I am often asked by students about compressor pedals. What do they do? Do you use them for solo boosts? If I want a lot of sustain, do I crank the sustain knob? Sustain is good, right? I always tell them more or less the same thing: Compressors aren’t sexy. They don’t make your guitar sound like a spaceship, and they don’t make it swirl, echo, spin, distort, or cry. Why buy a pedal that doesn’t affect your sound in such a dramatic way? To answer that, we have to understand what a compressor actually does to your sound. You actually can feel a compressor working more than hear it, and if you are hearing it, it just may be set wrong.

Lovin’ Touchin’ Squeezin’

Vintage tube compressors are still used in studios for their smooth sound.

First, we have to understand that a compressor, well, compresses. What does it compress? It compresses the dynamic range, or the distance between the loudest sound your guitar can make and the quietest sounds you can barely hear. Think of it like this: We have all listened to music on shuffle mode, and some songs jump right out while others are half the volume. You turn up the volume to hear the quieter song, and the next one jumps out of the speakers making you rocket out of your seat.
Now notice that this never happens on the radio. The radio station uses a device called a compressor to even out the volumes of every song, so one isn’t louder than the other. Here is another example: Ever hear a singer that whispers and screams in the same song, yet both are able to be heard above the music? Now we know outside the recording studio, a whisper would never be heard above a rockin’ band. However, the vocals are compressed on the recording making the volume of both whisper and scream just about the same. So a compressor takes loud sounds and makes them quieter, while taking quiet sounds and making them louder.
Applying this to guitar, it can make hammer-ons and pull-offs the same volume as picked notes. It can make one string sound as loud as six. It can make the volume of heavy rhythms consistent. Play lightly, it boosts the volume. Dig in, it lowers the volume. This is why I think you usually feel how a compressor works more than hear it.

What do all those knobs do?

Just turn them all up, right?

While compressors used in recording studios can have many controls to tweak, I’d like to focus on the controls you are most likely to find on the average pedal compressor. The Seymour Duncan Vise Grip has four knobs and one switch. We will start with the most obvious, which is the volume knob. As we learned, a compressor takes quieter sounds and makes them louder. It also takes louder sounds and makes them quieter. So the overall volume is reduced a bit. The volume knob makes up this difference in volume by raising the gain of the compressed signal (when the pedal is on) to the uncompressed signal (when the pedal is off). Some people make the compressed signal even louder than the uncompressed signal- this is good for a boost during solos.
The sustain knob is the amount of compression. You might think that sustain is good, so you should crank this way up. As a note dies down, the compressor automatically raises the gain louder, letting notes ring a little bit longer. But there are a few tradeoffs to be aware of. It will also reduce the initial attack of each note, and every little pick scrape or thump your hands make on the guitar will be amplified as well. Hum from pickups and excessive distortion will be more apparent as well. Excessive compression also robs the guitar of touch-sensitivity, which, depending on your style of music, might not be desired. For country chicken’ pickin’ or metal rhythms, a lot of compression is great. For blues or jazz, not so much.

Perfect for bass or any clean signal.

The attack knob will adjust the initial attack of the note. Short attack times (with the knob turned down) clamp down hard on the signal as soon as you play it, sometimes sounding like someone is turning down the volume very quickly whenever you play a note. Longer attack times (like when the knob is closer to center position) allow the initial attack through. To my ears, this sounds the most natural, as I can still shape the dynamics with my pick or fingers while having the end of the note more compressed.
The blend knob actually blends in the uncompressed, dry signal with the compressed one. This way, I get to keep the initial attack and bloom of the notes over time, while hearing the compression on the tail end of the note. The blend knob can also smooth out a compressor that is trying to keep up with very percussive, tight playing. This is not only great for guitar, but for vocals, bass, and keys as well. As a guitar pedal, it is usually the first in the chain of effects on a pedalboard.

Set it Right the First Time

The Waves Renaissance Compressor is one of my favorite plug-in compressors for recording.

A compressor set badly is …something to behold. As the guitar signal is turned up, the compressor turns it down. As notes fade out, the compressor raises the gain. You get feedback on the end of notes, but the attack is too soft and quiet. The laws of physics don’t apply here as a harder attack with the pick results in a quieter signal. You can hear the compressor working, and if you think it is bad for a guitarist, a vocalist can damage their instrument trying to work against a badly set compressor. So take time setting it up. You shouldn’t hear it working so much as feel it. You might not hear a huge difference between when the compressor is on or off in terms of actual tone, but it should make playing a bit easier for you as it evens out the dynamics of taps and arpeggios.
Some people (like me) use it for sustain in solos as well as a solo boost. Every recording we ever hear uses compression, as well as every TV and radio station. Compressors are everywhere in recorded and broadcast music, and it is a great tool to harness no matter what style of music you play.

What is your first pedal on your pedalboard? Do you ever use a compressor?