@pogo here is something from folks who design amplifiers with very low output impedance:
from
https://audiophilestyle.com/ca/bits...eaders-qa-with-lars-risbo-bruno-putzeys-r815/
@Sagittarius: Does the high damping factor of class D amplifiers negatively affect the subjective sound quality of some types of speakers as some reviews seem to suggest?
[Damping factor is defined as 8 ohms divided by output impedance, high DF means low output impedance and vice versa -ed]
Bruno: Oi, we had to battle for this high damping factor! Before anyone worked out how to put global feedback around the output filter, the damping factor of class D amplifiers was actually much lower than that of class A/B. And much good did that do… In particular, the big problem was at high frequencies, where the frequency response became enormously load dependent. Most speakers have a highish impedance in the top octave, and early class D amplifiers would thereby produce a very clear lift in the top end, which explains why they were often perceived as sounding harsh.
Lars: Have we got an indication where this idea of high DF being bad for sound might have come from?
Bruno: I’m not sure. Maybe in the past, high DF was associated with big, sluggish amps? I wonder what speaker they were using. If they’re listening with a broadband speaker with no crossover filter for instance, a high output impedance could bring some of the benefits of current drive and reduce hysteresis distortion in the iron in the speaker motor.
Lars: You’d have to really go overboard.
Bruno: An SET amp with no feedback typically has an output impedance equal to the rated load impedance so you’d be looking at nearly 6dB less hysteresis distortion in the midrange. But for any other speaker this trick is quite useless. Crossover filters are simply designed with the assumption of a voltage source. It’s a matter of standardisation. Otherwise how this particular speaker sounds when connected to that particular amp becomes rather unpredictable.
Lars: Isn’t that precisely why hi-fi shops do all that mixing and matching?
Bruno: It’s quite possible that shop owners would have less fun if all amps had sensibly low output impedance. But beyond “sensible”, damping factor is completely overrated in my view. Once output impedance is low enough to keep response changes due to load variations to within a small fraction of a dB it’s low enough. The term damping factor is seriously misleading because some folks think that an amp with a DF of 1000 is ten times better at stopping a moving cone than one with a DF of 100. It doesn’t make a jot of difference. In both cases the resistance of the voice coil, the crossover filter and even the speaker cable will dominate totally. In actual fact you wouldn’t even want to have infinite damping because the speaker designer counts on the natural resonance to define the bass response. So it’s rather a good thing that the speaker has its built-in resistance in series with the amp.
Lars: Having a super low output impedance does have one real benefit: it makes bi-wiring work. The whole point of bi-wiring is to isolate the tweeter and woofer portions electrically. If you have a common impedance in series with the speaker, distorted currents produced by the woofer will turn into a distorted output voltage which in turn is seen by the tweeter section. So if you want bi-wiring to work its magic you really do need an extremely low output impedance, which is the same as super high DF of course.
Bruno: Good point, a hundred might not be enough then. I think people resort to bi-amping when their amps don’t have low enough output impedance for bi-wiring to do the trick. So for a serious audiophile having really high DF is a real advantage, if only financially.