The 50-65% seems like the "average" range I've read in lab tests at 8 ohms and 0.1% distortion going back 20 years (when the transformer size is stated, like in the AH reviews).
As agreed, those are good average, typical numbers for efficiency. That is however, as I mentioned before, not always a reliable way to estimate an amp's transformer's VA rating. Efficiency is just one factor, albeit an important one as class AB is so much more efficient than class A. For transformer VA sizing, It is more about the designer's approach, in terms of how much they want to take advantage of transformer's overload capability and the nature of music and movie contents that are not continuous sine waves at constant levels. So while an AVR manufacturer may be comfortable with a really low factor of 0.7 VA/W as cited in the Elliot Sound article, or a reasonable one of 1VA/W, whereas the much more robust 2 VA/W 8 ohm rated output approach may be adopted by more expensive amps such as Bryston, Parasound Halo, Anthem's P and STR series power amps. There are also the >2 VA/W approach at the extreme end such as your Citations, and some Passlab models, with the likes of Anthem's MCA series, Adcom, Monolith, Outlaw amps that seem to fit in between, just loosely and as examples only.
There are very few articles on the web on transformer sizing, but Elliot Sound has at least one article on that topic. Other than that, one would have to resort to forums where knowledgeable people hang around, and/or do your own calculations.
Here's the Elliot sound article on Linear Power Supply Design:
ESP - The Audio Pages. Linear power supply design information - Learn how to design your own high performance amplifier power supply.
sound-au.com
It is a long one to read, but you can always jump to near the end and just read the VA Rating paragraph pasted below.
"VA Rating - Class-AB
The minimum VA rating suggested is equal to the amplifier power. A 50W amp therefore needs a 50VA transformer, or 100VA for stereo 50W amps. Larger transformers (up to double the amp power rating) will provide a 'stiffer' power supply, and this may be beneficial. For continuous operation at full power (never needed for hi-fi but common for guitar amps), the transformer should have a VA rating of up to 4 times the amplifier power.
It is suggested by some transformer manufacturers (and no doubt gleefully adhered to by many amplifier makers) that the VA rating needs only to be 0.7 of the maximum amplifier power. While this will work well enough in most cases, you will not have a 'stiff' power supply - a more appropriate term would be 'soggy'. The DC voltage will collapse as more current is drawn. It's unlikely that you'll hear any significant difference between the smaller and larger transformers with music."
Here's a DIY thread that has some useful suggestions, I think Nelson Pass, Gedlee do join in some of those conversations occasionally.
What should be the transformer rating in respect to wattage of a power amplifier, As we all know nobody will be feeding a sinewave signal to the amplifier,nor would anybody use it at maximum power , hence what should be the wattage of a transformer for a poweramp of say 250 watts for optimum...
www.diyaudio.com
I still have one of Pass's F5 amp to build yet. I have only ordered the amp kit, will soon have to figure out the power supply transformer need soon. Being class A, I think I would go with 300 VA, or may be 400 VA maximum.
Would a specific Class A/B amp model's output also depend on how much class A bias it was designed for?
Yes, definitely, the higher the bias the warmer it runs, all else being equal. I am not an amp designer but I have read enough to know that it doesn't mean you need bias high enough to be in class A for the first several watts to sound the best or to achieve minimum crossover distortions. If designed well, it could be just high enough to ensure there is a good overlap such that both output devices in the push and pull cycle are fully on and conducting long enough so there is no dead zone around the zero crossing point; and of course good use of feedback will be key to keep distortions low. Benchmark claimed they also use their feed-forward technology, to virtually eliminate cross over distortions.
