I couldn't find the power consumption figures for the Emotiva so quoted the transformer (not an accurate consumption figure).
There is nothing wrong in quoting transformer ratings. In this case, it is given by Emotiva in the published specs anyway and we can access such information on their website.
For clarity, I should explain just a few things about transformer ratings.
1. Transformer output rating on the nameplate literally refers to what it is rated to output in VA that is simply voltage (in Volts) X current (in amperes). It is not how much power in watts it delivers, and not how much power in watts it will draw from the house power outlet.
2. A 600VA rated transformer is rated to output 600W continuously into a 100% resistive load, again only if it is 100% resistive.
3. Power (in watts) consumption by a load depends not only on the applied voltage and current draw but also the power factor, that is the cosine of the phase angle between the current and voltage phasors. For a pure (theoretical only) inductive load, the current lags the voltage by 90 degrees and for a pure capacitive load the current leads the voltage by 90 degrees. For a complex load such that of a speaker, it could be anywhere between 0 and close to 90 degrees lagging or leading and would also vary with frequency.
4. Transformers are typically rated in VA because this way, regardless of the power factor of the intended load you can calculate the power output in watts for a specific load using the formula:
Power output (Watts) = Voltage (Volts) X Current (amperes) X Power Factor (see 3. above).
5. A transformer rated 600VA such as the one for the UPA-5 will draw slightly more than 600VA from the outlet when operating under rated load condition. Exactly how much more will depend on the specific transformer's efficiency but in general transformers are very efficient, typically >90%.
6. Transformers generally have very good overload capability and can run under overloaded (to a point) conditions on short term basis. The output voltage will obviously drop more as the load current increases but it will not just blow up unless you really push it hard and long. Depending on how well one is made, you could run it under say 10% overload condition all day long and it will not likely blow up in a hurry, but you will shorten its life in the long run if you overload it continuously.
In the case of an audio amplifier, you have to factor in its efficiency in any power consumption calculations. As you already know there are many different designs such as classes, A, B, A/B, D, H etc. Efficiencies vary between those classifications, but even within the same class their overall efficiencies are not necessarily the same. So when people say class A/B efficiency is about 45 to 55% they are talking about 'rule of thumb' kind of thing.
The EMO XPA/UPA amps are class A/B so you could assume an overall efficiency of between 50 to 70% under rated to slightly overloaded conditions. Consumption figures are not always a good indicator of an amp’s rated output because as Audioholics staff had discussed before, you cannot be sure under what kind of rules/standards/load conditions they are taken. I have yet to see a single mid range amp manufacturer who publishes their product's power consumption under rated ACD condition. Those who do provide consumption figures occasionally, such as HK, Yamaha would say something like 'maximum' power consumption. Such figures are not much more useful as they do not tell you whether the amp was measured at rated load under ACD, rated THD+N, phase angle between the load current and voltage, frequency/or range, and for how long, continuous or not etc. It would be difficult to try working backward to get the amp's rated output from their given consumption figures.
Transformer rating could be a reasonably good indicator of the low and top end limit (still very approx only) of an amp's power output if you know the design class of the subject amp and assuming the amp section is not the bottle neck. If you can in fact find the power consumption of the amp under rated conditions as stated above then yes it would be more accurate.
Generally, with a A/B amp the power consumption an of an amp an accurate indication of the power rating from the manufacturer. For example an 7 channel A/B amp drawing 400 watts may output about 57 wpc all channels driven an thats pushing it,
May be I am missing something here, from what you have said yourself, you do know that a class A/B amp (XPA/UPA) does not have 100% efficiency? So if the amp draws 400W it will output nowhere near 57WX7. Even if the load is 100% resistive, say an 8 ohm resistor, it will at best output a little more than 40WX7, most likely less. Even then, it would be just for a resistor. A loudspeaker has complex load characteristics (resistive, inductive, and capacitive). Its impedance is also frequency dependent. So the output of the amp driving such a complex load when 'drawing 400 watts' (your words) is going to output less than that calculated by just allowing for losses (efficiency factor) alone.
but the manufacturers accurately quotes the 100wpc spec into 2 channels (each channel outputs about 1/4 of the total output for stereo and it sounds better to the consumer).
Apparently some provide such numbers by following some 'FTC' standard but we do not know for sure and even if it is a fact that some do, others may not.
Class A amps are less efficient and class D amps more.
You are correct. In fact they are probably at the opposite ends of the spectrum. Class D amps could run as high as 90% efficiency while class A couldn't do much better than 20%.
So true Josuah. Many here say all amps are the same, which is ludicrous. Your brave making this controversial statement.[/
I think both sides could be considered brave making such general statements such as all amps sound the same, or, different.. I have seen slightly more specific comments such as "all well designed amp operating within it's design limits......" and "amps can sound different if/because.....................".
