isolation in power supplies (preventing noise accumulation)

I'm worried about noise in mic preamp power supply that gets the +24V and +48V phantom from the same 24VAC transformer secondary. I would've liked to use a dual secondary (48VAC center tapped) transformer for isolation between the two supplies but could not find one that fits in a 1RU chassis. The 48V comes from a doubler which taps the AC before it gets to the full wave rectifier for the 24V supply. I'll try to post the schematic in The Lab when I have a legible copy drawn.

Is extra noise really something to worry about? Should I use an external supply with the center tapped trafo instead?

Also, as the subject of this post says, feel free to discuss other ways to reduce audible noise and hum in a linear DC power supply. I know about using IEC inlets with EMI shielding and that it's good to filter out any DC offset before the trafo primary. Any other fun tips?

Thanks!

I've found most noise on the 48 volt supply disappears if you do three things:
1. Hang a 100 ohm resistor followed by a 1,000 uF capacitor (simple RC filter) after the 48 volt regulator.
2. If you're using an LM317 regulator, couple the ADJ pin to a nice quiet ground using about 220uF of capacitor, and remember to put in those little protection diodes in there.
3. Add a metal shield around the transformer (even mild steel works for this though MuMetal would be better).

I don't get hum or noise on the phantom if I do these two simple things.

-Dale

I'm already doing 2 and 3 but the filter is a good idea. You're talking about a low pass, right? So the 100ohm is in series with +V and the cap is connected from +V to GND immediate following? Thanks, Dale!

Yes. That got rid of the last bit of hum in my phantom supplies on all of my DIY preamps.