Re: damping factor
For any signal to be transferred properly, it is always assumed that the difference between the output impedance of the outgoing device and the input impedance of the incoming device should be 1:10 (i.e. the input impedance should be at least 10 times greater than the output impedance).
If this is so, signal transfer will be accomplished with little loss. Just as it's true for any link, so it is true for the amp-speaker line. But here more so than elsewhere, because the speaker is no ordinary load, it's far more complex. The speaker has what is known as a Q factor, where Q stands for quality. Not to get into another topic altogether, just take my word for it that it's highly desirable that the amp controls this Q factor, and to do that, it must have an output impedance at the very least 10 times smaller than the speaker impedance. This difference is called the damping factor.
If your amp has an output impedance of say 0.1 ohms, and the speaker has a nominal impedance of say 8 ohms, then your damping factor is (8:0.1) 80:1.
Now, here come the problems.
First and foremost, on the amp side, the smaller the output impedance, the easier for the amp will be to deliver larger currents (assuming the rest can cope with them, of course). In that respect, we want as large a damping factor, or as low an output impedance, as we can get. But it's not at all the same thing HOW you achieve that, nor where.
As to how, just add lots of negative feedback and your output impedance will drop, or your damping factor will rise. Of course, this will cause problems elsewhere, so it's hardly the answer we want. Add parallel output devices and your output impedance drops almost linearily; add another pair of same transistors, and you almost literally halve the output impedance, or double the damping factor.
But the damping factor (output impedance) is a composite matter, because it depends on amp topology, number of output devices, power supply output impedance and your transformer impedance. All these combine to produce the output impedance.
This should explain my strong preference for multiple output transistors, quality parallelled (which also reduces their own inherent output impedance) filter capacitors and large, seemingly overpowered toroidal transformers. All these combine to give a small output impedance even without negative feedback, let alone with it.
However, the damping factor is NOT linear. It's always much larger at the low end, say 0...1,000 Hz, than at the top end, say at 20 kHz. Several factors combine to produce this effect, such as diminishing effects of negative feedback, output inductor, rising output impedane of the power supply, etc. Typical ways to overcome this is to use better power supplies, wider open loop (without feedback) amplifiers, more output devices, avoid use of output inductor (and that implies a supremely stable amp, no easy job), etc.
On the speaker side, as we all know, their actual impedance is far removed from their nominal impedance. It's quite usual to see nominally 8 ohm speakers drop as low as 3 ohms or so, while some otherwise excellent speaker have a 4 ohm rating, and drop to 1.8 ohms. In addition to this, the speakers can be very reactive, meaning that they show large phase excusions, which tax the amp heavily, and a good way of reducing these interactions is to have a good damping factor. The better the damping factor, the less succeptible the amp to speaker funnies in very broad, general terms.
But work backwards here. A damping factor of 10:1 in case of admittedly rare but still present 1.8 ohms means an output impedance of (1.8:10) 0.18 ohms; actually, since you would want to have at least some margin of safety, that should be say 20:1, or 1.8:20=0.09 ohms. Into the full 8 ohms, this would be a damping factor of (8:0.09) 89:1, or to round it off, say 100:1.
More is better, but it's just as important how you get as it is to have it. And it can be had; for example, my favorite prototype has a damping factor of 150:1 into 4 ohms at 20 kHz. But, I use four output pairs in a wide open loop bandwidth design, large high quality capacitors and oversized, low loss toroidal transformers, because of which I can afford a low negative feedback factor and still have an excellent damping factor. At 100 Hz, it works out at better than 600:1 into 8 ohms closed loop (with negative feedback). I have to go that high if I want it to be able to do what I set out to do.
So, to conclude - a good damping factor makes an amp less susceptible to speaker load variations and ensures a good signal transfer, with little dropouts at frequency extremes and good speaker cone control. But it is very important how it was achieved, whether it has been forced by excessive feedback, or whether it is designed in, inherent to the design as such.
In my experience, amps which have been fed back to kingdom come to achieve impressive results will tend to sound less powerful than their specifications and measurements show, they will tend to sound weedy and aenemic, as if they lack real power. Conversly, amps which use lower feedback factors, larger power supplies, etc, will tend to sound full bodied, confident, with true grit and would seem to have more power than their specifications and measurements suggest.
Some months ago, I took my DeZorel filter to a friend's place, where it was connected to his Naim amp. That amp is rated at 30W/8 ohms, but as is it sounded like a typical 70-80W job. We added the filter, which of course made the amp's power supplies far more efficient, and it sounded like my Yamaha AX592, which is rated at 100W/8 ohms (though in all fairness, that Naim cost about twice what my Yamaha cost), but it's an obvious case of a job well done. The late Julian Vereker really did know his stuff.
Cheers,
DVV
