Dynamic Headroom

Cyclonicman - Thank you for you valuable summary.

Al - I like percents or ratios instead of dB. In case of 500W vs. 30W ratio of perceived loudness is:

R = (500/30)^(1/3.5)= 2.23 where "^" means "to power"

Eldartford wrote: "Almarg...I don't agree with your description of the power supply as "weak". It may have been designed that way."

I'm not sure what you disagree with. Nobody designs power supplies for class AB amps to be able to sustain sinewave at full power for unlimited time - otherwise it would require heatsinks and transformer of the size of class A amp. This would not make any sense since average music power is only few percent of max power. Power supplies being much smaller are already compromised (weak). Al is just saying that within given power supply size (wattage) you could design for higher output voltage (getting better headroom) sacrificing output current and making it weaker for average power demand.

Eldartford - I believe standard calls for 5 min rms power. http://ecfr.gpoaccess.gov/cgi/t/text/text-idx?c=ecfr&sid=129b71f1c02566f96bb00521fdf0bd03&rgn=d

I remember that amp I used once at New Year's Eve party quit after about 10 min - thermal protection started switching output OFF and ON constantly.

My current amp Rowland 102 is rated 2x200W but Icepower modules inside are rated for 40W continuous and 55W FTC rated power. 150W that my amp outputs with my 6ohm speakers is only 33% louder than FTC rated 55W.

I remember listening to one amp that was very dynamic and had a lot of headroom but on heavy orchestral piece sound just sagged - I suspect that higher momentary power was achieved by higher voltage and power supply caps but when momentary power demand extended to few seconds whole headroom collapsed (small transformer?).

Luxman mentioned by Cerrot was top quality amp imported from Japan until they attempted to sell mass market product thru Costco and lost "image". Competition with cheap brands was not successful while Audio Dealers stopped carrying it. I would expect very good performance and honest ratings from Luxman.

Eldartford - I don't listen very loud, perhaps 50% of max what makes already 1/10 of the power and because music does not contain peaks alone but most likely less than 50% volume most of the time it is another 1/10 of power. Music has also gaps - it is not continuous sinewave making it even less. I would assume that average power delivered to speakers is less than 1% of the max rms (around 1.5W).

Output stage of my amp is supplied from regulated 47V making 370W peak at 6 ohm. It's peak and not rms so overall headroom is not very big but it sounds big. It might be because of immediacy of response but also because of my moderate listening levels.

Edartford - It is class D amp with full H-bridged Mosfets. There is practically no voltage drop on them (possibly a volt total).

As for amps being design for "huge peak-to-average" - that's true but this average might vary. Some amps will handle Jazz nicely but give up at heavy orchestral piece (much higher average).

"A class A amplifier has by definition 0 db of headroom."

I'm not so sure of that. Class A is often designed with bias current in order of 150% o max output current to guarantee that transistors won't become nonlinear - hence some headroom might be possible.