Solid state design options...

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.

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.

Bombaywalla, great post. Please do not assume I have bought into the "all negative feedback is bad" camp, as that is not the case. I merely mentioned VAC's site as interesting food for thought, and much of what they have to say is probably applicable primarily to tube amps due to the presence of the output transformer. I will say, however, that two of my favorite amp designs presently are Pass X and Ayre V_x, both of which are touted as "no feedback" designs, and both of which sound purer than most anything else out there. And I would venture a guess that both of them have quite high DF to boot.