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In Reply to: RE: The definition posted by pictureguy on June 26, 2012 at 10:31:31
What you are talking about with the Pass is not headroom. It is simply the amount of 'A' power as opposed to 'AB' power.
The idea with headroom is that the amp can make more power than rated for a brief period of time. So it favors amplifiers with weak power supplies and class AB or B operation.
Yes, charging caps for bursts is easier than full-time high current supply neede for 'a' operation.
I did note the headroom idea in my first reply::
However, don't count out a/b just yet. My favorite example would be back to the Pass amps.
The xa30.5 and the x150.5 are very similar. I don't know how 'under the skin' this goes, but they each weigh about 75lb.....and are both offered as integrated amps. I suspect the major difference is in bias PS and how that is managed.
The 'a' amp? Stereophile got about 100 a side while the 150 was honestly rated. Remember me at Christmas.
Now, a question for 'ya.
My 'd' amp is rated at 250rms, yet that is for 60 seconds. the FTC rating is much lower.
Now, do you call that headroom or specsmanship or ??? The amp uses B&O modules with included SMPS.
I'd also think that unless you were simply a fan of 'a' amps, that listening and intended use would be one of the arbiters of amp choice. I know I never tap the RMS part of my amp, regardless of how it's rated. I may use 20 RMS and allow for a 10x peak factor. Easy for the amp and it never warms. (4 ohm panels)
Too much is never enough
The 60-second rating is specs-man-ship, not actual headroom as it was originally defined.
In a class D amplifier heat is the main issue as often they don't have much for a heatsink. The idea is the output devices spend no time in the linear region so they don't make much heat. It works fine unless you play organ pedal tones at high volume- extended high power tones can heat up the output and possibly drain the supply.
60 sec rating IS heat related. Even figuring 83% plug to speaker efficiency, that leaves something around 75 watts of heat.....quite a bit....A human emits about 100 watts worth, if I'm remembering correctly.
For the Big kilowatt module? Time limit is 30 seconds.
Both per B&O datasheets.
Heat is, of course, the enemy. Which is why some people will do a first pass judgement of amplifier quality by using a scale. PS and Heatsink, of course.
I don't know that my amp has a capacitor larger than my thumb and therefore doesn't store enough energy to drain. Being a smps, the tranformer is also small, gaining from the high frequency at which it operates.
One day I'll put my Killawatt meter on the amp, crank it up and see how much juice it DOES draw, when I'm pushing it. At the same time, I'll look for PowerFactor which is something neither of us has touched upon as yet. I don't know if that is germain to the headroom issue, however.
Too much is never enough
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