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9th January 2004, 03:00 AM  #11 
diyAudio Member

The short explanation is this: the amplifier's power is limited by two curves: Irms^2*R and Vrms^2/R. If you plot these two curves, the amplifier must be operating in the area under the curves. Where the curves intersect is the maximum power.
Here is the plot for my AlephX amps with 14V rails and 1.54A idle current per side. The blue line is the current limit, the red line is the voltage limit. If you can replicate this plot, you will have a good understanding of amplifier power and load impedance. PS: I consider this a 40W amplifier. 
9th January 2004, 05:31 PM  #12 
diyAudio Member
Join Date: Jul 2002
Location: Seattle

Thanks to panelhead and to jwb for the explanations. It is a tough row to hoe, but some of this stuff is beginning to make sense to me! As Harry Connick, Jr. (and maybe Bobby Darin before him) sang: "I'm beginning to see the light". From jwb's graph I infer that any given amp/setting has a well defined operating area, if that's what you call it. Panelheads arithmetic leads me to conclude that amps designed to stay tough into low impedances must have prodigious bias current (current capability?). I am reminded of a statement that Nelson made in the A75 article, something like: 'we know that about a hundred of you will write in and ask how to bias the amplifier for class A in 1 ohm'.
Larry Wright Seattle area 
9th January 2004, 06:20 PM  #13  
diyAudio Member
Join Date: Dec 2002
Location: Houston

Quote:
There is an even bigger issue here also. A 4 ohm rated speaker usually has the impedance vary from 2.8 ohms to 20 ohms or so across the audio spectrum. Plugging all those real impedances into the equations make the math worse than my confusing examples. George 

9th January 2004, 06:27 PM  #14 
diyAudio Member
Join Date: Jul 2002
Location: Seattle

For me, its easier. I have one set of Maggie MG IIA's with 6 ohm purely resistive impedance, and one set of Maggie MG12's, with 4 ohm purely resistive impedance. So your equations have made things much easier for me to figure out which way to go. ( Both systems are biamped with a subwoofer, so power in the deep base regions is not a big issue either. )
Larry Wright Seattle area 
9th January 2004, 07:10 PM  #15 
The one and only

In many systems, the lowest impedances are found in areas
where the audio power is likely to be quite low, for example below 20 Hz and above 10 KHz, and so this is not usually a real problem. 
10th January 2004, 03:03 PM  #16 
diyAudio Member

Ok, thank you all for your help. Now I'll try to make a schematic.
AudioAngel 
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