Dynamic Headroom

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

I like to think about this issue in terms of voltage. The amplifier delivers an electrical output which is the same shape as the input signal, but at higher voltage. When the output voltage is applied to a load (speaker) current will be drawn, and if the amp can't deliver enough current the voltage will fall. This limits amp power. Amp power may be limited by either the voltage it can output, or the current it can supply. It is better to be limited by current because music typically includes voltage peaks of such short duration that the power supply can provide the necessary current for a brief interval by drawing from its capacitance.

The "continuous rms power" by which audio amps are rated is really unsuitable for audio. The "headroom" measurement is important.

Kijanki...Agreed that "Unlimited time" would be more than "Continuous" in the definition of power. I don't know how the spec is exactly defined but I think that "continuous" could be 5 seconds or so. This would be long enough to reflect any power supply sag, but not long enough to get into heat sink issues.

Since the average power while playing music is a handfull of watts, but the peak may be hundreds (I have made measurements) I do believe that ability to follow peak voltage is very important. Power supply design is influenced by the need to advertize high "continuous rms power".

Using my MG1.6 speakers playing LOUD I measured average rms voltage in the 5 to 10 volt range but I still saw brief peak voltage corresponding to almost 600 watts. I was using 600 watt amps so I was able to see this voltage.
In the end I decided that while 600 watts for a MG1.6 is not ridiculous, 350 would probably be OK (so I bought CT D200 monoblocks).

Kijanki...So it seems that the FTC power spec IS supposed to cover thermal considerations. That might be very reasonable for pro amps which play highly compressed and peak limited music at maximum volume. I play music on my system very loudly (but not compressed music) and my digital amps get barely warm. My comments above are aimed at home audio amps.

Kijanki...The 47 volts you cite is DC. Allowing for a 3 volt drop through the output transistors this suggests that the amp can swing +/- 44 volts. But that 44 volts is the peak of the signal. For a sine wave signal this would allow 228 watts RMS.

Almarg...We agree. My point is that an amp which is current-limited for high voltage output is a logical design for signals that have huge peak-to-average ratios (like music). Having current to spare is no help if you can't swing the voltage. We use amps with huge power specs because that assures the voltage swing. Having that swing continuously is not really necessary.