Who makes

Ralph,

Now if the speaker is only going down to 4 ohms, the fact that the amp can't double power into that impedance does not mean that it is not a voltage source. This is due to the fact that the feedback of the amplifier will make it act like a voltage source independently of the amp's ability to double power.

ok. feedback keeps the output impedance low such that the lower impedance of the speaker is still much higher than the amp output impedance & the amp acts like a voltage source.

Now its understood that adding negative feedback to an amplifier reduces its output impedance, right? But right here we see that this really is not the case at all.

i'm having a lot of trouble accepting this. There's a closed form equation that clearly shows that negative feedback reduces output impedance. output impedance is reduced by a factor of gain*feedback factor. Now, if gain of the amp falls off, then you can keep adding negative feedback & it will not reduce the output impedance much at all. Most power amps are AC-coupled amps so their response at the low end is a high-pass. is that why amp gain is rolling off at low freq & negative feedback is not having the desired effect.

If a circuit really has a lower output impedance, it can therefore drive lower impedance loads without loss of performance. So if negative feedback really did reduce output impedance, you could make any amplifier drive 2 ohms without losses just by adding more feedback!

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.

From this we can see that the term 'output impedance' as used by the Voltage Paradigm does not in fact refer to the actual output impedance of the amplifier at all! Instead, it refers to the how the amplifier *reacts* to its load impedance with its voltage response. That is something quite different.

I have no idea what you've written here!

So in our example of the inexpensive solid state amp that cannot quite double its power into 4 ohms, it is still a voltage source as its feedback causes it to *limit* its output power into lower impedances, based on what it can linearly do into higher impedances.

i'm not sure that this making any sense. your statement seems to imply that this example power amp has intelligence in that it can figure out how much power it can output linearly into a higher impedance & store that in its memory & then restrict its output to that same power level when it encounters lower impedances. Nah, I don't think that happens. I believe that your example power amp will simply run out of ability to drive a lower impedance when it draws all the current it can based on its power output stage & its power transformer.

Will somebody out there please explain why I became a tax attorney when this EE stuff is just sooooo cool??? :) Ralph, if I went back to school to become a EE, would you hire me?

Tech guys -- let me put a face on the dynamic headroom discussion. My amp is a tube amp rated at 150 wpc, as measured off the 8 ohms taps into an 8 ohm load. The power supply is rated at 1040 joules. ARC couldn't fit another power supply cap into the chassis -- no more real estate. If relevant, the sensitivity rating of my speakers is 92 db. And yes, if I push the volume, I bleed out of ears. That is if my wife doesn't make me bleed from the top of my head with a rolling pin first. :)

So in plain English -- what does that mean? Is the 1040 joule stat even relevant?

Hi Bruce,

Kudos on your interest in the technical aspects of this stuff. Most people's eyes just glaze over :-)

Regarding the 1040 joule energy storage capacity of your amp's power supply (that number perhaps being particularly appropriate considering your occupation :-)): What would happen without adequate energy storage is that during musical passages requiring lots of energy, especially high volume bass transients, the amount of ripple on the DC output voltages of the power supply would increase significantly, and since it is that DC which powers the audio circuits, the result would probably be some degree of contamination of the audio signal.

Now, is 1040 joules overkill for a 150 watt amplifier, that presumably has well designed audio signal pathways? I don't know. Particularly in audio, it seems that the line demarcating good conservative design (i.e., design that provides comfortable margins relative to the expected needs) and overkill can be very blurry.

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

Best regards,
-- Al

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.