>.to many other things to explore
There's not that much to explore - it's very simple. There's a lot of articles on damping factor, most don't get to the balls of the issue they only add confusion in peoples minds - that ESP article is one of those. Forget all the stuff about back EMF etc. while some of it is true that view doesn't let you see anything clearly.
With solid state amplifers you cannot easily control the output impedance with the type of amplifier or the external component values; it makes such a small different it is of no consequence. You have to *create* an amplifier output impedance using feedback and that requires a feedback connection which is different to the standard ones.
* What damping factor does:
Damping factor represents the output impedance of the amplifier. High damping factor = low impedance. Low damping = high impedance. View the output impedance of an amplifier as an ideal amplifer with an impedance in series with the output.
The impedance of a guitar speaker varies with frequency. If you drive that speaker with an ideal amplifier it will have a certain frequency response (as would be documented by the manufacturer). If you now put a resistance (Rout) between the amplifier and the speaker the speaker impedance forms a divider with the added resistance. The frequency response is modified according to:
Vspkr/Vamplifier = Zspkr / (Zspkr + Rout)
When the speaker impedance is high you get more "boost". A guitar speaker has a peak in the bass region and a rising impedance in the high frequency region. Consequently the lower the damping the more bass boost and more high frequency boost. You can see this on the second picture of this document:
http://www.aikenamps.com/JJ_EH_tube_tests.htmlThat's all there is to it. You should be aware that the resultant frequency response with low damping will depend on the speaker and also whether the speaker is in an open or closed box.
* How to change the damping factor of a solid state amplifier:
The way to do it is to modify the feedback so the output voltage depends on the output current. A standard way to do this is represented in figure 2 of this page:
http://sound.westhost.com/project27.htmThe components R26 and R23 deviate from the normal feedback connection of a non-inverting amplifier. R26 provides a current sensing and R23 routes the current sensing voltage to the amplifier feedbacks. The components R26, R7, R6 and R23 are the voltage and current feedback components (one of the values is redundant, it only scales the others) . It is possible to write down an equation which gives the output impedance of the amplifier, Rout, in terms of these part values. There's another equation which gives the gain.
Checkout a number of modern solid state amplfiers on line: Marshall, Fender, Hughes Kettner, Crate, Peavey, Laney (some discrete some chip). They all use this feedback configuration.
You can do this with most amplfiiers, it's particularly straight forward with the standard discrete amplifies like the example. It is equaly straight forward for chip amplifier that have standardard inverting/non-inverting inputs like opamps.
* Why bother with it
Tube amplifiers, especially guitar amplifiers, have low damping factors. The low damping factor interracts with the speaker impedance (as described above) and this interraction is part of the sound/equalization of a tube amp. Without it it sounds flat.
Mimicking the output impedance on a solid-state amplfier ensures the frequency response is more like a tube amplifer.
[Added note: when you use feedback to create the output impedance/resistance there is no additional power dissipation and no loss of voltage swing at the output terminals - unlike adding a series resistor..]