When guitarists confront the issues of dynamics and sustain, The Attack and Decay, of the notes they play there are many possibilities to consider. If the problem they're trying to address has to do with a thinness or weak sounding guitar voicing or notes dying off to quickly, options might include increasing the gauge of the strings, using a booster or an overdrive in front of the amp, replacing the pickups they currently use for a set with more mids, purchasing a graphic EQ, or any number of other remedies. A piece of gear sometimes overlooked or passed over, either because of misunderstanding it's concept or design or functionality, is the Compressor. What actually is a compressor? How would a guitarist utilize this device to achieve their desired results?
Compressors have been used in recording studios for quite awhile now. In this environment they're used to remove the peaks and valleys within a recording. They're meant to be transparent. When applied properly someone listening to the recording shouldn't even know it's there. Knowledge of a studios use of compression often leads to a players misconception of how a compressor would be used within their rig.
A compressor, as used in the studio, and a compressor, used within a players signal chain, is an example of how a tool can be used in a different setting to achieve a completely different result. For the guitarist who fully grasps the concept that everything on ten is an incorrect approach and that sometimes a little of this or a bit of that can make all the difference in the world, a compressor might be the perfect solution to their issue with dynamics and sustain.
A compressor works by first boosting the signal and then limiting the amount it initially lets through. Note* The opamp, (boost stage), used in one of the vintage models was the JRC4558D. This is the same opamp used in the TS-808 Tubescreamer as well as some of the early versions of the TS-9. As the signal decays it releases its hold and this creates the sonic impression of a note having a longer tail than usual. The note sustains longer. It retains its volume longer as it fades slowly away. On the front end of the signal, the attack portion of the picked note, a compressor acts a bit like a peak limiter. It takes the strength of you pick attack and slightly alters it's effect from simply an increase in volume to a percussive quality that can impart a feeling of strength in definition or power. Notes enter like the roar of a bear or the kick of a mule and then slowly decay like day turning into night.
Compressors are normally placed as the first pedal in the signal chain. Many players like to place an overdrive directly after the compressor. In essence their signal is boosted, limited, and released by the compressor and then boosted again by the overdrive. This type of setup results in a thick, rich overdriven tone. It can produce a guitar voicing similar to that used by Santana on his sustain laden guitar melody Europa. Another setup used by many players from the 80s right up to present day substitutes a distortion pedal in place of the overdrive. This can result in a guitar voicing similar to that heard on the Steve Vai masterpiece For The Love Of God.
So a question arises. If a compressor can pull this off why isn't it used by more players? First off, this type of physical note definition isn't always appropriate to the type of music being performed. Secondly, compression, as used within a guitar signal chain, is seldom applied properly by players inexperienced in it's proper settings. It's purpose is to take the knife edge off the front and sustain the tail of a picked note. Many players become so fixated on how long higher settings will sustain a note that they overdo the levels to the point of destroying the attack of the note. Their note definition is completely squashed. Also, extreme settings on a compressor will result in extreme noise both prior to picking a note as well as when the note starts to decay at the end. To these players I would say; I understand your desire for notes to sustain into another lifetime, but, without an appropriate attack to define the front end of the note and a decay that can tail off into quietness you create more of a noise laden hum than a true vocal quality. Back off on the amount of compression until the front end pops and the tail end is elongated. When these two tonal qualities have been reached your compressor is set properly.
How many knobs would you like with that?
Some quality compressors only have one knob labeled Compression or Sustain. While this is nice and simple, it really doesn't let you use the boost stage to it's fullest. If you purchase a single knob type, it's best to start with a compression setting around 9:00 o'clock or 9:30. If you are planning on only using the compressor with a separate gain pedal you should turn on the gain pedal while you make your settings.
With the guitar on you can slowly increase the compression. As you increase it, you'll notice the dynamics of the picked note become more percussive. If your running it through a gain pedal you'll notice both a thickening as well as an increase in it's sustain.
A two knob compressor is much better because the second knob is typically a control for the gain level produced within the pedal. It may be labeled as Output or Gain. A quality compressor pedal, if set with the output very high and the compression very low, will produce tones very much like an overdrive. Having this option is a definite plus. As you raise the level of compression, if noise develops, you have the added option of reducing the gain to delete the noise. This allows for higher levels of compression, sustain.
Three knob Compressors add an Attack knob to the other two. You set your gain/output, and your compression/sustain, exactly like you would on a two knob version. If your running a guitar with single coils set the Attack between 7:00 and 11:00 o'clock. If your running humbuckers set the Attack between 2:00 and 5:00 o'clock.
Important: Remember that even single knob compressors have their own onboard gain section. As you turn up that single knob, your compression and your volume will increase. For clean sounds, this isn't much of an issue, but if you also use a boost pedal in the chain after the compressor, the volume increase from compressor off to compressor on can be significant. If the desired amount of compression results in too much volume, reduce boost pedal's the output level, (not the gain level) to compensate.
The other by-product of compression is noise. When you start hearing noise when the guitar signal chain is not receiving anything from the guitar pickups, it's a good indicator that you surpassed the optimum setting on the compressor. In this case the noise is originating in the compressor. At this point, you can either back off just enough to remove the noise or live with the noise if the sustain you're getting is more important to you than the noise. Personally, I back off a bit and, if using a gain pedal, slightly up the gain setting on the gain pedal to reach the desired amount of sustain.
Popular Compressor Pedals
Compressor pedals are available from many manufactures. Electro-Harmonix, Boss, Carl Martin, AMT, Toadworks, Diamond, T-Rex, etc.. While many of these pedals deliver a good compressed tone, most professional players that use compression pedals tend to choose from one of three tried and true builds either in the form of a vintage original or a clone model. The three favorite vintage types are the original MXR Dyna Comp, the gray Ross compressor, and the Dan Armstrong Orange Squeezer. All three are excellent at their primary function as well as adding a tonal voicing or shade of their own. These vintage models are getting harder to find and their price is constantly going up.
Some very well made clones of the original three are available. Both Retro-Sonic and Pro Tone offer clones in the under $200 range that come close to the originals. Analogman and Keeley both offer excellent clones of the originals in the $165 to near $300 price range depending on the model. Clones of the Orange Squeeze tend to be a bit cheaper in price, they do tend to add a bit more noise, while clones of both the MXR Dyna Comp and the Ross compressor will run you a bit more. Analogman even offers a pedal that has both the Orange Squeeze clone in one half and the Ross compressor clone in the other half.
Which one is for you
If all you play is guitars with single coil pickups, the two knob compressor pedal is perfect. The two knob pedals are voiced perfectly for brighter, more percussive, single coils.
If you play guitars with humbuckers, I highly recommend a three knob compressor pedal that includes an Attack control knob. Humbuckers lack the front end attack of single coils. So if your going to be using humbuckers an Attack control will make all the difference in the world.
It really is a shame that guitar compressor pedals, their use as well as their proper settings, are so misunderstood by players seeking answers to the very problems these pedals are built to address. If you've already purchased three or four gain pedals hoping for that thick magic tone to develop and you're less than satisfied with the results I strongly recommend the purchase of a compressor pedal. When applied directly in front of your prior purchases it will be like adding four additional, warmer, thicker, and longer sustaining, gains to your arsenal.