In the early days of guitar amplification, guitar "amps" had one volume control knob and a few other controls to adjust tone. Today's guitar amps are often loaded with sound effects and controls to further process and alter audio signals, allowing musicians to fine tune their sound. The inclusion of pre-gain and post-gain controls on an amp can create a wide tonal range, from a laid-back mellow sound to screaming distortion and long sustain.
Guitar Amplifier Configurations
Guitar amp circuitry is organized into two main sections, a pre-amplifier, or "preamp," and a power amplifier. The weak signal coming from a guitar first enters the preamp stage, where it is strengthened. Some processing of the audio signal is also done in the preamp. Circuitry to alter the treble, bass and mid-frequencies is common in preamp sections. Sound effects may also be featured in the preamp section of many guitar amps. Although the guitar audio level has been strengthened by the preamp, it is not yet strong enough to drive speakers. Upon exiting the preamp stage, the guitar signal is sent to a power amplifier section, which increases the power to sufficiently operate speakers.
Pre/Post Gain Achieves Distortion At Low Volumes
When guitar amps are turned up loud, there is a natural distortion produced, which guitarists find desirable for certain music genres. Overdriving tubes and transistors, the active electronic components in an amplifier, can cause a distortion that creates harmonics and can add sustain to guitar notes. Turning an amplifier up to full volume might be fine for a live performance on stage, but not always desirable when you are in a recording studio or just practicing at home. To get the distortion and sustain you want at low volume levels, a feature on some amps is the addition of "pre-gain" and "post-gain" volume controls.
Setting Pre-gain And Post-gain Controls
The pre-gain knob adjusts the volume entering the preamp section, while the post-gain control sets the level coming out of the preamp and entering the power amp stage. Start by turning the post-gain control to zero, or minimum. Turn the pre-gain knob to maximum. This will cause the guitar signal to overdrive the preamp section, generating the desired distortion and sustain sounds. Slowly turn the post-gain knob up until you reach the desired volume level. Even though your guitar's volume in the room may be low, you will simulate the sound qualities as when the amplifier is running at full volume.
Pre-gain & Post-gain Stompbox Configurations
The terms pre-gain and post-gain are also used by guitarists to refer to signal routing of a guitar through a series of sound effects foot pedals (called "stompboxes") as well as rack-mounted special effects units. Multiple stompboxes can be placed in series (in a row, with the output of one connected to the input of the next). They can be placed in any order, however, there are certain effects that are generally placed first in line, and these are considered "pre-gain effects." These include compressors, harmonizes, pitch shifters, wah-wahs, filters, overdrives and distortion units. Typical post-gain effects, placed in line after those previously listed, include chorus, phasing, flanging, delay (echo), reverb and tremolo. With the huge number of guitars, amplifiers and effects boxes available, and the ability to place effects boxes in any combination, guitarists can experiment to create an almost unlimited variety of unique sounds.