What does Current mean in a power amp???

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!

Rwd - Seandtaylor99 has it right, the first question you should be dealing with, and the one that is not answered here - because you have not supplied us with the info to do so - is: Why am I being told I "need" a "high-current" amplifier in the first place? What are your speakers (and room size, and listening tastes and volumes), and what is your present amp? Who told you that you will need a different amp, and why? Do *you* hear a problem? Can you listen the way *you* want to happily? If not, why not?

You shouldn't be thinking about "numbers to look for". This will only lead to the kind of confusion you see above (not that the many worthwhile attempts to explain the whys and wherefores of the "current question" aren't valuable or interesting, or appreciated), and will not necessarily help you to choose a new amp you will be happy with, should that prove advisable. You should be thinking about sound results, and maybe also the agenda or qualifications of your advisor. The most important thing to realize, if you are indeed unsatisfied with the sound you're getting right now, is that nothing anyone tells you, either here or anyplace else, if going to be a proper substitute for listening to some options and deciding what you like for yourself.

If you want some applicable perspective on your situation, please respond with specific answers to the background questions, so we can actually assess and address *your* audio needs, desires, and questions in a more pertinent fashion. The ability of any amp to output "high" current into a given load if called upon to do so is probably always a 'good thing' if looked at in a vacuum, but audio design always involves trade-offs (especially as cost, preference, and system-matching are concerned), and that attribute in and of itself will never alone dictate which amp you'd be most pleased with for your system, tastes, and budget.

[P.S. - BTW, Sdcampbell, I have always been led to believe that the correct 'water' analogy - as far as it goes - is that voltage represents 'electrical (water) pressure' and amps of current the 'rate of flow'. Your description strikes me as the reverse of this. Care to comment?]

...now we're speaking in full engineer terms but let me clarify further:

First, we need to understand further the concept of IMPEDANCE rather than RESISTANCE. In case with DC we call it RESISTANCE. In case with AC we call it IMPEDANCE since it has a reactive components such as capacitance and inductance. The resulting complex value of impedance consists from active resistance R and reactance which is a complex value:

Z = R + i/2*pi*f*C + j*2*pi*f*L

where R is active resistance C is capacitance L is inductance i, j are directional vectors since capacitive reactance with inductive reactance is out of phase by 90degrees, f is the freequency and pi is the famous constant. The sum of members with directional vectors in the formula is called reactance which is the value excluding the active resistance. Please note that reactance consists of vectors and doesn't follow the arithmetic adding and certainly the Impedance is a vector value and has a direction.

So the Ohm's law for the alternated current will yeald: U = Z*I where U is voltage(volts) Z is IMPEDANCE(Ohms) and I is current (amperes)

As to the Bryston discussion it realy has not enough current for continues power and clips! Their professional models don't clip but sound pathetic. If you have Bryston amp you'd better not use low-impedance speakers, it will clip.

Bryston 4b-st for instance is not even capable to deliver 200W into 8Ohms as specified. Plinius SA100 rated 100W/8Ohms will rock much faster and easier and even louder with the speakers of the same sencitivity and impedance no $hit!

To finalize this situation I believe that 80% of that issue is depending on power supply and the rest is to electronics capability to deliver the current.

Marakanetz: Since music is typically comprised of multiple instruments being played at the same time, the signal produced can be extremely wide in terms of frequency response at any given moment. All of these notes / instruments can be played simultaneously.

As you know, most speakers are "multi-way" units i.e. have woofers, mids, tweeters, etc... and some may have multiple drive units within each range. Besides the amplifier trying to load into all of the various drive units simultaneously at any given point in time, it also has to deal with the signal dividing crossover network. As such, it is possible for an amp to see very different impedances and levels of reactance at different points within the audible bandpass at the same time. With that in mind, we are now using a complex signal ( multiple notes over a wide frequency range ) driving a complex load ( multiple drivers working within limited frequency ranges ). As such, each driver / frequency range as provided by the crossover network could present a different picture with various challenges to an amp at any given time. You simply can't "sum" all of those variables into one simple equation.

On top of this, reactance of a speaker / individual drive unit can vary with different drive levels applied. Obviously, not all drivers saturate at the same point nor do they have the same amount of excursion capability. While better designs seek to eliminate such variables and / or reduce the potential for what would appear to be a "seam" between drivers in a multi-way system at any given drive level, such is not always the case.

If the "lack of seamlessness" situation does arise, you can bet that part of what you hear is not only attributable to the speaker design itself, but also to that of the specific amplifier being used. The amplifier is responding to oddities in terms of loading within the "problem" region. As such, some amps may respond in various manners to loading conditions and the result is why we have audible differences.

This is the reason that you may think that a speaker has a certain sound to it yet hear a very different presentation when you change amplifiers. The electrical constants of the speaker have not changed. The ability of the circuitry to deal with those electrical characteristics is what changed when you swapped amplifiers.

Given that both amps were being driven within their range of linear operation and were not clipping, how can you mathematically calculate the discrepancies heard and explain why one amp sounds "better" or "different" than the other with the same load on them ??? You can't and there is no specific formula to do so.

If i were to "pick a formula" for a "good" amp, the amp would have a helluva power supply. I am not talking about just a big reserve of staggered value capacitors placed at strategic points within the circuit, but a very powerful transformer with high rail voltages with extended duration current capacity. The circuitry would be very fast in terms of rise time. In order to to have a fast rise time, you have to have wide bandwidth as you can not have one without the other. This insures good linearity within the audible bandpass since the unit is fast enough to keep up with signals above and beyond that range. The amp would also have a very fast slew rate. This would mean that it could respond to changes in amplitude very rapidly. Between having a quick rise time to duplicate sharp directional and / or polarity changes in the waveform, the high slew rate would allow us to do that regardless of intensity or amplitude of those changes. On top of all of that, the amp would have a very low output impedance. This would minimize the ability of the speaker to actually "modulate" the output of the amp. Some call this the "damping factor" of an amp.

The end result would be an amp that was quite stable and retained consistent loading and sonic characteristics into whatever you threw at it. Obviously, "passive" parts quality and circuit layout does count, so one would have to take such things into consideration. If you can get those basic things right, chances are, you'll have an amp that will work well into just about any load you can give it AND sound pretty good doing it. Sean
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