Fast Amplifier

a "fast" amp is one that has the ability to output "naturally" a quickly rising transient input from recorded sources like piano. while the "quickness" of an amp is affected by characteristics that include bandwidth, the "objective" measurement of "speed" is generally expressed by slew rate (theoretically, the higher the expressed value, the faster the amp). as with damn near everything in audio, tho, your ears are the ultimate measuring device. once you understand what "fast" means, you'll know it when you hear it.

Cornfed pretty much summed it up. The three terms that summarize a "fast" amp to me would be rise time, slew rate and ultra wide bandwidth. You can't have really wide bandwidth without having pretty solid measurements on the other two categories though, so it is kind of redundant. If the device can't due at least 100+ Khz with excellent linearity, you will have "side effects" that are audible and measurable within the normally accepted "audio range" of 20 Hz to 20 Khz. This can be seen as rounded edges of square waves, ripples or sags on the tops of the square waves, ringing on the trailing edge of the square wave, etc... Most of these things would be quite visible on a good quality scope with a test frequency of 10 Khz or so if the amp was "too slow". Sean >

No offense to the two previous posters, but let me try to answer you in less technical terms. Perhaps some will disagree with me here, but this is how I define it. Terms like "fast" and "quick" refer to a component's ability to reproduce the natural dynamics of a sound. Dynamics or transients are the changes from quiet moments in a passage of music to loud moments and back again. Musical instruments when heard live (for example, think of plucked guitar strings, percussion instruments, etc.) have a certain snap to them. There's an immediacy to the individual notes. They sound, well, "live." Recorded music sometimes lacks that natural level of dynamics and--by comparison--can sound like you're listening to the music with a curtain between you and the musician. Instead of a sharp raise of a plucked string going from jet black to instantly there before you, it's as if there is a slow (relatively speaking--we're talking about the speed of sound here) build up of the sound. Good audio systems can come close to capturing that same level of quickness that live music has. So a "fast" or "quick" or "dynamic" component is one that is able to render those soft to loud passages in a realistic manner. At least that's how I've always defined it.

Sean, in my experience quite a few of the manufacturers also include a low pass filter in their high bandwidth amplifier designs. If an amplifier's pass band extends well above 100KHz it is also capable of amplifing RF signals which may not be a desirable effect.