[Craig Sawyers]

Ususally with an oversized core, and specifically arranged to have minimum acoustic hum.

For a domestic power amp playing music (rather than continuous sine, or some other artificial signal), the supply can be 70% the rating needed for the nominal power output. So a 2x100W amp will only formally need a 140VA from the supply.

Now a simple supply (bridge and reservoirs) pulls pulse-like current out of the transformer, so it needs to be 50% higher VA rating than the 70% rule - so say 200VA for a 2x100W amp.

In higher quality audio gear, (or pro audio which is turned up to 11), the transformer is significantly larger than that formally needed. This is to compensate for real loudspeaker loads, which have significant impedance (including phase) variation, and so can suck significant current.

So audio grade transformers tend to be much larger VA rating (and smoothing caps of larger value) than you might expect from a naive calculation. In the above case of 2x100W you might typically fit a 300-400VA transformer.

A good (and over the top) example is the Naim Statement, where each monoblock power amp can produce 746W into 8 ohms, 1450W into 4 ohms and 9kW pulsed into 1 ohm. That uses a 4kVA transformer. Each one (there are two of course) weighs a total of 100kg, a decent chunk of which is the transformer and the rest of the power supply.A fair chunk of the rest is the massive convection cooled heatsinks, which if you run the thing at 1450W into a 4 ohm dummy load will have to cope with ~600W of waste heat. (class B is ~60% efficient at full whack)

Craig