15-08-2020, 03:09 PM
Yes, "Watts RMS" is a marketing term, as "continuous average sine wave power" is a bit of a mouthful 
Being charitable, I suppose you could say the term indicates that the quoted value was derived from an RMS voltage measurement. The usual procedure is to wind up the power until the THD measures 1%, record the voltage, then calculate the power based on that and the value of the dummy load used. But some manufacturers might go for 10% THD. Given how abruptly the THD rises as you reach clipping with a typical design with plenty of negative feedback, there isn't really much of a difference between 1% and 10% THD. But for a valve amp, or some other low feedback design, that approach can be quite useful. But with all that in mind, I tend to regard power ratings with no distortion value as incomplete measurements. But power ratings are usually very conservative because power supplies are most often unregulated, so the power you get at 1% or 10% THD will depend on the mains voltage at the time of the test. Plus, manufacturers are very keen to not get caught out by (the small number of) reviewers who actually bother to measure gear. A typical 50W amp might measure 55 or 60 watts on test - perhaps more.
Peak power is the same, only the peak voltage rather than the RMS value was used. Obviously, that's a straightforward doubling of power for a sine wave. Understandably, many are cynical of this, but in practice, music is rarely sinusoidal, and the practice of focusing on "continuous average sine wave power" is a arguably not exactly representative of how an audio amplifier behaves with real signals. But it is an easy test to do, I suppose.
The peak to mean ratio of a sine wave is 3dB, whereas real music can be much larger - perhaps 20dB. So a 100 watt amp that is just hitting clipping on the peaks might only be delivering an average power of 1 watt to the load. This is good news for designers, who can save a lot of money on heat sinking and power supply components - the non-audio engineers will cynically decry that, but those in the industry know that building a 100 watt audio amp that is capable of delivering a 100 watt sine wave continuously would be massively over-engineered (and costly) for the intended application. In a business sense, over-engineering is as bad, perhaps worse, than under-engineering.
But all this means that audio amplifiers deliver higher voltages with music than they do with sine waves, and this introduces the concept of "dynamic headroom", which could be a couple of dB in practice. This is one of several reasons why regulated power supplies in audio power amps with a mains power supply are rare (and actually not a good idea). Again, non-audio cynics will (understandably) scoff, but in practice it's welcome.
All that only scratches the surface, but suffice to say, power is complicated when it comes to audio. But it's one of those nice scenarios where real-world use does actually favour the designers for a change. Not just of amplifiers, but also loudspeaker manufacturers. If you think amplifier power sounds tricky, speaker power is a whole other world!
I'm worried that by discussing audio power specifically, we've straying from the topic here. If this develops further, or if the OP feels it appropriate, I'll break it out into a new thread.
I'm also thinking about moving this thread to "General Chat". Mike, do you have a specific reason for wanting this to be in the private section? If so, I'll respect your wishes, but as a general point, I'm starting to feel that we might use the private areas a bit too much - which means that non-members miss out on potentially interesting and valuable discussions - discussions that might attract new members, perhaps...
Being charitable, I suppose you could say the term indicates that the quoted value was derived from an RMS voltage measurement. The usual procedure is to wind up the power until the THD measures 1%, record the voltage, then calculate the power based on that and the value of the dummy load used. But some manufacturers might go for 10% THD. Given how abruptly the THD rises as you reach clipping with a typical design with plenty of negative feedback, there isn't really much of a difference between 1% and 10% THD. But for a valve amp, or some other low feedback design, that approach can be quite useful. But with all that in mind, I tend to regard power ratings with no distortion value as incomplete measurements. But power ratings are usually very conservative because power supplies are most often unregulated, so the power you get at 1% or 10% THD will depend on the mains voltage at the time of the test. Plus, manufacturers are very keen to not get caught out by (the small number of) reviewers who actually bother to measure gear. A typical 50W amp might measure 55 or 60 watts on test - perhaps more.
Peak power is the same, only the peak voltage rather than the RMS value was used. Obviously, that's a straightforward doubling of power for a sine wave. Understandably, many are cynical of this, but in practice, music is rarely sinusoidal, and the practice of focusing on "continuous average sine wave power" is a arguably not exactly representative of how an audio amplifier behaves with real signals. But it is an easy test to do, I suppose.
The peak to mean ratio of a sine wave is 3dB, whereas real music can be much larger - perhaps 20dB. So a 100 watt amp that is just hitting clipping on the peaks might only be delivering an average power of 1 watt to the load. This is good news for designers, who can save a lot of money on heat sinking and power supply components - the non-audio engineers will cynically decry that, but those in the industry know that building a 100 watt audio amp that is capable of delivering a 100 watt sine wave continuously would be massively over-engineered (and costly) for the intended application. In a business sense, over-engineering is as bad, perhaps worse, than under-engineering.
But all this means that audio amplifiers deliver higher voltages with music than they do with sine waves, and this introduces the concept of "dynamic headroom", which could be a couple of dB in practice. This is one of several reasons why regulated power supplies in audio power amps with a mains power supply are rare (and actually not a good idea). Again, non-audio cynics will (understandably) scoff, but in practice it's welcome.
All that only scratches the surface, but suffice to say, power is complicated when it comes to audio. But it's one of those nice scenarios where real-world use does actually favour the designers for a change. Not just of amplifiers, but also loudspeaker manufacturers. If you think amplifier power sounds tricky, speaker power is a whole other world!
I'm worried that by discussing audio power specifically, we've straying from the topic here. If this develops further, or if the OP feels it appropriate, I'll break it out into a new thread.
I'm also thinking about moving this thread to "General Chat". Mike, do you have a specific reason for wanting this to be in the private section? If so, I'll respect your wishes, but as a general point, I'm starting to feel that we might use the private areas a bit too much - which means that non-members miss out on potentially interesting and valuable discussions - discussions that might attract new members, perhaps...
