What is a 'fast' amp?

Not asking about specific amps, rather what does a 'fast' amp sound like vs a 'slow' one? This terminology is thrown about a lot and I don't know how to tell the difference.

I got to thinking... is an amp fast when it doesn't rush the music (ie, it is faster than the music, so the music appears relaxed) or does it rush the music, making it sound fast? I peronally would think that neither is too good, but I would take the relaxed over the rushed sound, of course.

?? Perplexed.
The way my friends and I use the term "fast" is an amp that has the ability to reproduce great and accurate details. Some people associate "fast" with bright and forward sounding,not good virtues,however the details and dynamics that a fast amp has to offer doesnot have to come at the expense of a smooth and liquid sound signature. Examples of "fast" great sounding amps would be Edge and Goldmund. Hope this helps.
fast 'rise time' or a fast waveform response vs a slow one.
Look at the square waves in test teports.
A 'fast' amp has a near vertical rise in the squarewave. A 'slow' amp has a curved line in the vertical response to a square wave.
Some amps start out the squarewave with a steep, near vertical line and 'tire out' and start to curve off on the rise.
Some cannot maintain the power and curve off the square top of the waveform towards the far edge.
The graphic chart is not all there is to a power amp, but the response to a squarewave tells a lot about how robust the circuitry really is. Advertising claims aside.
What the 'fast' means in sound is powerful transients. Crisp attack.
Slow means warm. Sound is rounded out. mellow.
If you want to hear what's on you album or CD, would you rather hear as closely as possible what's actually recorded or something smoothed over?

Think of it this way. An amp is taking in what's coming to it, amplifying it and passing it along to the speakers. The faster a good amp reacts, the more realistic your presentation.

It's called the amp's Slew Rate. If you've heard about Class 'A' amps, this is their bread and butter. If an amp is running wide open all of the time, it can react much quickerto the input.

The problem for the user is that the amp is drawing a lot more current from your wall outlet because it's running at full power all of the time. It's also why the amps run warm to hot.

Krell has tried to make this more efficient. You can look into Class 'A' and Class 'AB' amps more if you want to.
Wow, good description, Elizabeth.
Spectral is a very fast amp it has very fast rise time
What about slew rate?
I can relate to Teajay's definition. For me speed implies how well the amp can portray the leading edge of a note, i.e., the attack. And how much it handles the stacatto qualities of intricate piano and guitar playing. I have yet to hear a tube amp that does this even remotely as impressive as the CAT JL2 and JL3.

I do not see how a bright or forward sounding amp can have any relation to speed. I have heard many such products and they smeared the details as well which implies a very poor response time. A Classic example of this is no pun intended, the ARC Classic 150 amps.

There are other things to consider, even when assessing square wave response vs. music:

- If an amp has "overshoot", or it takes the initial transient beyond the original wave's amplitude, then you may get a sound that is harsh or overly etched. Upon hearing this type of sound, some listeners may be impressed, as the amp sounds "fast", and "detailed". However, over time, fatigue sets in, leading to increased sales of used equipment online.
- Similarly, if an amp exhibits a type of "ripple" on the trailing end of the wave, then there could be phase anomalies. Imaging may suffer, or there could be a 'disjointing' between high and low frequencies, causing a smearing when both are present. I think when an amp does this, it causes the music to run together, and makes the quiet moments between notes noisier.

- The best amps are not just the fastest, but are uniformly accurate in terms of frequency AND phase across those frequencies. By responding quickly and releasing just as quickly, the amp lets the music sound like music, and the spaces in between sound as they are supposed to: silent. Ironically, this is how a really "fast" amp can make the music sound SLOWER: "unrushed", and can contribute to a sense of rhythm, pacing, and overall musical enjoyment.

This debate has been around since the 1970's at least, when Kenwood marketed its line of "high speed" amplifiers. Sorry for the long response, but I just wanted to clear up the subject as best I could.
The Atma-Sphere MA 2.2 has a slew rate of 600V/microsecond, which, when I last looked, was the fastest slew rate of any amplifier. Is this still true and is this what we're talking about? Its sonics certainly jive with this remarkable figure.But then I'm biased, as I have one and think it's a remarkable piece of design.