Who makes

Ralph, thanks for the thorough explanations. I too was having some difficulty understanding some of this, but after reading your two posts on the subject a couple of times I think I follow what you are saying.

What I'm still not quite seeing, though, is the SIGNIFICANCE, at the system level, of the distinction you are drawing between acting like a voltage source as a result of feedback and truly having low output impedance. Putting aside the effects that feedback may have on amplifier characteristics such as distortion, gain, and bandwidth, and PROVIDED that the amplifier is operated within the limits of its current, voltage, power, and thermal capabilities (whatever they may be), if we consider the amplifier to be a black box of unknown makeup, wouldn't it behave in the same manner with respect to its interaction with the speaker regardless of whether it achieved a given "output impedance" (as the term is commonly understood) with or without feedback?

And if so, isn't it reasonable to think of feedback as resulting in the amplifier having lower output impedance, as long as it is operated within the limits of its capabilities?

Best regards,
-- Al

Perhaps Ralph or Bombaywalla will have some additional thoughts on your question, but that's the best I can do on it.

Thanks, Al.
like Ralph & you, I was trying to understand what the strength of the ARC Ref150 power supply is when Bruce wrote 1040J. Tell me if I'm wrong:
150W/ch into 8 ohms - I calculate that the secondary is at 35VAC.
Energy = Voltage * Current * time
Then, 1040J/35VAC gives me a Current * time product = 29.71.
So, *supposing* the Ref150 power supply can provide 10Amps, then 29.71/10Amps = 2.971 secs. That's a long time to supply that much current.
How I read this is that with 1040J of storage energy, the Ref150 can supply 10Amps for 2.971 secs while maintaining 35VAC on the secondary.
With music program material no transient is going to last that long meaning that the Ref150 power is pretty darn robust....

Hi Al, the significance isn't. IOW, yes, to your first question. As to the second, no, simply because, well, the term is IMO mis-used.

To be clear here what I am saying is that the actual impedance of the circuit is not changed. IOW what is reasonable is that the amplifier using feedback and operating within its capabilities probably means that it is able to act as a voltage source. Now if the load is entirely resistive its probably not of much consequence. At any rate we are still talking about servo gain in the amplifier if it has feedback- and that servo gain does not actually affect its output impedance.

It is a lot easier to understand this when you look at the example of an amplifier trying to drive a lower output impedance, especially one that might be too low for the amplifier to do efficiently. Then its easy to see that feedback has no effect on output impedance.

Because of this I have really been of the opinion that a different term needs to be used, so that understanding of what is happening comes a little easier. Remember Bombaywalla asking me about intelligent amps? Because the feedback affects voltage response, it works out that it will cause the amp to make less power into higher impedances, which can work nicely if that higher impedance is a peak brought on by resonance.

The problem of course is that feedback also contributes to unnatural brightness in amplifiers, by adding trace amounts of odd-ordered harmonic distortion up to the 81st harmonic (see Crowhurst). Because our ears use these harmonics as loudness cues, we are very sensitive to them- so much so that amounts that are nearly impossible to measure with current equipment are not hard to hear.

So if we can be clear about what is happening, we will have an easier time charting our way to making the equipment do what music and our ears expect.

If we are confused about what the effects of various design considerations are on the behavior of the amp are, its guaranteed to be a muddle, and that is what the majority of the audio industry has been in for the last 40 years or so (by this I mean that the ideal in amplifiers under the Voltage Paradigm really has not changed all that much in that time- just look at how well an old Citation 12 can do against modern solid state).

I have no doubt that this is part of why tube amplifiers are still very much with us 50-60 years after being declared obsolete. Heck, tubes have been obsolete for longer than when they were the only game in town. Obviously, the use of that term is also mis-applied :)

12-06-13: Almarg
Ralph, thanks for the thorough explanations. I too was having some difficulty understanding some of this, but after reading your two posts on the subject a couple of times I think I follow what you are saying.

Thank you, Al, for this post. Good to know that I was not the only one having trouble interpreting what Ralph wrote. I know that if YOU are having trouble interpreting Ralph then I'm well within the right to be confused as well. ;-)

Ralph, thanks for your 2nd post - you have made things much clearer than what you wrote in your orig post on this matter. I think that I now follow what you are saying.

I like Al's question to you tho' as I was thinking along those lines as well. Please clarify further. Thank you.

Bombaywalla, your analysis of the 1040 joule number looks reasonable to me. Obviously the 10 amp number is just a rough guess, but even if it is off significantly the bottom line conclusion would undoubtedly still stand.

On the output impedance issue, I suspect that you didn't see Ralph's last post before submitting your last post, the two posts having appeared at around the same time. Given that clarification I think we are all on the same page technically, and what Ralph is essentially saying is that equating voltage source characteristics resulting from feedback with reduced output impedance, besides not being technically precise, reinforces lack of recognition of the tradeoffs that are involved. Given that interpretation, I see no issues.

Best regards,
-- Al