I've been musing on this one, still wondering why relatively low-power audio amplifiers need something like this when, in my limited experience, toroidal transformers in the 500-650VA range for stepper supplies seem to work happily without anything needed. Is it that inrush currents per se are not the issue with audio amplifiers; is it actually to avoid the switch-on thump which is annoying and potentially damaging to loudspeakers? I would guess that even a relatively high-power (domestic) audio amp is unlikely to be rated at much more than 300VA and unlikely to trip breakers or anything like that. Much larger transformers are likely to trip breakers at switch-on so might need special measures - although a suitable curve MCB might help?

Muzzer - happy with your explanation of saturation effects. Vague memories of B-H hysteresis curves seen through closed eyelids while the world's most boring lecturer copied his yellowing notes to the white board... However, I have no feel for the magnitude of these effects so bow to your more specialised knowledge. Clearly there are several factors at play including smoothing capacitance, transformer primary resistance, and so on, but I wasn't aware of core saturation effects. What I can say, and maybe this is directly relevant is that this last weekend I installed a 500VA toroidal transformer in my newly-acquired CNC lathe, with each 50V secondary directly connected to a stepper driver rated at 60V AC/80V DC. I assume that the driver contains whatever smoothing capacitance the designer thought it needed. While testing since then I have probably switched that thing on and off dozens of times a day and nothing has gone pop yet! I expect to spend more time making sure E-stop mechanisms are in place than I shall about inrush limiting, but that's my personal view. We all plough our own furrow!