For many individuals using amplifiers and pedals for the first time, assembling a rig can be somewhat daunting, particularly if their initial encounters with these devices have been in a digital format. While Gain, Volume, Presence and more will influence your sound level, they do so in two distinct manners—additive and subtractive; the specific impact they have is contingent upon their placement and, in some instances, the amplifier or effect being utilized.
Volume- The volume control is typically subtractive, meaning that at a setting of 10, it represents the full signal, and it diminishes as the knob is turned down. When this knob operates independently, it restricts the overall sound of the amplifier; however, when it is part of an effect that includes a Master control, it solely limits the signal at the input stage. This has tonal implications—if your input volume is more constrained, there is less signal available for amplification, thereby altering the character of the sound that is ultimately amplified.
Master- The master knob, also functioning in a subtractive manner, regulates the sound produced at the conclusion of the amplification process. Lowering this control maintains the same tonal quality, albeit at a reduced volume. This feature can be particularly useful for adjusting rhythm guitar tones that require heavy distortion without overpowering the lead line.
Presence- Presence operates additively. Its function is to enhance the incoming signal by boosting the high-frequency components, thereby providing the sound with increased attack. Techniques such as strumming, muted picking, rhythmic string attacks, and high-neck playing are all accentuated in a percussive manner; however, on its own, it will not distort the sound.
Gain and Drive- Both are additive controls used to amplify the signal, essentially serving similar purposes. Drive is typically associated with electronic circuits, such as pedals and solid-state amplifiers, while Gain is primarily focused on amplifying the frequencies of an analog tube. Although your HeadRush does not utilize tubes in the traditional sense, it simulates the effects that tube amplification would have on a signal compared to what drive accomplishes.
In tube amplifiers, distortion is achieved by overloading the tube with signal, resulting in a fuzzed processing effect. The overall tonal quality tends to be more open, as the sound is shaped by the tubes being driven by Gain. Consequently, in amplifiers lacking a gain knob, distortion is typically attained by adjusting the EQ settings and modifying the Presence level; the gain knob effectively automates this process for you.
In solid-state amplifiers, distortion is produced by pushing the signal beyond the electronic limitations of the circuit board. The key distinction is that these electronics are generally designed with the expectation that users will intentionally drive and clip these limitations, resulting in greater control over the signal as it is driven.