Solid state design options...

I usually take a peek at the slew rate. To me it says something about how careful the amp design is; if the amp sounds good to boot, then somebody did his homework. Higher is better but if the topology is sloppy, a high SR can mean instablity, distortion, and noise - especially with more output devices. IMO, this parameter gives the amp its speed (pace), adds to the illusion of realism by presenting transients closer to real time, and presents quieter silences. Of course, it doesn't pull everything together - there are many other factors (I wouldn't call them "marketing gimmicks", they are just specs, which show performance, which in turn only hints at the sound).

I was reading on the VAC website a while back about damping factor, and it caused me to rethink it a bit. Their point is that amps with high DF are essentially using back-emf as feedback to the amp, thus modifying the current delivery from the amp in real time. They then go on to make the points that (1) the voice coil is a lousy sensor, as it wasn't designed for this, and (2) the back-emf almost always passes back through a crossover network, which further changes its amplitude and phase characteristics. Interesting points for those who think high DF is the way to go. It may not necessarily be so. It may actually be better to have a zero-feedback output stage and simply reduce the output impedance as much as possible. Then it's up to the speaker to accurately reproduce the waveform being sent to it. Just some food for thought; I personally don't know which is better.

Karls,

Interesting feedback comments!

Likewise I was reading the GamuT Audio website (http://www.gamutaudio.com
On the right side click "FAQ" & then click "what is damping factor") & they claim that damping factor of just 25 is sufficient.

When I think of damping factor I mostly think "amplifier output impedance". The lower it is, the better off you are (& the more naturally you get bass control). I'm not aware of the various techniques of obtaining high DF but your comments have me pondering....

The more the speaker impedance changes, the more the variation in DF, which is not so good i.e. load dependent DF. I have inferred that those amps that have a constant high DF over the audio band have low output impedance over that same freq. range. Might be a simplistic view but it is one criteria I use to short-list an amp. Listening to it provides the final verdict.

A small point: DF is the ratio of the load impedance to the internal impedance of the amp. If we average the speaker load to 8ohms, just how do we get a DF of 25 or 50 (unless we're marketers)?? In other words, how low must the amp's impedance be to reach these beautiful ratios?
Where's Sean and other EE's?

The way you get really high DF (ie, really low output impedance) is with negative feedback in the output stage. This isn't the only factor, just the easiest way to reduce the output impedance to really low numbers. And with negative feedback comes the issues I mentioned above. Visit vac-amps.com; there are numerous thought-provoking essays on this and other topics. I'm not saying they're right, or that there aren't other issues, but they do offer some interesting points to think about.