agedhorse

It's no different between a line frequency based supply and an SMPS (with a bit of a distinction for PFC based supplies). It's all about total efficiency, the higher the efficiency the lower the input power that's needed to achieve rated power. Taking your hi-fi amp as an example, if it's rated at 110 watts/channel (RMS metrics), then it's going to be roughly 160 watts/channel at 4 ohms. The calculations would be as follows: Total audio power at lowest rated impedance is 2 x 180 watts (total of 2 channels) = 320 watts. The NRTL agency testing power will be (320 x .125) = 40 watts total. Say the efficiency is 35% for the amp and 75% for the power supply, the rated average power being drawn will be 40W/(35% x 75%) = 152 watts which is what the agency will be looking for as the "rated average input power". There's another number that can be included, it was more often used on hi-fi and more commonly used a while ago, but that is "MAXIMUM" input power which is of little use unless you are operating the amplifier continuously with a sine wave signal (your speakers may not appreciate this). Here, the calculations will be a little different in that instead of the 1/8-rated power specified by the safety regulations, you would use the total rated power at the lowest rated load impedance. Note that the efficiencies are typically higher in full rated power testing) In this case it would be ~320W/(45% x 85%) = 830 watts (this is higher than your given number, I suspect that my assumption of 4 ohm power is on the high side OR the amp is not rated for 4 ohm operation. With class D, you can run the same calculation but substituting the average efficiency of about 90%, and with an SMPS the average efficiency is typically about 92%. Thus: 40W/(90% x 92%) = 48 watts rated average input power. There is not a "something from nothing" scenario here, it's all about how the regulations require that we state the input power, and in reality the 1/8-rated power is not too far off until we get into heavily overdriven or compressed tones where the crest factor (an expression of the ratio of peak to average content of the signal) decreases. IF the amp is designed specifically to support such operation, especially if it is advertised as such, the 1/8-power factor may be increased to more accurately reflect the expected operation of the amp. Note that different countries have slightly different requirements for testing and labeling (called "national differences", but this is by far the most common way it's done. Hope this helps.

Thanks Charlie! This is very much the same process that we use when designing amplifiers, developing the combination of load profile, driving signal and thermal conditions, integrating these factors over time. This was even more the case when designing linear amps when we had to worry (a lot) more about SOA (safe operating area). Class D amps are more forgiving in this area than linear amps due to cut-off/saturation operation. While this is correct for voltage, it is NOT correct for power as you would be ignoring the squared factor in the power equation (P=V^^2/R) When RMS is used in describing power, all it means is that the voltage and current are based on RMS units, not that the power itself is in RMS units. It's a short-hand notation that's understood by most professionals in the industry and has been used for years.

I think you are misunderstanding what the rated input power number means and why it's required of any amplifier that is sold in the EU, as it must be tested and certified to EN60065. This regulation specifies that the rated power input be based on a minimum of 1/8 rated audio power (in RMS units) at the lowest rated impedance. It can be higher than 1/8-duty cycle if the amp is specifically designed for this operation, and passes the tests accordingly (many do not, or the companies may choose not to test to extended duty cycle beyond the legally required 1/8-power which is 12.5%) Designers have no choice but to follow these regulations, they are regulations that the European countries have specified that we designers follow. If you don't like it, take it up with the authorities having jurisdiction in your region. The reason for this extends all the way through the audio world, in pro audio for example, this number is used to assist in sizing branch and feeder mains circuits, environmental heating loads and such for installations where many amps may be installed. The same certification standard applies to bass amps. This is a commonly misunderstood bit of information, if you wish to denigrate any manufacturer for using it you might want to understand more about what the number legally means first. Once you understand what this number means on a product that carries an NRTL agency approval, you can then calculate backwards by using efficiency to discover the maximum rated power in RMS terms. I normally don't post here, but am available at TalkBass as many of you here already know.