What does Current mean in a power amp???

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
>

Sean, the equation I've mentioned is not as simple and yes, this equation is being used to specify the speaker impedance curve in responce to any particular freequency. The reactance is a directional part of an overall impedance and resulting vector of reactance can be negative and that's when the impedance falls can occur.

Indeed the music is a complex signal having lots of harmonics and subharmonics but it's more arithmetics still since the numbers we work here with is finite and can be calculated. The theory states that the signal of any arbitrary form or complexity can be represented by the sum of sine waves of different order(Fourier theorem) which means that the freequency(to be more precise a sinusoidal freequency components) of an arbitrary form signal can be defined using Fourier theorem.

As to your statement about the differences in sound with relatively same capabilities amps and the speakers first of all they're assume will be not too much audiable, second there is a specific formula but truly it doesn't make a sence to excersise it untill you find the family of infinitessimal values specifying these nano-ohms and nano-volts once again using a Fourier theorem along with Ohm's law.

As to the good amp formula so not to be so exhausted calculating limits using a complexed engineering formulas(certainly applied in more sophisticated amplifiers than audio) you're completely right and I believe that Plinius SA100 falls in this category.

In defense of the Bryston
Stereophile measured a Bryston 9B-SST (THX at the time) and said it had no problem delivering it’s rated 120 wpc with all five channels driven. I’m mentioning this amp because it was the only Bryston review on Stereophile’s web site. Say what you will about the musicality of Bryston products but I’ve read anyone complain that they did not meet their rated power.

Correction...
That last sentence was supposed to read, "...I've never read anyone complaining that..."
Sorry for the grammar error.

PS the Plinius SA100 look cool. How does this compare to other amps in it’s price range (~$5000)

Nikki, Brystons arn't bad at all as a budget amps having a huge 20Yr transferable warranty but I heard from many people that they can't drive the speakers that have impedance falls lower than 4Ohms -- they start to clip.