Amps (Amperes) measure the flow of current that a potential difference forces through a circuit. The symbol is always A. The name derives from the French physicist André-Marie Ampère (1775-1836).
Since current is a flow of energy it is able to do useful work. In many instances it is the magnetic effect of current that is used and the more current that flows the greater the effect. In motors and transformers this current flow is beneficial and is made intentionally high. In close conductors carrying potentially interfering signals high current flow is likely to be detrimental and is often kept as low as possible.
Current carrying capacity of conductors and connectors is directly related to size. Large currents need more metal.
Typical currents in low-level analogue audio signals are very small – pA (picoamps – 10-12 amps) to µA (microamps – 10-9 amps).
Phantom power for microphones is usually in the range of 2 – 5 mA.
Equipment using AA and PP3 style batteries draw <100 mA – though with digital equipment there may be peaky bursts which average to this rather than a continuous load.
Equipment driven by NP1s and similar batteries may draw 1 – 5 A and the power rails of much digital equipment running at 3 – 5 V also draw quite high current.
With power amplifiers the loudspeaker outputs (which are using the magnetic effect to drive the voice coil “motor”) are much larger – typically several amps, or 10 – 100 A in the case of large PA drivers.
Currents higher than 100 A require extremely large conductors and are avoided wherever possible.