How to guess the volt-amp rating of the transformer in my amp


I'm the owner of a Marantz PM 6006 integrated amp, which the company touts as having a "high-current toroidal power supply" without actually giving a rating of the volt-amps the power supply is capable of. It's something I'd like to know, because for better or worse I'm just a numbers-oriented person.
I've used Google, service manuals, parts numbers and Marantz customer support without success. Instead, I've come up with two very rough methods of guessing what the transformers VA rating might be. I freely admit these are error-prone guesses by a non-expert, and I'd like to ask if anyone with more knowledge than me could comment on the validity of these two attempts.

Method 1 (Use the approximate size of the transformer)
The transformer is in an iron case, with a radius of 11 cm and height of 7 cm. I spoke to someone at a company that builds custom audio toroidal transformers, and he thought a transformer of those dimensions would likely be in the range of 250VA -300VA.

Method 2 (Use the power consumption rating on the back of the amplifier)
The back of my integrated amp says "Power consumption 395 Watts". If I assume this represents the maximum, or transient peak power consumption, the Volt-Amp rating of the transformer would be equal to the product of 395 watts and the "power factor" constant, which could range from about 0.6 to 0.8. By that math, the VA rating would be between 237 and 316 volt-amps.

If correct, either of these methods would lead me to the conclusion the transformer is either 250VA or 300VA. Thanks in advance for any perspective you can provide.
johnanguswebb
Another way to guess:
Fuse Amperage rating X incoming AC voltage,
2.5A X 120V = 300VA
Just guessing :)
You could also multiply the mains fuse rating by 120 and that'll be in the ballpark.
Thanks for your help!
On the circuit board by the fuse (next to the transformer) the following is written:
100V T5AL 250V
120V T3.15AL 250V
220-240V 1.6 250V

I assume these are for different countries with different mains voltage. The bottom two both work out to 352-378 VA, which seems in the ballpark. The top one is 500 VA. I’m surprised it’s so high. I’m also thinking the fuse wouldn’t be set to blow at a current higher than the transformer’s rating, but that’s not my expertise. Could I ask how well these numbers fit the hypothesis that it’s a 300 VA transformer?
To answer my own question: I read online that a rule of thumb to find the fuse rating for a given circuit is 1.25 x Power/Volts. (I’m guessing 1.25 is some kind of fudge factor.) Applying that formula for 300VA gives 1.25 x 300VA/120V = 3.13A. The closest available fuse rating would be 3.15A, which is what’s on my circuit board. If I left out the "fudge factor" the rating would be 2.5A, as imhififan predicted. So I think I do have a 300VA power supply.
I still wonder why the power comes out differently following the fuse recommendation for 100V mains power, but I’m not going to try to figure that out.

The sizing of a transformer is actually a lot more complicated than just determining how many watts the unit consumes. Also, the fuses don’t necessarily indicate "actual power use" as they are usually sized to be over the actual usage to allow for those slight "overhead surges".

In your case, the description of "high current transformer" is most likely a marketing ploy. The PM6006 only really pushes 45 watts per channel into 8 ohms and 60 watts into 4 ohms, so it is not a "high current beast" anyways.

The power output of the amp is really determined by the combination of transformer size and power supply capacitance size. It will be a sliding scale. Let’s take two examples:

Parasound A23 - 125 watts x 2 into 8 ohms

1000 VA transformer (1kVA)

48,000uf capacitance in power supply


Odyssey Khartago basic version - 110 watts x 2 into 8 ohms

400 VA transformer

120,000uf capacitance in power supply


So, both have about the same power output, but one has a tiny transformer with huge power supply bank. The output of the Khartago is mostly driven by the power supply capacitance (bass hits) because the smaller 400VA transformer just doesn’t have enough brute force to power/push the bass frequencies.

The output of the Parasound is driven more from the massive transformer for the larger bass frequencies (current coming in from 60hz A/C waveforms), with the smaller capacitance filtering for ripple and faster midrange/high transient spikes.


So if you’re just asking out of curiosity, the answer is "it really depends". lol.

That being said, it is true about transformer size dictating output.  Based on what I know, I would agree that transformer is in the 250-300 VA size. 
Thanks, that’s really interesting! I just learned what the capacitors are for.
A look inside my PM6006 reveals it has 10,000 uF capacitors paired to its presumed 300 VA transformer, which is humbling by comparison.