48V phantom power from 12V AC transformer

[beatnik]

I am trying to come up with a power supply for a tube mic preamp using an off the shelf toroidal transformer.

This transformer has a spare winding 12,6V @ 1A and I want to power relays and LEDs with the unregulated DC voltage, a VU meter buffer with a 7812 regulator, and 48V for phantom power.

The first two are easy enough but I am wondering what would be the best way to obtain the 48V.

What about a voltage quadrupler circuit like the schematic attached plus a regulator with TL783 ?

On some tube preamps I have seen the 48V phantom derived from the HT rail and regulated with a zener diode, but I am wondering if it could be dangerous in case of a fault the HT voltage will reach the microphone.

Can you think of a better solution ?

 

[rogs]

Here's an image from the schematic for the dbx286s channel strip, which shows how they derive the 48vphantom power supply, using a voltage multilplier and a zener based regulator.
Source is 19VAC rather than the 12v you have available..... One extra multiplier stage maybe ? ....

 

[gyraf]

Don't rely on dropping HT voltage - too dangerous. You simply can't trust your pass element to fail open.

Look at e.g. the G9 project for 15V-to-P48 by a simple voltage tripler - you may perhaps need one extra tripler stage if running on 12V:

http://gyraf.dk/gy_pd/g9/g9_sch.gif