What does Current mean in a power amp???

This has probably progressed well beyond the original question, but it makes for an interesting topic of discussion.

I disagree with the statements that ohm's law is not at all applicable, because with the substitution of the term "impedance" for "resistance", ohm's law is very much true for a single-phase AC circuit. As stated by both Sean and Bigtee, the AC impedance is made up of resistance, capacitive reactance, and inductive reactance. The resistive component is constant at all frequencies, but the capacitive reactance is inversely proportional to frequency, and inductive reactance is proportional to frequency. These properties, along with the phenomenon of mechanical resonance, described above by Sean, explain why any given speakers impedance plot can vary so wildly, both above and below the nominal impedance.

However, in order to look at a "simple" model of a loudspeaker, one must have some way to express the relationship between voltage, current, and impedance, and ohm's law provides that relationship. The other relationship required to model the circuit is a basic power equation, which is also different for an AC circuit. For a single-phase AC circuit, Power=Voltage*Current*Cosine of the phase angle between the voltage and current waveforms. This difference in phase is, of course, a result of the net reactance at any given frequency. To apply the power equation without accounting for phase relationships, you must assume that the reactive component of the equivalent impedance is zero, yielding a purely resistive load. I assume (but don't know for sure) that this is how published amplifier ratings are derived, with a discrete frequency sinusoidal waveform applied to an 8 ohm resistive load.

If you accept the above, then in a very roundabout way, ohm's law does in fact have an effect on whether an amplifier can drive a given speaker. If we could build a "complex" mathematical model for a given type of speaker paired with a certain amplifier (and don't forget the role of our choice of speaker cables in this model) ohm's law could describe, at any particular frequency, how much current our voltage source (amplifier) could supply. We could then look at the phase relationship between the voltage and current, apply our power equation, and we would have our value of power at clipping for any frequency that we cared to look at. Since I didn't do very well in differential equations, I will leave this modeling process to the wonderful people who design audio electronics for a living.

BTW, I noticed that while I was formulating this response, a couple additional posts were added. Thanks to clueless for thinking on the same line as me (kind of scary, huh?), and Seandtaylor99 hit the nail right on the head, although I think we are still just short of a full fledged pissing contest :-)

Perhaps "pissing contest" was a little harsh? :-)
The posts are interesting, though.

~~~~How does one technically piss a thread? Not that I would ever consider it, though I have had threads pissed on, technically speaking. How would one be determined the winner? What would be awarded, medals, trophies, or a review in T.T.P. Magazine?

Jest The Facts

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My point was that Ohm's law ( used in a generic manner ) is applicable at very specific points. Obviously, one can measure and calculate what is taking place electrically at any given point in the operating curve and break it down mathematically. However, the measurements at any given point may have nothing to do with what is taking place elsewhere within the parameters of operation. This is what makes a loudspeaker a "complex" load. Even if one were to try to break the speaker / amplifier interface down using Thevenin's theory, which is far more complex, those parameters would still vary somewhat with frequency and amplitude.

With the above outlook taken into consideration, the amplifier / speaker interface is one of a dynamic nature that changes with amplitude, frequency and impedance. One can't model a "simplistic" speaker / amplifier interface based on just a few measurements that would otherwise work fine in a circuit with set parameters and limited variables. As such, trying to break the entire amp / speaker interface down to something as simplistic as Ohm's Law would be next to impossible unless the speaker maintained a constant impedance across the entire frequency spectrum at any given amplitude and the amplifier acted as a true "voltage source". Does anyone know of such a speaker ?

The parallels that one can draw using a resistor as a load and that of an actual speaker are far and few between. For each "benign" speaker load that you show me, i can show ten that are not quite so simplistic. As such, how an individual amplifier and the corresponding circuitry will respond to each load that it sees with varied frequency and amplitude becomes completely unpredictable. Hell, some amps even change frequency response aka "tonal balance" as the drive level increases on a dummy load ( non-reactive "perfect" speaker ) on the bench. Who could predict how such an amp would respond with the variables encountered with different speakers and their associated reactances during real world operation??? If you think i'm making this up, John Atkinson noted this in a recent review of a Rotel amplifer in Stereophile and Moncrieff had covered this 20+ years ago in IAR.

While some of the variables can be minimized by having a true "voltage source" as an amplifier and a speaker that was linear in impedance, this still does not take into account how this combo would actually "sound". As i've tried to stress, there are just TOO many variables to try and sum things up "simply". While many folks don't have the technical background to know why such things take place, they have enough experience to know that you will never know exactly how various components will mesh until you try them out within the confines of your system. We can call it system synergy, complimentary colourations, etc... but it still boils down to the fact that there is NO set formula other than the old "trial & error" method that most of us have had to do to get where we are today. If things could be summed up easily using a simple formula, i think that there would be no need for forums such as this. Sean
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PS... I'm not trying to be argumentative or step on toes, so please don't take it that way. I'm simply trying to say that you don't know what to expect until you try it. I've been "confounded" way too many times before to know that things that should work sometimes don't and vice-versa.

There's one other factor with an amplifier that envolves its feedback circuit. This also affects how the amplifier "sees" its attached load. Some manufactures install small coils of wire (almost like a choke) on the output to prevent the amp from ringing into loads it doesn't like.
As Sean said, this is much to complicated an issue to be summed up in a simplistic Ohms Law formula that was designed for DC circuits. You can't even apply it to the resistance in a piece of straight wire. Once you apply AC to it, the wire can react rather strange(depending on its composition)
They use to use actual current limiting circuits to keep the amp from overheating.
My best advice is to decide what type of speakers you are going to use an experiment around until you find a good amp to match. Way too many variables to say an amp is right based on specs. Amps that "Really" double down are a good place to start. Some of the best sounding amps---well you're not going to worry about current(eg. the Pass Labs Aleph 3)One heck of a good sounding amp at 30/ch into 8 ohms and about 45 into 4. It drove a lot of speakers well as long as you didn't want to rock a hall!