Fast Amplifier

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.

Everyone has offered valid points. Perhaps the only thing I can add are some real world examples. In my experience(especially within a manufacturer's line), the smaller, less expensive amps are faster. The bigger ones are stronger. If you have demanding loudspeakers, requiring the power, you need the larger amps. But if you speakers are more amplifier friendly, the smaller amps can give you more musicality(I know I am going to be ripped by some for saying this). In my opinion, it is related to the power supplies. Massive transformers, and coffee can sized caps make a lot of juice available. But it is often slow and unweildy. An analogy would be comparing a dump truck to a Toyota MR2. The dump truck has the torque, but is slow and ponderous. The MR2 doesn't have the power of the dump truck, but is zippy, light on its feet, agile, and FAST. I think that the easiest place to find these differences in audio amps is to compare North American and European amps. Euro amps are smaller, lower powered, and faster. Examples would be Musical Fidelity, Electrocompaniet(not all of them), Jadis, Kora, Audio Analogue, LFD, Cambridge, Pathos, etc. American/Canadian amps are bigger, more powerful, and slower. These brands would be Krell, Mark Levinson, Jeff Rowland, Classe, Bryston, Audio Research, VTL, etc. For Thiels, you need the current of the larger amps. For Triangles, you can use the smaller amps. I have been through a complete metamorphosis over the past decade. I used to be on one side(big amps), now I am on the other.

trelja: my experience doesn't quite match yours. with relatively inefficient speakers (< 87 db), like those iv'e owned in different versions for the past 10 or 12 years, high power/current amps are required. when i listen to different amps by the same manufacturer, they tend to have better and better transient response (rise/decay) as the output/current increases. brands i've experienced this with include accuphase, boulder and jrdg.