Class AB / Class D ratings?

[teemuk]

That said, what I heard was that the peak power rating on a class D amp is more applicable as a rating than RMS than on a class AB amp because of the faster power ramping on a class D amp.

I have never heard this. Are you sure you are not confusing "burst" and "continuous" to peak and average respectively?

The thing with most class-D amps is that they can not sustain high average power output for long periods of time without overheating. Therefore manufacturers can't provide "continuous RMS" ratings for them.

However, musical instrument amps do not drive motors so they don't even need such "continuous" performance and in addition the "music" signal has such high crest factors that momentary "burst" power figures are actually a rather good approximate of the amp's performance in terms of output power.

IIRC, something like an ICEpower module, cooled properly, can sustain its full rated output power for about a minute or two. After that the thermal protection kicks in and starts to severily limit the output power. Naturally you can't give such an amp a "continuous" output power rating but its amplifying your bass, not driving a motor so in expected application this does not matter.

When you pluck your bass the initial transient of note attack may easily be eight to ten times as high as the average signal level. If the amp can produce this momentary "burst" at high power without clipping the rest is easier because in average the amp is not even demanded to produce such high power levels.

There is a lot of unneccesary elitism about musical instrument amps supposedly having to be able to sustain their highest rated power "continuously" and indifinitely and burst ratings being dishonest in some regard. It doesn't work like that.

 

[agedhorse]

Speakers will also suffer from damage when fed continuous sine wave signals, so the duty cycle de-rating apply there as well.

 

[Bob Lee (QSC)]

Talking simply, RMS is the average power output over a long time.l and peak is the highest output, even if only for a split second.

Not necessarily. "RMS" power is only a colloquialism. RMS properly applies to voltage or current, not power, but it's sort of been applied for decades by marketing departments to describe power calculated from RMS voltage measurements into a defined load resistance. There is no time definition to "RMS." In fact, I remember about 25 years ago looking at the specs of a questionably designed power amp from a brand that briefly made an appearance on the market and went away again. Their specs said "RMS" power but used the IHF method for measuring: a 1 kHz repeating tone burst (at the point of clipping) 20 ms long followed by -20 dB for 480 ms, repeating every 500 ms. This 20 ms burst was presumably measured for its RMS value, and they calculated a power rating from it. It wasn't conventional, but I can't say they lied about a term that has no standardized definition.

That said, the FTC Amplifier Rule doesn't apply well to a lot of implementations of Class D amps today that are engineered to have high dynamic power. Klas Dalbjörn at Powersoft came up with a really interesting test signal series for describing power amp performance. Unfortunately, at QSC I couldn't get anyone outside of R&D to give it much of a consideration.