Vocabulary question

What does it mean when people use the term "fast" for an amplifier?

the rise time to a certain power is fast.
Hi Marakanetz,

Can you explain that sonically?

Some amplifiers will not have the power reserves to reproduce a sudden burst of musical energy. An amplifier which does possess this reserve power is able to reproduce this difficult passage of music. When the sudden burst of musical energy occurs the amplifier reproduces it immediately and completely. A lesser amplifier will trim detail and not provide the sudden impact of this piece of music. This is where the expression 'fast' is derived.

I hope this makes sense to you. You might also run down to Barnes and Noble and grab a copy of Robert Harley's "Complete Guide to Highend Audio." His book has many of the definitions a person might be seeking.
For me an amp which is fast is one in which the leading edge of a note (burst of sound) rises from nothing to full reproduction in a very short time. Think of a graph with a flat line which, when there is a burst of energy, can move perpendicular to its original path, i.e. 90 degrees (which would be as fast as it can get) or could rise on an lesser angle which would make a duller sound. (The correlation to rise time is how long it takes an amp(and speaker)to release a sound, i.e. damping). An amp which is fast can greatly enhance the sense of pace and rhythm of music if your speakers are up to it.
The most colloquial description I can think of is that sudden dynamics in the music seem to leap out of the speakers at you. For example rimshots or a hard snare drum hit, or the pluck of an acoustic guitar. It's hard to nod off when listening to such an amp.
In contrast a "slow" amplifier might have an equally detailed sound, but simply puts you to sleep. This might be a good thing if you like to relax to music.
It's all a matter of taste (though my personal taste is that "fast" amps sound closer to live music). If an amp gets too "fast" it can be very tiring to listen to for extended periods, though it could sell very well because of the wow factor during short demos.
Thanks guys for all your response....This really give me a better feel for a "fast" amp......With this in mind, does a tube amplifier sound slow?.....(e.g. Hovland Sappire only rated at 40 watt per channle)...
Both tube amps and SS can be slow or fast sounding - a lot has to do with how they integrate with your speakers. I think speakers are a bigger problem than amps - one reason that a lot of folks like panels and electrostats - they are faster and do not retain energy as long as most dynamic speakers resulting in a clean and crisp sound. I'm not familar with the Hovland and I don't think you mentioned your speaker.
I currently have the Pass x-250...and I heard that they are suppose to be fast..and that's why I wanted to know what fast really mean so that I can analyse how it sound. I had it for about 3 weeks...and currently driving the Sony SS-M9ED speakers. I don't have a preamp so I'm pluging my Shanling CD player directly into the x-250.

I'm thinking to get the Hovland HP100 as a pre to mate with the pass x-250...and eventually maybe upgrade the x-250 to the Hovland Sappire since it is so pretty....

To me it means how quickly the output signal follows a changing input signal (which is the definition of slew rate). The "faster" the output rise means (generally) that transients and silences are more accurately depicted. The biggest impact, IMO, is that percussion instruments (pianos, cymbals, etc) are better defined with respect to the overall pace of the music.
Perhaps Peter Moncrief of IAR (Intern'l Audio Review) said it best during his reviewing a fast amp, perhaps the fastest in 1998. To paraphrase Moncrief's description of this fast amp:

"(This fast amp) effortlessly reveals layers of musical nuance, is more extended, with the rise and fall time on music's transient details such that each fast nuance is executed more individually, with better intertransient silence, yet at the same time each fast nuance sounds more delicate because it does not sluggishly linger at the peak or get clogged up there, as most amps do to varying degrees."

Mr. Moncrief goes on to say:
"Some amps try for musicality and delicacy by softening and defocusing the music, smudging and veiling everything. (This fast amp) doesn't need any such trickery as it can go for full articulation and sharp focus, yet still sound accurately musical and delicate, because it is so capably fast and transparent... and is so capable that it handles the entire spectrum, and all of music's demanding complexities, with the remarkable sense of relaxed ease that is the hallmark of a truly great audio component."

Moncrief then relates this sense of relaxed ease to Fred Astaire's dancing abilities which Fred seemed to do so effortlessly. Other dancers might come close to Fred but they sweated like pigs trying to do so. Thus the other dancer's straining and sweating were a distraction for the audience toward their performance. Where as, with Fred, the audience saw only the performance. Not the performer.

I think Michelle Kwan has much the same abilities in figure skating as Fred Astaire had with dancing.

Does slew rate affect how fast an amp is?...My NAD S200 slew rate is 100v/us and my pass x-250 is only 50v/us?...

Which amp is better in this case?...I love my x-250 but I still think very highly of the S200. I still owned both amp.