Grasping all of the features behind an amps settings can be difficult. Many musicians have an okay understanding of what knob adjustments such as EQ and Gain do, but knowledge of the exact function of such adjustments can hold a greater understanding of how to shape your tone, and how each amp is different. These settings are usually similar with each amp, but for the purpose of explanation I’ve decided to augment my descriptions with examples from a Marshall MG100DFX.
The channels are fairly simple in concept. Most amps have at least two channels: A clean channel and a distortion/overdrive channel. Both channels usually are sent to a master gain, and are used for shaping the signal and changing the output level.
A clean channels gain has a higher threshold for clipping, allowing for less distortion of the signal being sent to the master volume.
A distortion channel has a lower threshold, and creates a clipping effect at lower gain levels. To compensate for the difference in signal output, or volume, the channel is run through a second gain stage, which on the MG100DFX, is called the volume. This volume is then routed to the master gain.
Some amps, like the Mg100DFX, have the option of a secondary gain channel, which has an even lower threshold, and allows for more distortion.
Amp equalizers usually offer two or three options. Most common and basic are the bass and treble EQ. These functions work through shelving, which means they effect the signal above or below a certain frequency. The bass EQ shelves low frequencies, and the treble shelves high. These can vary from amp to amp. For example, the MG100DFX clean channel bass EQ shelves low at about 200Hz, and the treble EQ shelves approximately 800Hz and up (This is just collected through tests I’ve done. I’m sure a Marshall employee can tell you these numbers more exactly).
A third EQ option is a mid range band, which effects frequencies in between the two shelves. Mid bands are peak equalizers that effect a specific frequency, and pull others around it in a parabola.
The MD100DFX offers an equal-loudness contour setting. Since the human ear perceives different frequencies at different volume levels, this superimposes a curve to the EQ in an attempt to fix this issue. The higher the setting, the more curve is added
With a better understanding of these functions comes greater control over overall sound. With knowledge of which aspect of tone each setting effects, a musician can adjust his settings more wisely, and have more knowledge to draw from when choosing an amp.