Quote:

Originally Posted by

Thanks for the comments.

Addressing a few of them...

I admit to disliking the term headroom, because I feel it's misunderstood. Headroom, by common definition, is power beyond what you think you need to deal with unexpected signal peaks. Say you wanted 100 dB SPL at your listening position for each speaker. You factor in speaker sensitivity and loss due to distance. You decide 200 watts / channel will do the trick. Well, maybe you got it a bit wrong. Knowing this is possible, you decide you want headroom. So you double the power. But that gets expensive! Maybe it's cheaper just to reduce volume a bit. Headroom, IMO, is not dynamics. You should factor in dynamics into your planning - for example using THX's stated 20 dB of dynamic range.

**MichaelJHuman**Thanks for the comments.

Addressing a few of them...

I admit to disliking the term headroom, because I feel it's misunderstood. Headroom, by common definition, is power beyond what you think you need to deal with unexpected signal peaks. Say you wanted 100 dB SPL at your listening position for each speaker. You factor in speaker sensitivity and loss due to distance. You decide 200 watts / channel will do the trick. Well, maybe you got it a bit wrong. Knowing this is possible, you decide you want headroom. So you double the power. But that gets expensive! Maybe it's cheaper just to reduce volume a bit. Headroom, IMO, is not dynamics. You should factor in dynamics into your planning - for example using THX's stated 20 dB of dynamic range.

Haven't chimed in in a while...here's my take on the gentleman's point on headroom. You can achieve headroom two ways:

1) Brute Force (buy an amp with RMS rating far in excess of what you think your peak power need will be

2) Clever engineering (buy an amp with an adaptive rail (Class G or H) whose RMS rating is matched to your average power need but with significant dynamic headroom to meet your peak power need). Now you'll have a hard time finding an amp wioth 20 dB dynamic HR, so there has to be some degree of "brute force" over specification even with Class G&H to meet a 20dB figure.

The other factor that comes into play and is perhaps more significant is how does your amp perform when driving a load with impedance curves that dip well below 4 ohms and or that exhibits large phase angles? This pertains more to amps with constant rail voltages instead of those described above. Most of us buy an amp based on how it performs in continuous output into an

*8 ohm, time and frequency invariant load. A factor I always look for is does the PA "double down" in continuous and peak output when you halve the test load resistance? A PA with a power supply designed to do this will double the current it can supply when the test load is cut from 8 ohms to 4, from 4 ohms to 2, and very few can even continue down to 1 ohm.*

**assumed**Because your "8 ohm" speaker very likely does present a load less than 4 ohms at one or more frequencies, this is a significant consideration. AVR PAs spit the bit on this test. Here's a link to a Stereophile test done on a Parasound PA that has long been out of production, showing how it behaves in terms of peak output when driven with short tone bursts (duration not specified but probably in the 10s of msecs) as the test load is decreased from 8 to 4 to 2 to 1 ohms.

This PA is a friggin arc welder. It delivers bursts of 2200+ watts into a 1 ohm load. I maintain that if you performed this test on all PAs, you'd see the 3 curves corresponding to 4, 2 and 1 ohm loads pushed way to the left, as 90+% of PAs have not been engineered to source this kind of current.