bright cutting sound, you will hear more noise than anybody.
Is there any way to combat the noise? Well, you can turn down the highs on your rig. You can eliminate other noise sources in your signal chain, such as overdrive pedals or any sort of extra amplification/gain/EQ/boost stages. You can check for ground loops in your rig- they can be hard to track down, but ground loops are a major source of noise that most people do not even realize is going on in their system. You can put any noisy items after the compressor instead of in front. As a last resort, you can use a noise suppressor or a noise gate- but honestly I would strongly recommend doing what you can to reduce the sources of noise before adding any gear that will alter your signal even more.
Very often, noise is caused by ground loops or other grounding (earthing) problems. There is no universal standard for the way circuits/devices are designed in terms of grounding, which means that some devices which work great on their own, or with certain other gear, will work terribly in connection to gear which was just designed a bit differently. Also since often the ground is internally connected to the metal housing of a pedal or rack device, any two processors may touch and connect through the metal rack and housings, resulting in a common ground- which may cause noise depending on the specific gear. Here is a very good article on the subject of ground loops: Rane Note.
Noise gates can be useful tools sometimes, but generally they are terrible at solving noise problems. There are exceptions here and there, but in my opinion 99% of the noise gates I've seen included in compressor units are worthless because they do much more harm than good to your signal. They chop your signal on and off, wrecking the natural attack and decay of your notes, while allowing all of the noise through for the duration of each note. In general, unless you are making intentionally choppy-sounding music, or recording/performing in a very dense mix, I would avoid using a noise gate. Even in the best of cases it will not eliminate hiss or hum from your signal path, it will only turn down the volume in between each note. Noise suppressors are an entirely different type of device. They operate by filtering the frequency range that noise occupies, and that can work well sometimes. But even then, I still think it's better in general to solve the source of noise problems, whenever possible, rather than trying to mask them.
Did somebody tell you that a compressor would help reduce noise in your rig? They were wrong! :)
Is a compressor necessary? ("Another guy told me he never uses compression.")
Short answer: No, not for everyone. A compressor is a very useful tool, and just as with any other tool, the better you understand how to use it the more useful it can be. There are a lot of circumstances in which a compressor is the right tool for the job. But that doesn't mean you have to use one. Many people prefer to regulate their signal peaks as much as they can strictly through playing technique. And many amplifiers (especially tube amps) compress your instrument signal a bit just by the nature of the amp design. Overdrive and other types of distortion also compress your signal considerably. So under many circumstances it may not be "necessary" to use an external compressor. It's up to you to analyze your particular situation and determine if a compressor, as a tool, can be useful to you. Remember too that mis-using a compressor will get you bad results, exactly the same as mis-using any other sort of tool. If you apply a chainsaw when you needed a fine dovetail saw, it'll be bad news for your woodworking project. But don't blame the chainsaw!
Please read these very useful articles, they explain everything:
• From Harmony Central
• From Electronic Musician
• From Mix Magazine
• From Jim Carr
• From Rane
• A .pdf from dbx, "Compression 101"
• A .pdf from TC Electronic, "Compressor Applications"
• A .pdf from Rane, "Dynamics Processors"
It's mostly a question of personal preference, but here are some considerations:
• Overdrive, distortion, and fuzz compress your signal already, so additional compression is often unnecessary, and may even interfere with the tone of the distorted signal. So you may want to place the compressor before the distortion, or switch the compression off while using distortion.
• Most phasers, flangers, wah-wahs, and choruses are not too particular about whether there is compression before or after; however some of them may put out strong resonant peaks which could be kept in line by putting the compressor after.
• Pitch-shifters often track better after a compressor, since the incoming signal is kept at a steadier level.
• Delay and reverb are not impacted much by compression. If you use the noise gate feature built in to some compressors, though, it will cut off the trailing ends of the echoes, so switch off the gate or put it ahead of the effect.
• In the case of using a line-level rack compressor, your choices are fewer: it generally has to go between your preamp and power amp, as described above. If you use line-level rack effects in a loop, then you would use the same decision processes as with pedals, and weigh the pros and cons of the effect of the compression on your rack multifx as a whole.
I see this line pretty often. There are several factors that can cause this experience:
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