Who makes

"A number that generally says more about the robustness of a solid state (but not tube) amplifier than all of the foregoing is how closely its 4 ohm continuous power rating approaches being double its 8 ohm rating."

That's the most common spec out there to use to help gauge ability to handle a "difficult" load.

Not perfect or 100% reliable always (numbers are meaningful but no single number can tell the whole story) but is generally a useful benchmark to help weed out pretenders.

If a similar rating is even provided into 2 ohm, that is usually a good omen as well that shows the maker really cares about these things.

Even more bankable than the specs is when reviewers like Stereophile actually perform measurements with 8 4 and 2 ohms when bench testing a product as part of the review.

Mapman, I agree with your comments buy you might be missing something: how the use of negative feedback affects this.

As you probably know, most transistor amps (especially ones that can double power nicely even to 2 ohms) employ negative feedback. **All** inexpensive solid state amplifiers do as well.

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. If you have heard of the Wolcott tube amplifier, this amp employed enough negative feedback to also act as a voltage source, and it was completely unable to double its power into lower impedances.

Its right about here that I see where a lot of designers get into a little bit of trouble in understanding the effect of output impedance on how the amplifier responds to load. The thing that clears the air is something called Kirchoff's Law- the law of energy conservation.

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. 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!

Obviously that does not happen- if you really want to drive lower impedances you need things like more power tubes/transistors, bigger output transformers/heatsinks, etc. IOW Kirchoff's Law stands in your way. IOW adding negative feedback to an amplifier does not affect its actual output impedance at all, only its voltage response.

(Kirchoff's Law BTW is a basic law that says that the energy in an electrical circuit cannot be more or less than the amount of energy put into it. Its one of the first things you learn in electrical engineering.)

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.

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. This can be a bit confusing! On the ground what this means is that the example amplifier probably will not ever put out 60 or 65 watts unless the loudspeaker has a very flat 4 ohm impedance curve.

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?