Amplifier current vrs watts; why is current more important?


Lately when talking to knowledgeable people in the audio industry I’ve been hearing how current delivery is more important than watts in determining weather an amplifier will drive a speaker.
So what exactly is current and how does it effect speaker performance? How can a amplifier rated at 150 watts into 8 ohms vs one rated at 400 watts into 8 ohms be a better match for a hard to drive speaker?
hiendmmoe
hiendmmoe
viridian
So I do it backwards and choose the amp first.
That’s fine, so long as the speaker then chosen doesn’t cause that amp to become a "tone control"
To me I would think in that easy "Kantor Simulated Speaker Load" graphs I posted above, that Stereophile uses, that +- 1 or 2db deviation from flat would be acceptable.

That means easier loads than the "Kantor" horns, JBL ect the amp would be almost flat and on harder to drive than the "Kator load" the deviation would still be just acceptable at +-3db deviation from flat.
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Cheers George
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A speaker is an INDUCTIVE load.  What that means is that it proportionally resists the rise of voltage across it.  In Calculus, the voltage across an inductor is given as:

V = L x di / dt

L: inductance in henries and di / dt is the rate of change of current thru it.

When transients occur, the amp has to overcome the resistance of the speaker to increasing the voltage across it.  That is why it is difficult to drive inductive loads.  To be able to do that, amp has to be able to deliver loads of current.

Electronics 101: inductive loads.

PS: Music is not a straight sinusoid signal.  It is a MESS of transient signals.  A piano for example is almost like a steep 90 degree rise in signal.  To faithfully reproduce it, an amp must be able to supply LOTS of transient current, hence the need for an ENORMOUS power supply.

Having said that electrostatic speakers a different ball game.  They are capacitive loads.  Combine different frequencies and it is a miracle that modern amps actually work :-)




The issue is a little complex when discussing audio frequency modulated currents. Voice coil speakers use alternating electric current in copper coils in close proximity to the speaker magnets to create the mechanical motion of the speaker cone. Audio modulated AC current in any coil produces frequency dependent reactance and impedance not simple 8 ohm DC resistance that might be inferred by the speaker specification. The accuracy of overall sound reproduction is the ability of the amplifier to overcome the reactance and impedance across the audio spectrum otherwise sound distortion is introduced. There is also the issue of electrical resonance and the amplifier needs to be able to dampen such. In general all modern amplifier designs effectively deal with these issues unless you’re an electronics hobbyist building your own amp. 
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