Al has it right of course. If I can add something, those high Joule numbers mean that it will be harder for the output stage to modulate that power supply. This reduces IM distortion at higher output powers. Nice huh?
Bombaywalla, I think you have missed some things in my post and it may be because I did not write clearly enough. But before I restate anything, I want you to examine what you wrote below:
i'm having a lot of trouble accepting this. There's a closed form equation that clearly shows that negative feedback reduces output impedance.
VS.:
I don't think so. ability to drive a lower speaker impedance will depend on the output stage (more output current needs to be shared by more output devices), how much current the power transformer can supply, heatsinking ability (all these points you've mentioned in your next sentence). You can keep adding negative feedback but If the amp is incapable of supplying the current, additional negative feedback does nothing
These comments are actually contradictory- here's why (and please do not feel like I am in any way attacking you on this, I have seen very intelligent people struggle with this in the past until they looked at the math): On one hand, you have your formula, OTOH you acknowledge that you need more current ability to drive lower impedances as I had stated (which you also acknowledge). This is something to reconcile.
(I think the difficulty here is that the understanding of how this works is based entirely in the Voltage Paradigm. I have always been careful to use the word 'paradigm' for a reason. A paradigm is a platform of thought; quite often anything outside of that platform is regarded automatically as blasphemy. The take-away here is that life does not care about what we think- reality goes on doing its thing regardless.)
So here is the nub of it: we both agree that you need more current to drive a lower impedance. Now this is fairly simple, so if I were to ask you which of two circuits is lower impedance, the one that has more current, or the one without, what would you say? I am hoping the former rather than the latter!
Now with that established, we can see that it is a profound violation of Kirchoff's Law that by simply adding negative feedback, we can make a lessor amplifier somehow have more current! That isn't going to happen- all we can do is change its *voltage response* (which gives rise to the 'intelligence' of the amplifier).
To put an even finer point to it, let's start with an amplifier that has a high output impedance, such that to drive a 4 ohm load it makes less power than into 8 ohms. We can assume that this amplifier has a fairly high output impedance, right? So if we add feedback with the assumption that it reduces output impedance, it would then follow that we would see the 4 ohm output power increase. But it doesn't- the 4 ohm output power will be seen to stay *exactly the same*. So we can only conclude that the addition of feedback did not affect the output impedance.
The same logic also says that the formula to which you refer (which I assume is correct) is changing something else, which the Voltage Paradigm has identified (incorrectly, based on the above proof) as 'output impedance'. IOW, 'output impedance' is a charged term under the Voltage Paradigm vernacular, and does not actually refer to actual output impedance! Crazy huh? Now go back to my prior post, and reread that part where you said I was not seeming to make any sense. Negative feedback is all about voltage response, not output impedance. The term 'output impedance' really refers to a combination of the actual output impedance of the circuit, in tandem with the servo gain which results from the feedback.
Funny how we can easily use the word 'impedance' and everyone anywhere in the world of electronics understands its meaning, but when you precede it with the word 'output' *and* you are in the field of audio, suddenly the actual impedance of the circuit in question isn't so relevant :)
If this is still a problem for you, just say so and we can go through some math. Its not complex by any means.
This is one of the reasons that I feel the Voltage Paradigm has holes in its theory, and thus becomes a leading edge of how we can effect improvement in the art.
