> If PRR could build this supply out of a tube and a transformer I bet he would..
It's hard at 5V, or even 18V. At 130V, it was once common: every other US home had a hollow-state 20KV flyback boost converter in the TV.
> you can get a *near* square output from a 556
And...? We don't need a square, we need any kinda wiggle that we can jack-up. Square is often more efficient, but we only talkin' a half-watt per mike, nothing's going to burn-up even at 50% efficiency. And square's efficiency also means it throws spikes all over: a falttened-top sine-ish wave might throw less hash.
IIRC, you build those large motor controllers. They have to be just-right or they burn up or sell at a loss. This is a different world. No doubt there are sophisticated little step-ups in some audio boxes, built by sophisticated guys like you. But in DIY Audio we just stick things together and the simpler the better.
> an inductor-based type, the switched capacitor and multipliers types are very noisy
Hmmmm. I ran some noise plots on that MobilePre USB, and noted a small spike near 3KHz. Only open-input, totally masked by room and mike noise. I blamed the room, full of PCs. But now I'm thinking: 555, lowest-price diodes, that could have been the Phantom supply's 555 wiggle frequency. Yeah it is in the audio band but, without any sealed box, it was near-enough neglgible for a $150 box.
At a gross level, the switching spikes are the same. The input current toggles between the supply rails and is equal to output current times multiplication times about 2 for 50:50 duty cycle. The coil likes higher frequencies and probably won't be 3KHz, so may be easier to filter. Cheap diodes don't like to break 20KHz so a many-diode/cap multiplier may end up in Audio. I'm sure the diodes don't all switch at the same instant so it may leave a ripple like a crippled centipede. Both ways are ugly; are they uglier than running to the store for five 9V batteries every 8 hours? Or in wall-power work, doing something at 50/60Hz?
The real answer, since we are stuck with +48V, is to design the preamp for +48V.