Class A
Amplifier topology where transistors are always on. Very inefficient, runs hot, but produces very linear, pure sound.
Class D
Switching amplifier topology. Highly efficient, cool running, and compact. Modern Class D can sound fantastic.
DAC
Digital-to-Analog Converter. A device that converts binary digital data (0s and 1s) into voltage (analog sound) that speakers can reproduce.
Imaging
The ability of a system to locate specific instruments or voices within the soundstage.
Impedance (Ohms)
Resistance to the flow of alternating current in an electric circuit. Speakers with lower impedance (4 Ohm) are harder for amplifiers to drive than higher impedance (8 Ohm) speakers.
Roll-off
The gradual attenuation of frequencies at the extremes of a driver's range (e.g., bass rolling off below 40Hz).
Sensitivity (dB)
A measure of how loud a speaker plays with a given amount of power (usually 1 watt at 1 meter). Higher numbers (90dB+) mean the speaker is easier to drive.
Soundstage
The perceived 3D space in which the music exists. A good system creates an illusion of width, depth, and height.
Total Harmonic Distortion (THD)
A measure of how much the output signal is distorted compared to the input. Lower is generally better for solid state, though tube amps add 'pleasing' harmonic distortion.