Typically a resistor and a capacitor, used to compensate for the speaker impedance (to make it easier to drive).
Because loudspeaker cables with enough capacitance act as a zobel network.
Here I run out of answers, as I don't want to speculate.
Nope....
A Zobel network (or Bucherot Cell) is not there for any reason to do with the speaker load or to make it easier to drive or anything like that.
A negative feedback amplifier with voltage feedback tends to show an inductive output impedance, with associated phase shift. This is often accompanied by a speaker load which due to voice coil inductance
can have a steeply rising impedance at HF which is less and less of a load for the amp as frequency rises. In the case of a common emitter output stage (before feedback) the gain of the output stage itself is proportional to the load impedance and so the open loop gain varies hugely with frequency, as, therefore does the
loop gain and the amps stability. It is less awkward with an emitter follower output stage but the phase shifts are still present.
The Zobel network puts a capacitive load on the output of the amp which tends to counteract the inductive output impedance of the amp and its resulting phase shifts and hence helps to stabilise the amp. The resistor in the network shelves the response to prevent the capacitor looking like a dead short at a very high frequency.
The output inductor and resistor (I don't call it a Thiele network. It was in use a very long time before his 1975 article!) are there to ameliorate the effect of capacitive loads on the amp and the resulting phase shift and falling impedance with frequency on the amp.
It is the
inductance and not the capacitance of the speaker cable that can help it to form part/all of this network. A value in the range 2 - 10 uH will usually be chosen and must be optimised for a particular amp design of course. The resistor in parallel with this inductor is there purely to damp the "Q" of the output inductor... With a typical feedback amp using an output inductor it is very instructive to feed it some square waves and observe the ringing at the output... The ringing is often due to the inductor and not due to HF gain peaking in the amp! I tune my output networks for best overall response in my power amp designs
A few companies, including Naim and IIRC Exposure (there are several others) place a small value resistor of usually around 0.22 Ohm in series with the amps output, in place of the inductor and resistor usually used. The amp then feeds the speaker via a resistor of often higher resistance than the speaker cable!
The idea is that this resistance is enough to isolate the amps output from the load capacitance but in the case of Naim it can be not enough on its own and need some inductance from the speaker cable. It's a bad idea! IMHO anyway. Its only possible benefits being to give a more constant output impedance with frequency from the amp (hence more constant damping factor... same thing)... but achieved by nobbling it so it's much worse at all frequencies! Another possible advantage is the resistor acting as a load for speaker back EMF but this is fairly tenuos and again highly frequency dependent.
What it definitely will do is add 0.22 Ohm in line with the speaker cable and reduce damping factor to a theoretical maximum (assuming a perfect amplifier and perfect zero resistance speaker cable) of 36 into an 8 Ohm load and 18 into a 4 Ohm load....
For those with an amp which doesn't use an output resistor of around 0.22R you could try putting one in line with the speaker cable and see what it does to the sound
Even if you have a Naim or similar you could add an extra 0.22R (or more) to give a idea of what it's doing. I think you'll find it to be very audible and not in a good way.... more "confused", less separation between instruments, less control etc.
In some cases, depending on amp and speakers used, some may find a "sweetening" of the sound and that it can sound less "relentless" and "hard" but this is the exception rather than the rule and the loss of control and precision should be evident at the same time.