Fuse Amperage rating X incoming AC voltage,
2.5A X 120V = 300VA
Just guessing :)
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.